How-To: Size Your Pilot Jet, Pilot Screw Explained
Every carburetor know to man will have a way to adjust the air fuel ratio at idle. This adjustment will trim the mixture so that you can have a smooth running engine.
The first step is to determine if you have an AIR screw that meters air, or a FUEL screw that meters fuel. They work opposite of each other so it is important to know that before performing this procedure.
A carb will have an air screw or a fuel screw 99% of the time. There are carb models that have both a fuel and air screw, however I actually have never worked on one in 20+ yrs of wrenching.
The Goal
What you are after when adjusting this screw, is to reach peak idle RPM and smoothness. This is the engine telling you it is happy, and it runs best at this mixture.
How do you tell if your pilot jet is too lean, rich, or just right?
The pilot jet is sized correctly if the fuel screw setting falls between 2.5-3 turns out.
The pilot jet is sized correctly if the air screw setting falls between 1-2 turns out, with 1.5 being the sweet spot.
What does a pilot air screw look like? What does it do?
A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.
What does a pilot fuel screw look like? What does it do?
A pilot fuel screw will also be made of brass, gold in color, and will have a pointy “needle tip”. That is why you always want to “lightly” seat these, the tips can easily break! I use the smallest diameter screwdriver so I generate the least amount of torque. The pilot fuel screw meters fuel, so turning it clockwise/in will lean the fuel/air mix, while turning it counter-clockwise/out will richen the fuel air mix.
Where is the pilot air/fuel screw located?
Let’s define the carburetor butterfly or slide the dividing line between the air box side, and the engine side. A pilot air screw will be located on the air box side, while a fuel screw will be located on the engine side. This is true for most carburetors. I’ve seen the pilot air screw on the engine side on most Amal carburetors. When in doubt, remove the screw and see what kind of tip it has, that will determine what it meters.
If you can’t find the screw, it may be hiding under a welch plug. This video walks you through removing them.
What are some indications of a lean pilot jet and/or screw setting?
- If your air screw is in further than 1 turn, you need a richer pilot.
- If your fuel screw is out further than 3 turns, you need a richer pilot.
- Off idle hesitation or bog.
- Revving the engine in neutral will result in hanging RPM’s, or RPM’s will slowly drop back to idle RPM.
What are some indications of a rich pilot jet and/or screw setting?
- If your air screw is out further than 2.5 turns, you need a leaner pilot.
- If your fuel screw is in further than 2 turns, you need a leaner pilot.
- Off idle heavy feel or sputter.
- Revving the engine in neutral will result in RPM dip below idle RPM, RPM’s will drop quickly.
- Turning the fuel screw all the way in will not stall the engine.
How do you properly adjust a pilot air/fuel screw?
The proper way to tune a fuel/air screw is to make adjustments on a fully warmed up engine and at LOW RPM. Lightly seat the fuel/air screw and set to factory settings as a baseline. With engine running at low RPM, turn the screw in clockwise until the engine begins to stumble, then turn the screw counterclockwise until the engine stumbles, noting how many turns in/out both occurred. Now, adjust between these two settings to achieve highest RPM and smoothest running engine. Reset idle to recommended settings and test ride motorcycle. If engine bogs or hesitates during acceleration, richen idle screws 1/4 to 1/2 turn and see if that helps. Rev engine in neutral and note how RPM’s return to idle. If RPM’s hang, richen the mix. If they dip below idle, lean the mix.
Here is a short video I put together to help explain this information along with a FUEL screw adjustment procedure.
Here is an example of a an AIR screw adjustment.
Comments
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hi i have a yamaha grizzly 660 2004 what causes it to back fire when i let up on the throttle after driving at about 60 -80 km hr please and thankyou
Jim,
When you close the throttle and decelerate, the engine runs lean, and can pop through the exhaust. Sometimes this is considered normal with a race engine and open pipe.
However, if this is something new, check for exhaust & vacuum leaks, as they can cause this as well.
Give me a bit more info: stock pipe, jetting, fuel screw set properly?
Hi, this website is very helpful for me. Thanks before. But i have an issue on my mikuni bs. I’ve set the pilot jet to 17,5 and the fuel screw set 2 counterclockwise. Its fit while engine on idle. But when run test, the engine feel rich and the spark plug is black. Try to to lean the screw its not helping. So I’ve changed the pilot jet leaner to 15. Engine run idle on fuel screw 5 counterclockwise. The engine was smooth enough for running. But the spark plug is minor lean. What should I do? Thanks for advice. Cheers~
Those are 2 drastic results for changing pilot jet one size. With the 17.5, is the screw responsive? How many turns out for peak RPM and smoothness? If you screw it in, can you kill the engine?
yes indeed. its strange. last night i was try using 17.5 again and get good idle on 2 counterclockwise screw. and if I screw all in, the engine will dies. but same, i feel the engine so rich while running low. if there any possible the pilot jet was broken or the size is incorrect due imitation product? or maybe i need to setup the float more lean? thank you for the reply Matthew. Very appreciated
How many turns out for smoothest and highest peak RPM with the 17.5?
Why do you suspect it is rich?
the highest rpm is 2 turn out. if i turn out more 2,5 the engine start stumble and turn in all the screw, the engine dies. and the spark plug was black
It’s odd that you get 2 turns out with 17.5, and 5 turns out with 15.
I’d buy a new set of genuine mikuni pilot jets in sizes 15 & 17.5, and retest. Also make sure tips on fuel screws and not damaged.
Matt
okay, will check again the fuel screw and buy an original mikuni pilot jet 17.5 and will give you a feedback later. thanks bro~
Hello I have a 2005 yfz450 , air intake uni and a gytr pige on it… I have the pilot jet 45 and am out 4 turns , I change it to a 48 and and it dose go to 2 turns but the rpm dont come back down quick when I let go of the throttle??? The carb is 90% rebuilt the only thing that i have not replace is the air cut valve cut that be a factor as to why it doing that??
Maybe. Also you’ll want to check the condition of the vacuum plate seal.
Hey I have a 1982 Honda gold wing gl1100 been trying to get it in tune but it’s really hard to tune the pile when it’s hot is there some kind of special tool I can buy?
Not familiar with your model, but here is a 90° screwdriver made for those hard to reach fuel screws:
https://amzn.to/3iS2cJj
Hello, I have a YCF Pilot f150 (it’s a pit bike) it came with a airbox an a Nibbi 19mm carb.. 19mm carb is really too small of a carb for the 150cc engine.. I purchased a Nibbi 26mm carb to get the power the 150cc engine has.. the Nibbi 26mm came with a 40 Pilot an 110 main an run like garbage an bogs, sometimes dies right off idle.. I have adjust the aircrew an swapped out different jets up an down in sizes.. I been messing with this carb for about 2 months now an can’t get it right for he life of me an driving me crazy.. any help will be greatly appreciated
Is the airscrew responsive at all? I would focus on the pilot jet / AS first.
I have a 1994 honda shadow vt1100c I am sitting at about 3.5 turn out. Idle is fine however when i drive it if i ease the throttle it boggs if i open the throttle on full it just perfect what do you suggest stock pilot screw is 40. I have a 45 in it
I’d probably go with 50’s.
Is this a stock bike? Or air cleaner / exhaust modifications? Did you check for vacuum leaks? Did you raise the needles with a washer?
hi matt how are you i have a 99 srad 600
my idle jumps up and down
at pull away i stragle have to rev it up to pull away
i have read this blog but cant say if its lean or rich because if cold and i rev the rev wil drop lower than idle but when the bike is hot and i come to a stop the bike wil rev 3000 and if i drop clutch a bit it wil drop to idle again
Check for vacuum leaks. Are the carbs clean?
yes i cleaned the carb
and change all the vacum pipes
new spark plug
air fliter boks had some oil in clean that out
i also forgot when brakeing als back fires
Hello, I have a 2004 KLX400SR and it has a Stage 2 DynoJet kit, Yoshimura slip on half exhaust, a twin air filter and 3×3 mod done to the airbox. After the kit is installed, it claims that the fuel screw should be adjusted within 3 1/2 turns. My fuel screw is approx. 3 1/2 – 4 turns and it won’t idle down unless I drag the RPM down by letting out the clutch a little bit and then it will stay down at a proper idle until I give some gas again and then it continues to idle high. Is my pilot jet wrong or should I be looking at something else. I sprayed the boots and carb trying to see if there are any vacuum/air leaks but can’t find any.
If you continue to turn the fuel screw out, does the RPM increase? If so it wants more fuel. If you hit over 3 turns on fuel screw, increase pilot jet size.
The RPMS should fall quickly and not dip below idle RPM when throttle is blipped.
Matt
Hi,
I have an 82 Susuki GS650G, Canadian Model. The previous owner replaced the old air cleaner with four individual cone filters letting through allot more air. He also put on quieter exhausts. When he sold it to me he mentioned that I needed to increase the Jet size, Didn’t tell me which jets, and that I needed to raise the pins. Due to time problems I have yet to do either. Could these be the reason my Engine just randomly dies while on the Highway? Or could there be something more serious involved.
Thanks in Advance
Michael
Michael,
The pod filters may or may not let more air in at idle. They do let more air in at higher RPM’s.
Let’s start with the pilot jet. If your fuel adjustment screw ends up in the 1-3 turns out range, and have no bogging or hesitation off the line, it’s good to go.
The needle clip position and main jet is something you will have to test trial and error and read the plugs. Usually, I would expect the clip to be one position lower than stock and the mains 1-2 sizes bigger than stock. Get a hold of a service manual and note stock jetting. Open up your carbs and write down what you find.
Randomly dies on highway. Jetting may or may not be related to this issue.
I’ll be sure to do that this weekend, what do you mean by “Read the plugs”?
Reading spark plugs gives you indication of whether it is running rich, lean, or just right. See my post on “How-To: Read Spark Plugs” which covers this topic and also how to perform a plug chop.
All things being equal and no other apparent or obvious issues/problems, the most common reason a motorcycle inexplicably “dies” on the highway (and nowhere else) is fuel starvation due to empty float bowls caused by clogged filter screens above float bowl needle/seat intake valve assemblies. Remove the needle/seat assembly and use a small screwdriver or pick to remove the tiny little filters and clean them. If you can park the bike on the shoulder for 10 minutes after it dies (giving fuel time to refill float bowls) and then start up and run fine again (for awhile)… dimes to donuts, that’s your problem. It only shows up on the highway because the increased fuel consumption at highway speeds outpaces the fuel flow slowly dripping into the float bowl(s) through dirty filters and empties the bowls before they can refill. The most common reason for this condition is rust in the gas tank. Tiny particles can get through the fuel filters above the petcock (which also needs to be cleaned) or other inline filters (replace). People park their motorcycles outside, especially in the winter, with untreated bare metal interior, part-empty fuel tank causing the inside surface of the tank to rust. Warmer days and colder nights affects air pressure in the empty space above fuel in tank which expands and contracts drawing in cooler moister air in the evening and overnight through overflow/breather = rust.
I have and 83 virago 500 which has 2 carbs. How does tuning two carbs differ from one carb if it differs at all?
Both carbs should be tuned to the same number of turns out on the pilot fuel or air screw. This will result in a smooth running engine. Obviously, when tuning, you can only work on one side at a time.
I have a 2005 polaris predator 500. I removed the spark arrestor from the stock exhaust and this caused my quad to backfire frequently. it has stock jets. should i go to a higher jet i think its got a 150 main stock
Joey,
When does it backfire? On deceleration with throttle closed? Have you adjusted the fuel screw after removing the spark arrestor?
Mostly when letting off of the throttle. I have not yet I just replaced my valves and am debating if I will need to rejet
OK. I will tell you that some popping on decel is normal with an open pipe. I would try readjusting your fuel screw. If you are close to 3 turns out, bump up the pilot jet 1-2 sizes.
On the main jet, you’ll have to a WOT plug chop and read the plug.
What is a WOT plug chop? Eventually I’ll be upgrading to a k&n air filter eventually so I just want to get an idea of what my carb should be set at
WOT = wide open throttle. Search my site for “How To Read spark plugs”. With a K&N, you should probably richen the main by 2 sizes. Do you plan on running the air box lid? If not, maybe jump 4 sizes on main. The pilot is adjusted for max RPM and hopefully is in the 1-3 turns out range.
It helps to have all mods in place as you only want to do this once 🙂
Yeah I’m gonna keep the lid on. My airbox does have holes drilled into the back of it though
Sounds good. Check the fuel screw adjustment and report back. Main will need to be increased by 2-4 sizes for sure. You will be moving more air and need to compensate with more fuel.
I have a sherco 250 which is very flat/slow to pick up, would you advice a bigger pilot jet as adjusting the idle doesn’t seem to work too well.
Stuart, did you try to adjust the pilot air screw first? How many turns out? Does it bog? What does the plug look like after idling for a few minutes?
As it stands at the moment, the pilot fuel screw is at 2.5 out and the air is at 4.5, though I’m told anything past 3.5 makes no difference. Can’t say I’ve looked at the plug after idling but will do at the weekend. Does it bog ? No not really, just doesn’t pick up quickly from first applying the throttle.
I should have said it is a sherco 250 trials bike.
Stuart, you should have an air OR a fuel screw, not both. There will also be an idle screw. Get back to me on what you have. Most likely an air screw.
So not a tech question, but is that a Yamaha XS series bike in the video?
Matt, good eye. It’s a 76′ XS360
Sir.to increase speed in my bike(splendor plus). How to i adust the jet screw ,whether i want to turn clockwise or anti clockwise.
Clockwise.
Matthew your the man, great site!
I have the same xs750 “77” triple you have worked on.
Im about to fine adjust the carbs, and want to know the settings you ended up with? just for a basic start point when i adjust mine. I too have some small filters on, but have a aftermarket exhaust with more flow through.
cheers
jimmi
Jimmi,
Thanks for the compliments! The 77′ XS750 actually has an 850 motor in it that was punched out to 885 or 888. It has air filter pods and a MAC 3 into 1 exhaust. Pilot jet was one step larger (45?) and fuel screw 2.75 out. Needle in middle position. Main: 132.5. Runs good.
Great site ! I watched the how to about adjusting fuel/air mixture screws. I am currently tuning my carb and would greatly appreciate some guidance. I’ll give some info first.
2012 klr650 w/CV40 carb,just made changes from stock:
drilled slide 7/64
klx needle 2nd clip 1 #4 shim
142 jet (from 145)
new extended fuel mixture screw
slip on mid pipe back Lexx exhaust
race tip (big unrestricted one)
original tube spark arrestor swapped with free flow Spark arrestor
1″X 4 holes in air box
new uni filter
With all the changes, the bike transformed in attitude…great throttle response, and it just sounds mean!… Only thing is, I’d like to get it dialed in nice. It’s a little over poppy on the decel…( I expect some ) and the fuel mixture screw can be all the way seated and it doesn’t slow down idle.
It does run pretty good across the spectrum of adjustment, but better towards the closed side of adjustment. I did not change the pilot jet.
Any advice on getting this carb tuned in would be appreciated !
Again, nice work on the videos, real clear information for people to understand. Thanks.
Hi, Michael.
First off, thanks for the compliments, and the KLR is a cool bike BTW! I do quite a bit of work on a 87′.
Popping is to be expected with an open “race” pipe. You can get rid of it by richening pilot/fuel screw, but sounds like you need to go the other way, leaner, based off your fuel screw position.
Since your best idle is at almost closed fuel screw position, I would decrease pilot by 1-2 sizes and see if you can get fuel screw in the 1-3 turns out range.
BTW, can you give me a link to the adjustable fuel screw? It’s impossible to adjust stocker.
Hey Thanks for getting back to me, going to head to the bike shop in search of some pilot jets. I’ll let you know how it works out. As for the mixture screw, there are several KLR websites that sell the screw, but the same carb(cv40)is used on harleys a lot, so I searched outside of the KLR community. Found this place, the screw was cheaper, and the shipping was fast & free.
Thanks again.
http://www.cv-performance.com/products/
Thanks for the link, Michael.
BTW, I find it odd that you have to go leaner on pilot, usually these bikes are lean from the factory. Have you verified the pilot jet size in service manual?
Hi I am having trouble getting my quad to run and I’d like some advice.
It is a 2000 Kawasaki Bayou 220, we cleaned the carb in a dip tank and
made sure everything’s cleaned well, when we start it we use fluid because
it wont start on its own 50% of the time.
once it starts it idles for maybe 2-5 seconds and it dies, we’ve tried
tapping the throttle to keep it running white we fiddle with adjustments
but we have found no success, currently it sits at 2 turn out on the A/F
screw, and the idle screw does nothing regardless of position..
We’re at a complete loss as to what to do with it. I’m really hoping you
can help me because I’m losing my mind with this thing and I’ve got 3
more quads that wont run to fix still.. and those are:
a 94′ bayou 220
a 88′ Blaster YFS200
a 87′ LT250R QuadRacer
I’m sure I’ll have more questions for you since all of them are having
carburetor related issues :/
Jared,
If it will only run on starting fluid, then the carb is not delivering fuel. When you had carb apart, did you spray carb clean through all passages and verify they are clear? Did you hold all jets up to a light source to see if you can see through them?
Have you performed a compression test?
Hi,
The first time I cleaned out my carbs I didn’t know I would get them out of tune. I got it working really well afterwards, a lot of power, but I heard a clicking sound after about 2 months and I thought maybe it was something I did in the carbs. I opened up my carbs once again and cleaned them. When I put it back together, the pilot fuel screw wouldn’t make a difference when I would turn it and my motorcycle only runs max 55 miles now and has very little acceleration. I can’t seem to adjust it. My pilot fuel screw is different from the picture though, mine has a spring on it and it screeches when I turn it. Supposedly its normal, unless it’s not a fuel screw. Should I keep playing around with it or throw it at the junkyard? I don’t know what to do. If I can’t fix it myself, I don’t want to keep it. I don’t trust mechanics, I took it to one and he charged me 400 dollars and there was no change to it. He didn’t even give me a receipt. Supposedly he worked for Universal Studios fixing all kinds of machines… anyway thank you for help. Oh and I didn’t know about the air screw. I don’t think I ever saw that on my bike, is that inside the carburetor? Do we adjust that too??
Thank you,
Allan
Allan,
The fuel and pilot screw will have a spring on it. In most cases, the fuel screw will have a washer and o-ring also. The purpose of the images above is to illustrate the differences in the tip, which will indicates what the screw meters.
If you here “clicking” in your engine and the power dropped off, perform a compression test. Also, hold a screwdriver to ear and engine and try and locate the noise.
Hello, i have a kawasaki mule 2002 with the 620 motor. it never has run right at idle so after some time replaced carb with a new one from kawasaki. the new carb seemed a little better it gave me some idle adjustmant for a while but still seems to be quite lean at tickover. now it has the idle screw right in and strugles to idle and stalls. the top end revs seem good and there is power. also when punching the throtle it bogs. i have looked on various sites and they say there is only one size of pilot jet and there non adjustable due to some regulations?
Mark,
There probably is some way to adjust the fuel mixture via a pilot air or fuel screw. It may be “hidden” under a welch plug or cap. I’ll send you an email and maybe you can send me a pic of the carb.
Hello I need a bit of help I have a Kawasaki prairie 400 idles fine quarter throttle is ok around 3/4 to wot I have a loss of power for just a second it cuts out then picks back up .it sorta feel like a ignition problem but when I take air box lid off it runs fine no cutting out no backfiring through carb .but replace lid it’ll spit cough backfire.ive re built carb rejected smaller . finally new carb .and it does the same thing .I’ve used a multimeter to test ignition when problem happens no change in volt meter battery at idle is 13volts reved it climb no hight then 14 volts I’m stumped and advise.tjank you Mitch .mitchpizzinio@gmail.com
Is the carb clean? Main jet stock size?
Thanks for the reply, success i have found a pilot fuel screw hidden under a plug and it is running much better now. thanks
Great job!
Hi Matthew,
I’m from South Africa.I got hold of a Sachs Madass 125 with 3000 k’s on the clock.As far as I could see on the internet, these bikes are fitted with Mikuni carbs,whereas mine has a TK carb on (round/slide).Main jet=94,Pilot jet=40.Could you please tell me how many turns out, the air screw should be?Can’t get it to idle propperly THANKS
Should be about 1-2 turns out as a baseline. You will have to adjust while engine is HOT and tune for highest RPM.
Thanks a lot ,now I can work from there!!
I have a 97 gsxr 750 sport bike that has 4 CV carbs. Do the setting for them have to be the same or different. I know the bike is running rich for sure(throwing fuel out) and it has a off idle hesitation. Happens a lot while cruising around town.almost feels like I’m riding a horse. I have tried every gear and all different rpms and its no better no worse. Seems like any gear or rpm if I hold the throttle at the same spot it hesitates. It also does it really bad if I pull in the clutch then get back on the throttle lightly. My manual says crazy numbers for the factory setting on these carbs. Its something like 4 1/4 outer and 2 3/4 inner. Runs terrible at those settings. I’ve got mods and the carb was professionally jetted. But it was done at a way different altitude then where im at now. So I’m hoping to just adjust the mixture to fix it. Thanks!
Brent,
Alright, let’s take this step by step.
All carbs should have the same fuel screw setting, for example, all should be 3 turns out.
Please explain what you mean by rich and throwing fuel out?
The hesitation, is it a bog (lean) or rich (sputter)?
List all mods and what your current temp and altitude are.
Sounds like you’ll have to take carb apart and record all jet #’s and compare to stock and go from there.
Thanks for the reply man I really appreciate it. The bike has a full Yoshi exhaust header and silencer. The silencer has been cut shorter as well. It has an ignition advancer +2 and k&n filter. My main jets are 132.5 outer and 130 inner. Stock is 130 outer and 125 inner. My needle is on the middle clip with a shim under it. And my pilot jets are all 2 steps up from factory which is 12.5 so they are all 14.5s and my mixture screws are all messed up. Bike runs amazing if I’m flogging it. But off idle it has a stumble I guess. How can you tell a stumble from a hesitation? I’m 100% sure its not a bog let’s just put it that way lol. And by throwing fuel out I mean if I rev it I get some black smoke just a little bit but then it puffs a lot more once it comes down to idle. And I had it at the drag strip last night and right at every shift it puffed black is what my brother was telling me. I’ve tried every setting I can try with the mixtures besides going way in like between 1-3 because of what my manual says they should be at stock. Should I just disregard the manual? Thanks again for you response I really appreciate it!
Brent,
Lightly seat all pilot fuel screws and turn them all out the same, about 2 or 3 turns. With the bike HOT and idling around 1100 RPM or so, turn each 1/4 turn in at a time until idle begins to stumble. Turn each out 1/4 turn at a time until it stumbles. You should be able to get the highest RPM somewhere in between.
All screws need to be the same amount of turns out.
Do this and we go from there.
Also my temp is around 70s right now and altitude is 1400s above sea level
Sounds good. I will do that tomorrow. Should I put in new plugs? Since I know it has been super rich for quite a while. Could to much fuel have fouled them or be throwing off all my work if I do try to set the mixture? Thanks
If the plugs are not fouled don’t bother changing them until you have it running much better. I like to spray a bit of water on each header pipe to see if the cylinder is firing. Water should “sizzle” off at the same rate. No sizzle mean cylinder is not firing.
I am positive all cylinders are fireing. I have a temp gun and all header pipes read in the same spot closest the the head in the 300-500 range so I know they aren’t foul’d completely just I’m sure very black from a few years of extremely rich mixture. But I will do what you told me to do today and see where the mixture screws land. Thanks again for all you time. Just wish that Suzuki manual didn’t lead me so far off where it should be lol.
OK so I did what you told me to do. My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall. Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary. Thanks
OK so I did what you told me to do. My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall. Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary. Thanks
Brent,
Good to hear you made progress. I would take a test spin at 1.5/1.75 setting and see how it runs. The temp should creep while standing still. What is the temp while riding it? Report back after test ride.
I took a spin around the block and it is going in the right direction for sure. I had it at 1 3/4 and it still stumbled just a little bit here and there. So I took it for a long ride. Temp is 70 degrees here and the temp at 60 down the highway was about 170-180s and in town it would creep up to 200+ one thing I did notice is that I was able to slowly ease off the throttle without it bucking and kicking and it was much smoother at cruising speed in 6th gear at 60 giving like a 1/16th throttle. So I’m confident were heading in the right direction. I really appreciate your time and expertise! I’m gunna go down to 1 1/2 and see if its better. I’ll keep you posted. Thanks!
*Update*
Went to 1 1/2 turns out and it still is doing the stumble. About the same as 1 3/4 honestly. I’m gonna try another 1/4 turn in. I’ll keep you updated. Thanks!
Brent, if it is stumbling/bogging/hesitating, you will need to go richer. Explain how you get it to stumble? Are you flogging it when rolling slowly?
When I get it to stumble I am either just taking off from a stop at very little throttle. Or if I’m rolling down the road and I pull the clutch in then let it back out and ease on the throttle. Always at around a 1/4 throttle or less. It will also do it in between gears while shifting right when I get back on the gas. Wide open there is no bog. I can cruise in 6th and go wide open and she gets right up and goes. Its just low speed and very little throttle that it does it. Thanks
Brent,
OK, let’s talk about this stumble. We have to determine if it’s too rich or lean. If you add more throttle during stumble does it go away? If so, it might be a rich spot. If it is a lean bog, more throttle will not help it recover.
You can also install new plugs and go ride it around and cause it to stumble over and over again and pull plugs to read them.
It seems this stumble is occurring when on the pilot 100%. The fuel screw setting at idle trims the pilot fuel, and considering the fuel screw ends up at 1.5/1.75, the 1/8 throttle position (all 100% pilot flow) will be rich. You might want to drop a pilot jet size.
More throttle does make it go away. But different gears and different rpm doesn’t.
Its a really weird kinda studder I guess really. It almost seems like a misfire in a car. If I hold the gas at the same spot while its doing it I can get it to buck and kick and carry on as long as I want. Sounds like wah wah wah wah wah all the way down the street until I give it a little more gas. I have tried a plug chop before to no avail. But then again I was also like 4 full turns further out on the mixture screwes thanks to the Suzuki manual lol. Thanks for all your guidance. I’m gonna try 1 1/4 and see if its any better or worse. I’ll keep you updated.
OK. Keep in mind that at 1/8 throttle you are fueling the motor on 100% pilot jet SIZE, the fuel screw is out of the picture by then for the most part. You can experiment with different fuel screw settings, however most likely it is all pilot by then.
Also, is the timing advance adjustable? Too much advance can cause bucking like you described.
The timing advance is just the factory key for Austria. The bike when it was sold in the us had a retarded timing to meet regulation. And no its not adjustable. It is sold by a company called factory pro. And from what I hear they are pretty replutable. And it also did this before I did the advance. I have the next size down pilot jets so I will try that if the mixture change dosnt work. How exactly does the pilot size not get metered at 1/8th throttle? Thanks
The pilot jet actually feeds fuel to at least 2 passages. For example, with throttle blades closed, one passage will be on engine side, while the other on airbox side. The engine side passage is further “trimmed” or “metered” by the fuel screw. When throttle blades open to 1/8 or so, it uncovers the other passage and it will be 100% on pilot. So if it needs to be leaner or richer at 1/8 you have to swap the actual jets.
Does that make sense?
Yes it does. Thanks. Now here’s a question. If my idle was set to high would that maybe cause the blades to be open all the time and possibly cause this? Because I was idling it at 1500 before when my mixtures were way out. but since adjusting the mixture it seems to idle smoother at lower rpm where it was opposite before. So could my idle be set to high and cause samething?
If your idle is set too high (throttle blades open too much), then any adjustments made by fuel screw may not be noticed. I like to have idle as low as possible when tuning fuel mix screws. Small RPM changes are much easier to notice.
My idle was below 1000 rpm when I made the fuel screw adjustment. But right when I was done I put it right back by habit lol. I’m thinking when my fuel screws were super rich I had to idle it up to keep it even running. Because now that I leaned them out I can idle smooth at way lower rpm. Before if I tried to idle lower it would surge the whole time. So I’m wondering if that would be opening that un-meatered hole and causing my stumble this whole time. Once it stops raining I will test my theory and let you know. Thanks
Alright Brent, please report back your findings.
So the weather in Ohio sucks and its been raining or to cold to ride ever since my last comment. But finally I was able to go another 1/4 turn in and take it for a ride. So here are my findings. Idling lower like in the 1100-1200 rpm range it is almost perfect at 1 1/4 turns out on all 4 screws. Any higher and the hesitation comes back. I also noticed something else that is different then before. The tip of my pipe Is turning brownish tan instead of black the way its been since I got the bike. So I’m thinking that is a good sign. I will be taking it to the track tomorrow to see if its any faster or easier to get off the line and will report back. Thanks so much for your help Matthew I really appreciate it!
Sounds like you are headed in the right direction. So the stumble was probably a sputter, not hesitation? I say that because you went leaner and things improved.
Now that your fuel screws are nearing 1.25 out, you can get a leaner cruise (1/16-1/8) throttle by dropping pilot jet size. You will have to readjust fuel screws and they will end up about 1/2 further out. If you decide to do this, your will be at the same fuel ratio and the bike will be getting a little less fuel when 100% on pilot.
Does that make sense?
Makes perfect sense. I will be putting in the next smaller pilot jets this weekend. I will report back my findings! Thanks!
So I put in the next size smaller pilot jets and it took a massive leap to the better side. I did exactly as u said and went a half turn out to compensate and now the only time it sputters is when trying to go like 5-10 mph which I would consider exceptable from where it was at. I also noticed it smoother in general throughout all rpm range and stretch links and all off the line I’m on the back wheel to the 60ft. Thank you so much for all your help!
Good job Brent! Glad to hear it worked out. Are you going to move on to the needle and main?
I have a 1999 Honda Shadow Aero 1100 with Shotgun pipes with the baffles cut down from 12″ to 4″. About 2 months ago I had the carbs cleaned due to a drop in fuel mileage, from 120 miles to about 85 miles. Since the was done, my mileage has increased but, when starting from a stoplight, there is hesitation and an occasional stalling. Also it is now backfiring in the pipes on decal that wasn’t there before the clean up. I am not mechanically inclined and the mechanic that originally did the work in no longer in my area. What are your thoughts on this. Thanks in advance.
John, it sounds lean. Go ahead and lightly seat your fuel screws noting how many turns it took. Get the bike HOT and tune the fuel screws for highest idle. Report back how many turn out you end up with and whether it helped.
I have an 82′ Yamaha xs400 heritage special…It hasn’t run since I got it, its got a brand new battery, the carb was recently cleaned, but before I got it he took out the air filters and put in foam air filter cones…we got it to start and run for a few seconds while holding the choke but then it would die…I’ve tried adjusting the pilot screw and it hasn’t done anything…the other thing that concerns me is the rubber hoses connecting the carb to the engine are slightly cracked, could that be an issue as well?
Dom,
What rubber hose? The boots that hold the carbs in place, or the vacuum line that runs from carb boot to petcock? Both of these should not have any cracks otherwise it will introduce air leaks (vacuum leaks). I have a video on how to diagnose vacuum leaks. Check for them and report back.
http://www.howtomotorcyclerepair.com/how-to-diagnose-motorcycle-vaccum-leaks/
The boots…I read that if there is a leak in the boot that putting electrical tape over them will temporarily fix the problem? Is that an accurate way to see if a leak in the boot is what’s causing my problem?
Dom,
If the boots are cracked, replace them. Electric tape, RTV, etc..never seem to hold up.
Hi there ive got a vespa scooter and when I asjust the air screw all way in or all way out does nothing at all..125 cc with simmoni pipe.been told by mechanic its got a air leak
Tony,
It is easy to check for vacuum leaks:
http://www.howtomotorcyclerepair.com/how-to-diagnose-motorcycle-vaccum-leaks/
Report back what you find.
1985 honda shadow 700.
I have k&n filter, and the chamber between muffler pipes has been removed and holes closed off.
I have 135 mains, and 45 pilots i believe…
the popping is finally gone (about 3 turns out on pilot screw). Or would popping on decel be normal with that exhaust setup?
Bike tops out at 95mph, when it could easily do 105 last year before i played with jets to get rid of popping. And also the bike stalls when gearing down and coasting to a stop sign at times (never used to do that). Buddy said it smells rich when he rides behind me…. Is this a pilot issure or high speed jet issue. And will pulling the baffles help to correct the problem by leaning the mixture out a bit and allowing me to turn a/f screws in without having popping. or is popping something i have to learn to live with?
Hope that didn’t break your brain
Darren,
Popping with open pipes is to be expected. You can somewhat remedy this by continuing to richen pilot jets, however will consume more fuel at cruise. Try going up one size on pilot.
I know that the stock air cleaner is very restrictive. Did you replace the air cleaner or just the element? Any needle changes? What are stock main sizes.
Matthew, first off, great site and really great and easy to undrestand articles.
Secnodly, I have a problem. I have a 2008 Yamaha XT125 with a 150 ccm big bore kit and an Arrow exhaust and I’ve recently fitted a Stage 6 R/T Dellorto VHST 28 caruburettor on the bike. The Dellorto carb doesn’t seem to use the same system as is described in the article as it has a metal Air mixture screw with a spring on it (I could link you a picture for a better idea, but I’m not sure if your site allows it).
My problem is that I have this screw screwed all the way in and the bike behaves very weird in idle/low RPM. In idle, the bike is very loud, doesn’t run smoothly and sputters a bit, but when I give it a bit of throttle (around 1/8, it sputters quite a bit more, sometimes the RPM fall down a bit and after I let go off the throttle they hang for quite some time and decrease very slowly back to idle RPM.
I’m guessing I’ll have to buy a smaller pilot jet (idle jets as they are called on the manufacturers website).
Any ideas/suggestions?
Andrej,
I believe that carb is fitted with a pilot air screw on engine side of carb. To verify, remove the screw and it it has a blunt tip then it meters air.
Have you tried to turn the screw out to achieve highest RPM?
Matthew, thank you for the swift response.
The screw is blunt, however the diameter of the end is less than a milimeter. The most stable state, if you could call it like that, with the highest RPM is when the screw is fully screwed in.
That screw meters air. If it idles best all the way in, it wants more fuel.
Does it run better with choke on? Try blocking the air intake with masking tape (50%) and see if it runs better to confirm lean condition.
The funny thing is that it runs worse on choke, also dies with a minimum amount of choke added. It runs better without the air filter, and also if I turn the screw out the exhaust starts to smoke heavily.
OK. What color smoke? How does the plug look?
The smoke is mainly a darker shade of gray to black. The plug is wet.
Andrej,
I’m not too familiar with the carb you have, however did a bit of reading and that screw meters fuel, not air like I stated before. Since you can turn in all the way in and it will still run, I would recommend dropping pilot jet 3-4 sizes. They are cheap so buy the the next 4-5 smaller sizes to experiment.
Thank you for everything. I have already ordered 3 of them, the one that is currently in is a size 45 (the biggest sold for this carb), and I’ve ordered a size 40, 38 and 36. Will report how it went after I get them, maybe it’ll help anyone else. Thanks again 🙂
No problem. It will be trial and error, but I’d start with the 38 and go from there. Post back with results!
The jets came in tomorrow, but as it turned out, the 36 is still too big.
It runs much better now, but still won’t die with the screw fully screwed in.
I have ordered another pair, this time sizes 34 and 32 (32 being the smallest one available) hoping one of these will be the right one. If not, I don’t know what else I can do.
Install the 32 and report back. Hopefully it will work for you.
Hello Matthew,
I have a Virago 535 2002 with popping from front cylinder, Bike has Highway Hawk Fat Pipes ( think they are highway hawk, no marking on them)
the exhaust had very small baffles in which I have replaced with 8inch baffles and baffle wrap ( makes chuffing sound now instead of loud bang) apparently the previous owner had a Dyna jet carb kit fitted at same time, carb sync looks near enough, neither mixture screw has much effect, front is 3 turns out and rear is 2 2/3 out popping seems to ease a little if front mixture screw screwed in until it is 1 1/5 turns out but neither effect engine revs, when the front mixture screw is fully seated with engine running then loosen it engine revs rise slightly but only momentarily then even out again. spark plugs good colour not sooted up or white. thought I had a vacuume leak sprayed carb cleaner on rubber carb mount to engine on front cylinder, engine revs rise dramatically,, stripped it down twice now and reseated/sealed all I could,, I`m now at a bit of a loss as to what to try next.any suggestions.
How it sounds below:-
http://vid1277.photobucket.com/albums/y493/jgl9/yammissfire_zps2db14cb6.mp4
Hey John,
You mentioned vacuum leaks. Are they all gone now? If not, you have to take care of them.
You should be able to get carbs synched perfectly, since there are only 2 of them, it’s even easier. Hook both gauges up to one carb to see if gauges both read the same. If carbs are hard to synch or won’t synch, then you have more vacuum leaks.
The fuel screws should be responsive. They should both be turned out the same amount. If you can close them and engine still runs, the pilot jets are too big.
An IR temp gun can be handy for aiming at pipes to read temps and help understand what is going on.
Hey Matthew, finally got the jets. Well it finally dies when I screw the mixture screw all the way in. I have it out by about 2 turns now and it runs quite smoothly. The only issue I’m still having is that the bike is quite prone to stalling when I add very little gas (as in from a standing start at a traffic light). Also when I’m riding it at about full gas and high revs, when I let go off the gas the bike is a bit nervous, makes a bit of a different sound and likes to kick forward a bit as if there is still some extra gas that ignites in the cylinder even though there shouldn’t be any (if that makes any sense written like this). Both of these symptoms were worse with jets that were too big. Do you have any idea what could this be?
Andrej,
If you slowly roll on the throttle will it die? Is the stall from a bog? Remove air cleaner and tape all but the bottom 1/8-1/4″ of carb inlet and see if conditions improve.
I will try and post back with results. Also, I’ve just heard from my parts seller, it appears that my carb is made for a 2-stroke but my Yamaha is a 4-stroke. Apart from the atomizer and its bush, the idle jets are also different. The ones that I’ve tried and have currently fitted in are all for a 2-stroke. The ones for the 4-stroke appear to have larger side holes. Could it be that this could be causing my problems?
Yes, the jet needle/emulsion tubes are different. I have no experience with swapping 2&4 stroke carbs. Here is some reading:
http://www.dotheton.com/forum/index.php?topic=29838.15
See if different needle jets are available.
I’ve read your link and I think it makes sense, especially seeing as it almost always gets flooded when going from half to 3/4 gas even though the needle is in the lowest position and the main jet is the right size (no problems at full gas, high revs).
The parts seller gave me info on which the 4 stroke parts should be (the atomiser with its bush, if I’m not mistaken, this is supposed to be the needle jet) although they aren’t sure whether the bike will run better with these 4-stroke parts.
I think I’ll go for it, hopefully it will finally work as it should 🙂
Andrej,
I think you are almost there. Try the the 4 stroke needle jet and report back.
Hello Matthew,
It appears that I have solved the air leaks ( no engine rev rise spraying carb cleaner now),, Carbs are synced perfectly,, I put my exhaust gas analyser in each exhaust pipe in turn, I then put both fuel screws too two turns out and the front cylinder is ok ( the front one is popping/chuffing at idle ) (before I put the larger baffle in and baffle wrap the front exhaust popped/crackled and banged ) but the rear cylinder is showing way way too rich so I turned it in one full turn and the reading came down significantly but still richer than the front. Turning the fuel screws in or out makes no difference to the engine revs at all, you cannot hear or see rise on the auxiliary tacho fitted for the purpose of tuning.
I`m confused as to why the front cylinder is the only one chuffing/popping. I understand that the pilot jets appear to be to large but when the previous owner had the exhaust fitted she also had a Dyna Jet kit fitted,, Would this not mean that the dyna jet pilot jets fitted would be correct for the new exhaust gas flow ( it appears that with the Dyna Jet kit you get new main jets and pilot air jets,, would it be this pilot air jet that needs to be reduced in size , there are two pilot air jets in the virago carbs, the Dyna Jet kit replaces the number two pilot air jet,,, or would it be the “regular” pilot jet that needs down sizing ) and weather or not it is,,, why do you think only the front pipe is popping/chuffing,, I`ve tuned loads of bikes over the years but have never had this problem. Your help, opinions and views are greatly appreciated,, I would love to solve this issue.
Think I shall look see what Dyna Jet jets are fitted.
P.S whilst syncing the carbs I turned ( unscrewed ) the adjuster screw right out and the engine revs increased dramatically,, is this normal ?.
John,
Let me back up a bit and ask what the compression is on each cylinder and are the valves adjusted if applicable?
Have you had these carbs apart? If not, maybe it would be a good idea to double check the previous persons work. Also, can you send me a link to the jet kit? Maybe I can go over the instructions so I can make better sense of what was changed out.
Hi, Mathew it’s Brent again don’t know if you remember me but my gsxr is running weird again. After I talked to you it ran good for about a month and then it started to run bad again. But it was more of a lack of throttle response. I bought a emgo sync tool and sync’ed they were way off. And now the throttle response is fixed. But the weird thing is I reset the mixture screws and at 2 1/2 turns in it dies and the idle keeps going up all the way up to 4+ turns out. My stock jetting is 127.5-125 main jets 12.5 pilot. My jetting in it now is 1 step up on everything so 130-127.5 mains and 15 pilot. It don’t make any sense to me why I would need to go up 1 step on mains and 2 on pilot for it to run right. Any help is greatly appreciated!
Brent,
If it ran good at a certain setting and all of a sudden started running bad, it is not the settings. Something else is going on. Explain in more detail the symptoms you have.
Not exactly. I’m thinking the carbs had never been synced and my adjustments were on it before it was synced properly. It did run good for about a month but then I started to lose throttle response so I synced and that problem went away. But when I left it right where it was. Which was 2 1/2 turns out on the mixture screws it idles terrible. Like constantly surging at idle. So I tried to adjust and I find to reach the smoothest and highest idle I am like 4+ turns out.
Is it possible for my main or needle to be lean and cause my mixture to have to be way rich to compensate? Thanks for helping me man. This bike is going to be the death of me lol
I should also add that with the mixture at 4 turns out when I take it for a test ride the lower rpm and smaller throttle openings are smooth and extremely responsive (more then it ever has) but since the sync the mid has been kinda lack luster. And at wide open throttle it pulls like a mule all the way up to Rev limiter. That’s what is leading me to believe the needle setting may be where I’m off.
Valve adjustment and carb synch should always be done before carb tuning. Also no vacuum leaks.
Now that you have them synched and you best idle is 4+ turns out, you need to go larger on pilots. Probably 2 sizes larger.
The needle does not come into play until 1/4-1/2 throttle, so no need to worry about it now.
Lesson learned lol. I was always under the assumption that carbs our of sync would lack throttle response and I always though it was very responsive. Until I synced then it was way more responsive. What I still can’t wrap my head around is why do I need so much more fuel at idle and my mains are only 1 step up. Don’t those normally coincide to an extent? Oh and I have the next size up and it’s 17.5 should I go 2 steps and get the 20? Thanks
Brent,
Carb synch is most important at idle and least important at WOT.
Sometimes bikes are very lean from the factory. I’ve taken stock bikes and had to up the pilot 2 sizes and still ended up 3 turns out. Give the bike what it wants and don’t get too tied up in the difference from stock.
Try the size up you have and see what happens.
If you carbs synch is dead on, you should be able to tap the start button with no throttle and it should fire no problem.
Gotha I will try the one I have and see where it lands me mixture wise. Thanks again for all your help.
Well here is a little update on the tuning progress i made. I put in the next size bigger pilot jet (17.5) and it made it run quite a bit smoother. I reset mixture screws after the change it smoothest idle was atleast obtained. It was 3 1/2 though so im thinking i will be putting in 20s next because it rode smooth and nice for about 10 minutes then it went back to its figgity self and was hesitating again. Still better then with the 15s but not fixed yet. I checked and rechecked synch and it has been spot on since i first did it and i still cant understand why this bike needs so much fuel at idle. Im only half a step up on main and when i put in the 20s that will be 4 steps up on pilot. Thanks for all the help i would never be even this close without your help amd write up.
Brent,
I would cruise around and cause the bike to hesitate like you mentioned. Try to have the bike do it over and over again and then pull the plugs (plug chop). Maybe the color will indicate what is going on.
Can you double check for vacuum leaks? Spray carb clean everywhere from cylinder head to airbox!
I put in the 20’s and i put the mixtures at 2 1/2 to start off. It started and ran pretty good and i let it warm up to try to dial in mixture and my highest idle and smotthest idle was 2 3/4 which seemed alot more like where it should be. It was way to late to do a test ride so i shut it off and waited till the next day. I tryed to start it the next day and was experimenting with a no choke start and it turned over 3 maybe 4 times and then hydro locked or atleast so it seemed. So i held the throttle wide open and it turned and fired atleast. Smoked for a few minutes. Wondering if maybe i had a float stuck and not all the way seated and filled the cylinder full of fuel or something. Started fine next morning and has been good ever since. But after that hard start i rode around the block and then re-tuned the mixtures ended up at 2 1/4 turns out and went for a half hour ride to really see how it would run. It ran good. Could tell it was alittle lean. Popping on decel real bad and had quite a bit of ping. Plus end of pipe was turning light greyish white. If i reset mixture and its over 3 turns out im gunna cry lol.
Brent,
You probably had a stuck float. Have you pressure test the needle and seats? I like to take 3-5psi of air and blow into fuel inlet. All should seal if float is depressed. High float levels could cause headaches.
Since you shortened the silencer, decel pop may be normal. Don’t try to eliminate this by increasing pilot jet size. You’ll end up way too rich at idle and cruise.
Has anything been done to CV slide and/or spring? Maybe the stumble you are trying to solve is the transition between pilot circuit and throttle valve. I know jet kits in some cases drill the CV slide and replace spring so they lift easier giving the engine more fuel/air. Something to think about.
Yes i did do a pressure test and all was good after that one little hiccup. Has been starting fine ever since. My float hight is factory which is 7mm from the gasket surface with the float just touching but not compressing the needle spring. As far as drilling the slide i know i havent done it but thats not saying someone else didnt. It was jetted but not with a jet kit by a shop i do know that. Im not looking to get rid of all the pop on decel but it was excessive in my book. And the tip of the pipe being white/grey was another lean indication i was thinking. Thanks again for everything. You have been a huge help btw. I tryed for leaks again like you told me. I sprayed everywhere and it is leak free. Ive actually done that like 5 or 6 times before just to make sure i wasnt missing something. Because to me it dont make any sense to need 20 pilot jets when stock is 12.5 but all tests i do come back negative. And compression is good and everything keeps pointing me to pilot jetting. I guess this bike just came mega lean from the get go but how didnt it hessitate when new? Or maybe it did who knows. But its only got 13000 miles on it so it should be able to run and ride smooth i would think.
Brent,
Sounds like you covered all bases.
I would drill slide hole as a last resort, however maybe someone drilled it out too large?
Maybe it would be worthwhile to find needles that have a smaller starting diameter (portion closest to clip grooves.
If i were to drill the hole out what size would i need to drill it to. And what size would be to big? Also what is the gain from drilling the hole? Would i need to start all over again with my pilot jetting? Im kinda wondering if this is some of the problem. Because when doing the carb synch i noticed the slides would flutter alot when revving. Would not getting enough air cause that?
Brent,
I don’t know if drilling the hole is a good idea or not. I’ve read both sides of the argument. I did however drill slides per instructions on some jet kits, which supply new springs as well.
However, it would be a good idea to check yours to see if someone has drilled them out. Most kits supply a 1/8″ drill bit for this, so stock should be smaller.
A larger hole will cause the slide to lift slower.
If your slides flutter, make sure they are perfectly clean and operate smoothly. Polish them with crocus cloth if needed.
Ok this bike is insane. It sat for about a week because it was raining. So i went out and choked it and boom hydrolocked again. So i held the throttle open and it started just like before. I checked needles and they hold pressure fine. So next day it sits over night and i try no choke. It starts and runs fine starting cold with no choke. So i check my mixtures and they are at 2 1/4. Still from that last long unsucessful ride and i start to adjust. If i get down to 1 or less turns it stumbles and dies. Anywhere 1 1/2 turns or more the idle stays the same till about 2 1/2 no change at all. Then above 3 it starts to stumble really rythmicly. So i think ok awesome im sure i got the right pilot 100% so i then put it at 1 1/2 to aim tward the bottom side of that range and see what happens. I take it around the block and it had been idling in the garage so it was 200+ degrees reading the temp guage. Ran amazing. I mean absolutly perfect…….until i started moving and temp dropped to 160-170 where it normally runs and boom stumbling and studdering like crazy so bad a fe2 times off the take off i thought it was gunna die out completely. How can the engine temp make such a huge differnce? I mean enough to make it run perfect then like crap just like that. This bike drives me insane. I could swear its a nitro rc car.
Brent,
Try tuning the fuel screws at engine temp of 160-170. Point some box fans at radiator to keep it cool. Stock thermostat is 180 correct?
Thats a good idea using the box fans. I always turn on the fan that i have wired on a switch bypass and it still creaps up to over 200 degrees. The fan dosnt come on till 225 the way the factory has it and drag racing it i always have that fan on to keep the temps down. As far as the t.stat goes i my fsm says its a range of 168-176 that it opens completely. Also with the mixture down at 1 1/2 like it is now i did a cold start try with the choke and it fired up super easy and the choke worked better than it ever has. With choke fully on the idle was at like 3k and as i eased it off it went down to 1200 perfectly. Before with the other jets if i choked it all the way the idle would be kinda low like 1k ish then as i eased it off it would go up to say 3k then come back down to idle. So im pretty sure i got the correct pilot size now. Its just tuning it in and i should be good to go. Thanks again for everything.
Cool. Sounds like you are VERY close if not there. So what size pilots are you currently at?
The bike should want some choke for cold starts. If it starts fine w/o choke then it’s too rich.
20’s and im at 1 1/2 turns. Trying a 1/4 turn more fuel tomorrow. I know its between here and 2 1/4 because at 2 1/4 with choke it hydrolocks when i try to start it and thats obveously not right lol. Ill keep updating until i get it. Just incase someone else has these issues and we can save them alot of headaches.
Brent,
I just want to say good job on not giving up and fixing it yourself! I’m sure you (and I) have learned a ton from this.
Leave an updated comment once you put some time/miles on the bike.
Ok now this one has got me scratching my head. Since my last post it has been to cold to ride until today so i went out and went a half turn more fuel. Which puts me at 2 turns out. I put alittle bit of sea foam in the tank and topped it off with vp110 and went for a ride. It will no longer accept full throttle. It breaks up really bad at wide open throttle. Almost like its lean on the main jets. But the only thing i have changed is the mixture. Im stumped. But plan on trying 2 1/2 but cant figure out how that would make any difference at wide open throttle.
Just as i had thought the mixture chnage made no difference. Still will not except full throttle. Seems like its running out of fuel or something. I did find something that may be throwing a wrench in the gears tho. I pulled off the vacuum line to the fuel valve and it was wet and had alittle bit of fuel in it. Im wondering if that diaphragm has a hole in it. And maybe thats why it seems to need so much fuel at idle when the vacuum is low. Just a thought. Let me know what yours are. Thanks
Hey Brent,
The fuel mixture will have very little effect at WOT.
Do you always run VP110? Probably overkill for this engine.
Interesting discovery on the petcock vacuum line. If there is an internal leak, then it will cause problems. During decel, vacuum is high, so you might suck up extra fuel and it may stall the engine. At WOT, vacuum is low, and you have a leak, so fuel delivery will suffer.
Hook up a vacuum gauge to petcock and see if it will hold vacuum. If it bleeds off, replace it.
Yeah i always run vp110 it is a drag bike primarily. So i run race gas almost always. I didnt figure the mixture would effect it i was just grasping at straws lol. And i will put the mitivac on it tonight and see if it holds vacuum. How else would fuel end up in the line tho? If im thinking right the only way is leaking diapragm i may be wrong tho. Im wondering if the whole time that has been leaking and what was making me need such massive pilot jets. Is that possible? Thanks for the input.
Well heres the update. I put my mitivac on that line and left it all noght and it never budged. So im thinking that diapragm is good. Not sure how the gas got up in the line. So i was testing a few things here and there and find the throttle position s3nsor’s resistance tested way out of spec. So now im ordering a new one. This bike will be the death of me. Could that be the cause of everything i have been experiancing? It says its supposed to read between 3.5-6.5 k ohms and mine was reading 16.28 ish. And when i losened the screws to adjust it anyway i went it made no difference at all. I didnt even know a carbed bike had a tps in the first place. Kinda seems like it really wouldnt control much or as much as a fi bike would. Just an update. I will update again when i get the new one on and test ride it. Thanks
Brent,
Honestly, I don’t have much experience with TPS’s. On a carbed bike, they usually have it to monitor throttle position and possibly control ignition timing and maybe a fuel circuit. On my dirt bike, there is a fuel cut solenoid and timing curve associated with TPS settings.
Let me know what happens when you get the tps working properly.
Well that did not go as smoothly as planned. I tested the new tps and it tested good and way different then my old one. So i installed it per the manual. I put the 15 pilot jets in and also put the needle back in the middle as a base line starting point. I took it around the block and it ran terrible. Bog off the line so bad i had to rev it and slip the clutch to get it to even move. Once i got above 1/4 throttle it ran smooth and clean tho. And the jerky throttle response is gone now. But this bog is what has me confused. My mixture is at 2 3/4 and if i go leaner it seems like the idle drops and isnt as smooth. I do know for sure that the tps not working made the bike run way rich because i pulled the plugs just grasping at straws and wow were they fouled. Black covered in soot. But i changed them with new ones and it still didnt completely fix it. It almost sounds like the tone of the exhaust has changed also. Way deeper almost v-twin sounding until i get past 1/4 throttle then it hits a turbo and most of the time blows the rear tire off. Kinda scary the couple times it did that lol.
Brent,
I can’t gather whether you are rich or lean on pilot. You mentioned plugs are black (rich) and it bogs (lean?).
Now that you have the TPS sorted out, reset your fuel screws for highest RPM, and note how many turns out you end up. That will tell you whether to move up or down on pilot jet.
Well heres an update. I tryed to set the mixtures and it was all over the place 1/4 turn idle up. Next 1/4 turn idle down. So i was talking to my father in law about it and he looked at the fould plugs and said for it to be dumping this much fuel in and not burning it like it was there would more than likely be alot of gas in my oil. Sure enough he was correct. I looked in the view window and it was way over full. I drained alot of gas mixed with a small amount of oil out of the crank case. It was probably 75% gas. No joke. How screwed am i? Would it ruin the clutch? Blow the engine? Or can i let it drip out for a while then fill with oil and call it a day? From what my father in law said it would make everything seem rich burning it off in the engine like that. Thanks for your advice man. Your a life saver.
Don’t sweat the fuel in the crankcase. Give it 2 drain and fills and it will be OK.
If you had that much fuel in the crankcase, the float became stuck open due to debris of worn float needle. Remember when you had to hold throttle wide open during cold start? That was probably due to all the fuel flooding the cylinders.
At this point just replace the float needles. They can give owners so many headaches with high float levels or flooding that you are experiencing. And yes, it will cause the bike to run super rich is the float needles do not seal properly and result in high float levels.
Another thing, you checked the vacuum petcock? Do you have a prime “PRI” setting? That setting bypasses the vacuum actuated setting and always allows fuel flow. Don’t use this setting other than when the carb bowls are dry to quickly fill them.
Well Matthew I honestly don’t know what else I can do. Installed the 4 stroke needle jet size 268 (smallest one available). It has 8 side holes on the top end, whereas the 2 stroke version doesn’t have any (it was a size 266). The bike only starts with a bit of gas, quickly dies without it, and smokes very badly (white smoke in lower revs, gets a bit darker if I rev her up). I had also installed a 4 stroke pilot jet of the same size as before (32) which has bigger side holes than the 2 stroke one and the smoking was much worse, which is why I switched back to the 2 stroke one and she runs like I explained in the beginning. The only thing that makes sense to me now, is that gas is coming in through these holes on the 4 stroke needle jet rather than air, which would happen if the gas level is too high. Maybe it’s related to the fact that the carb is in at a bit of an angle?
So yeah, she runs worse now, and I really don’t know what else I can do.
So with the jet needle change only it will not idle? What is the white smoke all about? That part does not make sense to me.
Yeah, I don’t get it either. Only the needle jet (atomizer with its bush) were changed, the idle jet is the same again. The only reason that would make sense to me why it dies would be that the excess fuel when I let go off the gas floods it to the point of dying alhough that’s weird because apart from the terrible smoke (very light gray if not white), it revs up without too much hesitation.
Also what bugs me is that I can’t adjust the idle screws because it won’t idle, but if I adjust them with the minimum amount of gas needed to not die, the revs are the highest when the mixture screw is all the way in (it meters fuel).
Well finally some good news (sort of). My theory proved to be right as far as it seems. I took some tape and closed the side holes on the needle jet, and it runs like it did before, just some sputter from 0 to 1/4 gas but other than that quite normal and also it idles with no trouble again. Also now I’m certain that the fuel level indeed is high enough to pour into these side holes as the needle jet was soaking wet to the top when I took it out, aside from the fact that it runs pretty well with the holes closed. The smoke is appearantly due to the mixture being way too rich.
So the next order of business for me is to lower the fuel level in the float chamber.
I currently have a needle valve #300 and a float with the weight of 9.5 grams and the fuel goes almost to the top of the needle jet.
I know that I’m streching the original post a bit, but do you happen to have any reccomendations as to how much smaller would be appropriate?
What size is the throttle valve cutaway? They are usually stamped on the bottom. Here is more reading on the topic:
http://www.howtomotorcyclerepair.com/how-to-tune-throttle-valve-cutaway/
If you don’t have a float level spec, I like to set the level about 2mm below the carb body and float bowl mating line. You can insert a piece of clear tube in main jet plug or float drain if equipped. Run the tubing in a “u” above fuel level and that will help you measure level.
The throttle valve cutaway is a size 45.
The fuel level is about 5 milimeters above the mating line.
I’ll be honest, I’m not too familiar with the carb you have.
I’d follow the carb mfg spec for float height and stick with that. I usually never change float height to mask a jetting issue.
I know throttle valves are pricey, but it may be worthwhile to try a larger/leaner cutaway with the 4 stroke needle jet. It will pull less fuel from pilot and jet needle at 1/4 throttle.
That seem to be the case, yeah.
I know this should be a little hard to say, but how much of a change does one usually go with the cutaways, considering I have the 45 and the others being 40, 50 and 55?
If you go back to the link I provided on tuning cutaway, there is a graph from Mikuni that shows each size changes fuel flow by 10%. On that particular bike, I stated with a 3.0 (lean) and went to 2.0 (rich) and it made a drastic difference. I ended up going to the 2.5. So I experimented with a 20% difference.
In your case, I would start with 2 sizes leaner. Plan on revisiting pilot jet and it will probably end up richer in size since throttle valve will make signal weaker.
I have an update on the gsxr. Its cold here so i have it on a battery tender and i went out after the oil filled with gas catastrophe to try to start it. I tested the needle valves with 5 psi and they held perfect. I have been watching oil level like a hawk to make sure it hasnt changed. But im starting to think i either have a battery issue or a starter issue. I go to start it and it turns over 2 or 3 times slower than normal and then stops. If i hold the starter button i can hear the starter motor almost sizzling but i only do that for a second. Instantly my battery tender goes from green light to charging and then it charges for about a half hour and green light comes back on. I go over and try to start it again and boom turns over fine and fires right up. Im not noticing any change in oil level so im not thinking its hydrolocking from filling a cylinder with fuel. Any thoughts? Thanks
Hey Brent, try shutting the fuel off and running the carbs dry before letting the bike sit overnight. In the morning, see if you have the same issue to rule out your theory.
Ok good idea. I will try that. And let you know what i find. Thanks
I’m having a problem finding the adjustment screw on my 2000 Kawasaki Vulcan classic 800. Please help.
Hi Tyler,
The fuel screw is located on the bottom of the carb near the carb boot. Make sure to check out my carb clean video for your model: https://howtomotorcyclerepair.com/vulcancarbclean/
Hello. Thank you for the information. I have a 1980 Honda Xl 250S. I start the bike and it runs but when I drive it and push the gas the bikes turns off. When I do not push the gas it doesn’t turn on. DO you have any idea what the problem is?
Does the problem get better of worse with choke partially on?
It gets worse. As soon as the choke is on and want to start the bike , it wont start. Without the choke it will start. Once it is one and I put the choke the bike turns off completely. Sorry for the late reply.
Pull the spark plug and take a look at what color it is. Email me a pic.
Hi great work on your videos, anyway been trying to tune a ktm 300 exc it’s a 2stroke, just off idle trying to smooth out the power, it’s a little bit hesitant, or not smooth, bit crackly I guess.. Thanks
Anthony, I will be coming out with a 2 stroke jetting guide soon. In the meantime, email me through my contact page and I can help you out.
-Matt
How would this work on a bike with two carbs? I’m trying to tune my 73′ Cl350.
Yes. Both pilots and fuel/air screw settings need to be the same.
I resurrected this 1985 Honda XL600R from the ruins for my nephew. The previous owner tried to make an attempt to do the same thing, but failed miserably. Too many armature mistakes almost caused this bike to be offered up as a eBay parts sacrifice. Some people should not even try to do something like without the correct knowledge and experience. (https://www.youtube.com/watch?v=5AqqQ6jnR5I)
Currently is hard starting and pops through the carburetors while trying to start it. I rebuild the carburetors, however the jetting is not stock due to the changes the previous owner tried to do, such as drilling holes in the stock muffler and modifications to the air box. Per the advice of my local Honda Dealer, I installed 122 & 188 main jets. I had a 62 and 70 slow jet. I decided to put in the 70 due the mods to both the air box and muffler. I do have a 58 and I am considering installing that to see if it helps with making it start easier. Any advice regarding this would be appreciated.
Hi Bruce,
I would look up and see what the stock pilot jet size is. I bet you will find that 1-2 sizes larger than stock is what the bike would want. Try adjusting the pilot fuel screw and see if it is responsive. If so, how many turns out for max idle RPM and smoothness.
Bike looks real nice!
Matt
Matt,
Thanks. The tank is from my 87 XL600R.
https://www.youtube.com/watch?v=FDRg49vzdxk
My bike has a 87 XR600R motor and carbs. I have been having issues with it running lean, white spark plug. I went from 122 mains to 125 mains and the color on the plug is headed in the right direction, light tan to pinkish. My next jump will be 128. I do believe the 45 slow jet is not enough to get the bike started reliably. I have not replaced the Air Cutoff Valve (ACV) yet. I have been reading that a faulty diaphragm can cause havoc. That’s what I have been experiencing with this bike.
Any advice will on this bike would also be appreciated.
Bruce,
The main jet is responsible for 3/4 to full throttle position running. You mentioned hard starting and idling issues? Well, that is all pilot jet/fuel screw. Get that sorted out and move on to the needle and main.
Matt,
I had to replace the pilot jet. The carbs are off of a XR600R using all the XL600R air box and exhaust. The primary carb had a 45 and I went to a 58. The bike now starts a lot easier after getting the air mixture right. Your video and additional information helped me to find my solution to this issue. Next will be getting the main jets where they need to be.
THX… BB
Awesome! Here is some main jet tuning info: https://howtomotorcyclerepair.com/how-to-tune-a-motorcycle-main-jet/
Hey Matt,
I’ve got a quick question for you. Could you perhaps elaborate on the difference between a lean hesitation/bog/stumble off-idle as opposed to a rich stutter? Thank you!
Sure, what specifically? How to fix it? Or?
Oops, sorry for the ambiguity…but more like the actual symptom itself. Like, how does a stumble/hesitation ACT compared to a stutter? How does each sound? How do they affect rpms as they are occurring?
I’ve got an off-idle….issue…if you will, and I believe I know which way of Stoich I am. But before being certain I just wanted to see your description to see if I can more definitively put a label to the behavior I am experiencing.
A lean bog will have the engine fall on it’s face and not recover. A rich condition will cause the engine to sputter, and emit black smoke from exhaust. Watch this video as it has a lean bog to get an idea of what it sounds like: https://youtu.be/6b9dMhgZT-c
I have a 2004 ltz400 and the quad runs best with fuel screw closed the pilot jet is the stock 22.5 and everywhere I read says this is correct. The quad has a full exhaust on it and the main is sized correctly. Is it drawing fuel from some other source besides fuel screw?
I’d look to see if your float height is too high, or worn float needle. Also, make sure choke plunger pad is in good condition and that the cable has some slack.
Hello M.I have a 1996 Kawasaki Vulcan I have a bullet filter for it .But it won’t run with it it will idel .Do u thank I need to change the jet???? Thanks Roland
Best thing to do is to buy and install a jet kit that is intended for a high flow air cleaner. If you are interested on how a carb is taken apart, check this video out: https://howtomotorcyclerepair.com/vulcancarbclean/
Hi, I own a 1986 Yamaha fazer. I paid 1,600 in carb work and when I got the bike back I messed with the fuel screw the one with the spring. Now bike seems messed up. Reason I messed with it is because it idled to fast. Now, I have to have the choke pulled out slightly to keep idle. And idle is too low. If I pull choke out more it dies. I’m a female who owned a Fazer years ago and my old one never sounded or idled like this one. I won it on ebay. It’s way loud and if the mechanic tuned to proper specs, somethings off cause my old one never idled like that. So I played with the screw. Only to find that supposedly this fazer has different pipes than my original one. At least that’s what mechanic says. But they look like my old ones to me. Also I hear a strange tick in a hose on the right side that has a metal spring over it. It sounds like the fuel is not working right or air is not. I’m not sure I’m a girl. And need help. There’s no motorcycle repair around my area. Anyone who can help, I will pay only if it’s fixed. I do have a few videos of my bike on YouTube. Cheryl Jenkins, the one with two cats in the picture. I live in Danville CA. 94526. Please Email me if you can set to specs. 4twowildkittycats@gmail.com 1986 YAMAHA FZX 700 FAZER.
That is a lot of money for carb work! Find a different shop!
First, adjust the IDLE screw so it will idle, then adjust the screws per the video I posted.
Hey Matt working on my snowmobile carb trying to get it to run right has hesitation of idle runs ok up high it is flat slide carb have 2 screws one on intake side one on air box side but should be same theory as motorcycle correct?
Matt, 2 stroke or 4 stroke?
Are you the Matt B that I think you are???
One screw should be idle, that raises/lowers the slide. The other should be the air screw (assuming this is an air screw carb).
Year make model?
Matt
Yes it is the Matt your thinking of. 2003 Polaris xc700 2 stroke. I believe they are 40 mm flat slides and from what I read online it has one fuel screw and one air screw I could send you a picture if you still have the same #
On your carb, the brass slotted screw on air box side is the pilot air screw. Stock is 1.25 out. This screw meters air and turn it in to richen.
The screw with white thumb adjuster knob on engine side, is the fuel screw, and stock setting is 2.5 out. Screwing this out richens the mix.
If the hesitation is a lean bog, try turning the air screws in to 1.0.
If the hesitation is a rich blubbery sound, try opening them up to 1.5 out.
You can always bring it by as well.
Matt
Thx bud I’ll keep ya posted. Need a little more snow though
OK. We need to get together soon. Been too long!
Hi Matthew, I’ve been working on bikes and cars for 40 years and never before ran into problem that I wasn’t eventually able to resolve through some basic applied knowledge and some trial and error but I haven’t had a lot of experience with multi-cylinder CV carburetion and I’ve been through all the knowledge, common sense and trial and error with this problem and after more then 2 months I’m right back to where I stated with out any resolution and I’m out of time. In reading through your postings I saw how you helped Brent Pfeiffer with his carburetor issues and I was wondering I provided you with all the information on the issue I’m dealing with along with the results of my trial and errors, if you felt up to analyzing the info to see if you might recognize a solution. If you don’t feel like doing it I’d understand, I imagine you are getting pretty burned out on people always asking for help, I know I would be. let me know,
Thx
Sure! Reply with all the details.
Matthew,
Not sure if you can help but I think the principle is the same. I have a 2001 Yamaha f80 outboard motor. Straight in line 1600cc (97ci) with four (BCF) keihin carbs. Engine ran fine until cracked block etc etc. Replaced powerhead with full reconditioned unit and reused my existing carbs. Prior to fitting them I dismantled and cleaned them and found that they had different jetting although they performed fine on previous engine.
Carb 1 (top) main 128, pilot 42. Carb 2 – main 112, pilot 45. Carb 3 – main 128, pilot 42. Carb 4 – main 112, pilot 45.
After fitting on new engine I cannot get Carb 4 to operate correctly, engine idles but only on top three cylinders. It will rev out and after 3000rpm all cylinders work but below and number 4 will not. also (engine side) Fuel mixture screw does not seem to make any difference at all even fully seated or out 7 turns. have proved by pulling plug lead on 4 and engine note still the same on idle. (on other cylinders, engine nearly stalls)
After idle, number 4 plug is petrol wet when pulled.
My engineer guy helping me, claims carb is no good and needs replacing, ($950 !) I am not so sure, have dismantled and cleaned again and cannot find any issues, no corrosion or blockages and looks identical to the others.
I should say that the manual states that for this 80hp engine the carbs should all be 112 main and 45 pilot
(For the 100hp model – 128 main and 42 pilot) This means that carb 4 is correctly jetted but not working.
Any ideas or suggestions would be greatly appreciated.
I am in Devon, UK and there is a lack of yamaha carb experts here! Lots of people selling new engines and laughing at me!
Replied to your email.
Hi I have a 1987 gpz 500 custom, I’m running single pod filters and stock cut down headers with slash cut cans.
I’ve dynojetted the carbs and almost got it running sweet!
It’s a bit of guess work as I have no rev counter, but it idles nicely mid to high revs seem good but it’s a bit boggy about a quarter up the rev range I would say! I’ve adjusted the needle hight to several positions but I can’t seem to iron out that boggy spot!
Would this point to a richer pilot jet being required?
I would say it the transition from pilot to needle that needs attention!
Any help would be greatly received.
Thanks
Did you tune the fuel screws? If so, how many turns out for peak smooth idle?
Hi Matthew, I have a 1980 Yamaha XS1100 Special with pods and aftermarket exhausts. The timing is good and I changed the oil and spark plugs already. The bike wouldn’t stay on everytime I give it a little throttle. The bike only stays on if the choke is on. Once I fully close it, the bike shuts off even when the engine is warm. The jet sizes for Carb 1: 110 (main), 130 (pilot air), 42.5 (pilot jet). Carb 2: 110 (main). 185 (pilot air). 42.5 (pilot jet). Carb 3: same as Carb 2. Carb 4: 110 (main), 125 (pilot air), 42.5 (pilot jet). I’ve had my fuel mixture screws turned out about 2 -2.5 turns, but it would bog everytime I throttle it (eventually it dies). It seems that the bike is running lean from the spark plug readings. I did try closing the fuel mixture screw all the way in, but the bike does not die. Should I buy larger pilot jets? Should the fuel mixture screws e turned out differently due to different pilot air jets sizes? Or what else could be the problem?
Don’t worry about the main jet or pilot air jet right now.
Focus on pilot jet and fuel screws.
So you are telling me that you can run the fuel screws all the way in and the engine will not stumble or die?
yes there is no change in engine rpm
That tells me that the fueling is off, and the screws can’t trim it out. Let it idle with new plugs for awhile and then send me a pic of what they look like.
Are you sure the carbs are clean and there are no vacuum leaks?
Hello, I need advice. I need to downsize my pilot jets (bike will not stop after closing fuel pilot screws) on my Yamaha XS 360 1977 Mikuni BS34 carb. Problem is, I don´t know if the pilot jets in the carb are stock (manual says 17.5) or smaller/larger. Is there a way to measure them so I know from which size to go down?
Thanks.
I have experienced the same with that model. Stock pilot jets were too rich. When you remove the jet, it will be stamped with a number, indicating size.
I bought the jets from Dennis Kirk. I think Mikes XS will have them as well.
Thanks man, love your work keep going!
Thanks for the compliments!!
Hi Matthew,
I’ve got an 85 GS700 that I have personally rebuilt the engine on 2 1/2 times. First time I did the top end and a screw backed out of the cam sprocket while driving falling into the crankcase and breaking the sprocket. Then I did the full rebuild and apparently we put a thrust bearing in backwards which seized the engine up 🙁 Now that I have meticulously put every bolt and bearing back in properly I believe all that is left is carb tuning.
The bike has aftermarket air pods and Vance and Hines exhaust. I’ve cleaned the carb completely, increased main jet/modified needle jet per dynojet instructions. Currently the bike is hard to start and when it does the revs hang and will not idle, runs great at 1/4 and above throttle. Saw your comments above and I’m really thinking new pilot jets will help this out but I don’t know how many steps. It has stock pilot jets in it at 37.5 and even with fuel mixture screw backed all the way out, 4+ turns, it will not idle. Do you think this situation would call for a 2 step increase or should I go with 3? I understand if you can’t give me a definitive response on that but if you can I would really appreciate it.
If everything mechanically is sound and no vacuum leaks, yes go ahead and buy 2 AND 3 sizes up from where you are at.
What is your setup here? Is the bike stock? If not what air filters and exhaust? Is it a dual exhaust or a 2 into 1?
The bike in video is a bone stock 76′ Yamaha XS360.
Thanks. I have a same year 360 stock besides aftermarket mufflers and cannot get it tuned right. Any recommendations? Seems like from light seat to 1.75 turns out it’s too lean and anything more it runs a bit better but seems too rich and doesnt want to hot start. The plugs have a little fuel on them. Can’t seem to find the balance. Already synced them up good and all the other usual non carburetor items have been checked.
Actually, this 360 ran pig rich with stock pilot jets at 17.5. I ended up installing #10’s, and that nets me 3 turns out on fuel screws. I replaced everything on the carbs including rebuilding the choke plunger pads.
Find a pilot jet size that will get your fuels screws happy at 3 turns out.
Thanks again. Where do you recommend getting good jets for these bikes? I can’t seem to find anything smaller than a size 30 unless you get the style with the bleed holes and that’s not the style the original ones are.
Dennis Kirk:
https://www.denniskirk.com/17339610.sku
Hi Matthew,
I’ve seen some of your videos on yt, great work! Keep it up:-) Thanks for sharing your knowledge.
I’ve got a Suzuki GSX1200 Inazuma which I modified a bit as a cafe racer.
I changed air filters to K&N (removed airbox) and free exhaust, bought a Six Sigma jet kit, but still having huge issues at low rpm’s especially while trying to accelerate rapidly, the engine chockes and backfires. As it gets to higher rpm’s it goes crazy smooth.
Previously it ran ok at 1 turn at fuel screw, problems started after carbs cleaning and also observed that the fuel pump keeps on pumping not like before, it used to stop when the pressured had build up.
The carbs are CVK32 with main jets 125 and 122.5 for inner cylinders, also one washer for each needle and a pilot jet size 40
Did you adjust fuel screws while it was running? 1 turn out is pretty lean.
What was stock pilot size?
Not yet, just hoped that previous adjustments would work. I’ll try to adjudst the fuel screws while running and let u know. I’m not pretty sure about stock pilot jet I suppose it was around 30…but I might be mistaken
Yup, I was wrong. Stock pilot jet is 35.
Hi, Matthew
The fuel pump was faulty, I swapped a new one, most symptoms on low rpm dissapeared 🙂
I did some adjustments on fuel screw while engine was running.
I got best results on 1-1.25 turns, if I go above i.e. 2-2.25 turns the engine starts to stumble.
What worries me, when turning the throttle rapidly there is some black smoke coming of exhaust, I suppose there’s too much fuel in mixture?
At 1-1.25 out, your pilot jets are too rich. Drop them back down to stock.
Hi Matthew, I’ve watch your videos in the past and they have help out a lot! But I have a carb problem that no one cause solve!I have a 1985 CB700SC with low miles. It sat for a few years. So I rebuild to carbs and had the carb bodies professionally “boiled out”. Now all put back together with new pilot jet screws #36 (old ones to clogged to save), it idles fine, which it didn’t do before. And revs up fine passed 1500 RPMs. But the problem is…getting it to rev off of an idle, it wants to stall. I’ve tried opening the pilot screws from 2 turns out to 4 turns out but still the stalling trying to rev off an idle. The float levels are fine and I synced the carbs per your video. Also, I’ve set the idle passed the bog point (1500 rims) Now it revs up from the idle but take a lot longer to return to idle now. Any ideas? Thanks! Kevin
Kevin,
Have you verified that all cylinders are firing? All pipes same temp? When you turn the fuel screws, does the engine respond with RPM increase or decrease?
Matt
I’m such all are firing, the spark plugs indicate that.
Nothing really happens when I turn the pilot jet screws in or out.
But I haven’t turn them all the way in or all the way out
Screw them all the way in the engine should die. The engine should also have a peak RPM at around 3 turns out or so. If you are not achieving both of these, something is wrong.
Did you check for vacuum leaks?
Matt
Hi Mattew,
Thanks a lot for help 🙂
I did go back on pilot jets to stock, huge difference. I ended up at around 2.25-2.5 turns, engine runs smooth.
Much better reaction on trottle.
But still having some issues when I go slowly at constant speed in lower range of rpms @ 1/8-1/4 trottle open, it jerks and chockes a bit.
For a quick test, see if the 1/8-1/4 throttle bucking/jerking goes away with a little bit of the choke in the on position. Better or worse?
Another thing that you can play with is ignition timing. Either retarding or advancing should help smooth things out.
Hi Mathew, I have a Yamaha FZ 750. K&N filter in standard air box and straight through muffler.
Bike run great after 3000rpm but up until that is hesitates under acceleration. While not under load it revs up well.
Im running standard jet but after reading some other comments it looks like i need to up the pilot jet a size or two.
Would that be the thing to try.
Thanks.
Adjust the fuel screws and note how many turns out. If over 3 turns out, you need bigger pilots. You may want to shim your needles .040″ up as well.
Hi Matthew
Hope you can help. I have a Chinese copy of a Honda XL125. It was a wreck when I got it (although it just about ran), but now the valves have been lapped in and I’ve replaced the ignition circuit – there’s a strong spark at the plug.
I can’t work out the carburetion though. It starts first kick on half choke. Opening the throttle does not increase the revs. After 2-3 minutes when I try to take off the choke it stalls at all throttle positions. But if I got to full choke it idles better.
I should add that I live in a hot climate 25-35 degrees centigrade and at 1400 metres i.e. pretty high. I’ve tried shifting the fuel screw from full in to 3.5 turns out, in quarter turn increments from full in, but I still can’t get the thing to idle without choke. Any suggestions? Thanks, Dunc
Take carb apart and verify pilot jet is clean. Check for vacuum leaks.
One correction to your otherwise excellent site, the Mikuni VM series carbs have both a pilot fuel screw and pilot air screw for tuning the idle circuit.
That is a pretty rare setup. I’ve actually never came across one out of the 100’s of carbs that I worked on. You are right, they do exist though.
I’m no expert, it just happens that the carbs off the 77 GS550 I’m rebuilding were made that way. Once again, excellent site.
Gotcha. Thanks for the compliments!
I have Mikuni Carbs a like in the video on removing the welch plug. It is from a Suzuki GS1100L – 1983. The Welch plugs are removed, but the screws are really frozen in there. Any suggestions on how to get them out without stripping them?
Soak it with penetrating oil and walk away for a few days.
Use a little heat (dissasemble everything else so you don’t melt anything) and try loosening them.
Buy a cheap flathead screwdriver and grind the tip so it fits the slot perfectly. Don’t use any downward pressure since the screw may plunge down and the tip will break in body. Also don’t go crazy in torque since the screw is brass.
Good luck!
Matt Thanks for the help.
Hi Matt. I am building a ‘97 XR70 engine with an 88cc kit, race head, 0.275” lift cam, and high-flow exhaust. I will be using a Keihin PE26mm carb that I had from a previous build.
Where would you start with jetting?
Btw, impressive work you’re doing here!
I don’t have exact jetting specs for your combo.
However, the pilot jet is correct when the air screw is 1.5 out. So read the info on this page and get that right first.
Then, do some 1/2-3/4 throttle riding and adjust the needle clip to get best engine response.
Then move on to main jet:
https://howtomotorcyclerepair.com/how-to-tune-a-motorcycle-main-jet/
Hi Matthew, first of all I’m sorry for my rusty English.
Thank U for your nice tutorials. They are well done.
I have a Kawasaki zrx1200r with 36mm CVK carbs.
I have some questions for you:
1) looking at this link ( CARBURATOR OEM KAWASAKI ZRX1200R)
https://www.kawasakioriginalparts.com/road-bikes.html?blockref=99912-1179.C-10&machineid=660&year=2005&model=ZR1200-A5H&country=XX
what about the solenoid fuel cut valve? Maybe it’s an electric valve that close main jet?
2) looking at this link (CARBURATOR PARTS OEM KAWASAKI ZRX1200R)
https://www.kawasakioriginalparts.com/road-bikes.html?blockref=99912-1179.C-12&machineid=660&year=2005&model=ZR1200-A5H&country=XX
you can see that pilot screw is in the engine side…so it should be a screw that meters fuel…but in the part list the screw is called “pilot air” (it has a different shape than what you showed in your tutorials)
I’m not sure but in my opinion it is a screw that meters fuel…not air.
what do you think about it?
thank u
Fuel cut solenoids are normally open passages that close and cut fuel at higher RPM.
Your screw is a fuel screw since it is engine side and pointy tip.
Hi Matthew,
First of all: thank you very much for your useful videos and articles!
Last February I bought a Yamaha FZS600 Fazer, and it was barely used one with all it’s pros and cons. The carbs were highly conteminated because of that and the needle valve stucked and my garage got a very nice petrol smell for a few months. 🙂 So I oredered a carb repair kit, and also also a Dynojet stage1, (mainjet and needle) kit for it. 3 weeks ago I finally have enough time for the carb cleaning and applying the new stuff in it. I can say that repair solved most of our problems: easy&smooth cold-warm start, nice idle, nice reaction at any RPM. So everything is fine except 1 little thing: fuel consumption. It’s increased to 9,2-9,3 liter/100 km (30 mpg) from the normal 5-6 liter/100 km (50 mpg).
My config:
Dynojet stage 1 kit, original airbox&filter, Mivv slipon muffler, new sparkplugs, pilot screw: 2,5 rounds
Do you think this fuel consumption increase is normal? Can I normalize it through only set the pilot screw?
Many thanks in advance!
A jet kit ADDS more fuel, so yes consumption will go up. If your fuel screw is 2.5 out, then going leaner on pilot will result in leaner running, and performance will be compromised.
Thanks for the answer! As a first step I’ll unscrew all 4 spark plugs and check the conditions. I changed them 700 kms ago, so they should be clean and maybe make the mixture a little leaner through the airscrew. If I’m not satisfied with the result then I’ll recheck the whole config (needle pos., mainjet size..) this weekend and order a K&N airfilter and replace the stock one.
Hi Matthew, I have been watching your videos and admiring your work on carburetors. I have learned a lot from your videos on pilot circuit adjustments. I have worked on a few carbs myself with good results. I recently purchased a 2000 Honda Sabre VT1100C2 with 16000 miles. The bike had not been ridden much the last few years and I expected to have to disassemble and clean the carbs. When the previous owner first started the bike for me it popped through the carb which I believed would be a lean condition due to a mucked up pilot circuit. I removed and disassembled the carbs completely and cleaned them. The pilot jets were plugged. The rebuild kits I bought include jets and the pilot jets were the same on the orifice side but the air holes in the tube side were smaller. I installed the new ones just the same. Fast forward and after all back together I started the bike and it ran much better, but, it still seemed a bit boggy/skippy at and just off idle. When I attempted to adjust the pilot (fuel side) screws backing it out made little difference in RPM. So I put a tach on it and the idle was way high. When I brought the idle down to the 1000 spec she wanted to die and did stall after 30 seconds or so. I can get a so so idle at 1200 RPM with the Pilot at 3-1/2 turns out. I test drove it and it is still boggy/skippy when taking off from a stop. After it gets above 2K RPM it takes off like a banshee! So, I feel I should take the carb back out and make sure the pilot circuit is cleaned properly. I plan to put the (clean) stock pilot jet back in and then see how it runs. I would really appreciate your feedback and recommendations. Does no movement in RPM when I back out the pilot mean it is too lean or dirty?
Before taking it apart, spray carb clean at boots and see if there are any vacuum leaks.
Another test is to turn the choke partially on and then do another acceleration test. Does it get better? If so, then it wants more fuel. I’d be inclined to bump pilot jets at least one size anyway. Bikes are lean from the factory period.
Ok Matthew, I warmed it up this morning and sprayed carb cleaner at the boots on both sides and no change at all i the idle so I think I am ok there. I had taken a good look at them when I had the carb out the first time and they looked good. So I pulled the carbs back out, completey disassembled, cleaned with Chemtool B-12 and reassembled with the original cleaned stock jets. Put it all back together and fired it up. To be honest is was a little better but still ran similarly. Question: by everything I have read on your site and others the “rule of thumb” so to speak is 2-3 turns out for the pilot needle is optimum. However, in my service manual it states initial setting 2-1/2 turns out. Then the instruction is to turn each needle out 1/2 turn. If the rpms increase 50 rpm or more to repeat the process until the rpms don’t increase 50 rpms. At which point you turn the #1 cyl. needle in until the rpms drop 50 rpms then turn it out 3/4 turn and that is the final setting. Then repeat for the other cyl. That being said, I start at 2-1/2 out, warm bike and start the process, but turning the needles out really doesn’t do anything until about 4 turns then it starts to sound worse. So I set both needles at 3-1/4 turns out and test drove the bike 10 miles or so. When I take off from idle it pops just off idle every now and again. Runs great at higher rpms. When I am coming to a stop sign and come down through the gears, when I go to take off, as soon as I blip the throttle to take off it will pop then come out of it. I took it home, pulled all four plugs out and was shocked to see the plugs were dark! Not really sooty but dark with a little white near the tip. I am eager to hear what you think. I was thinking my pilot was to lean but now I don’t know! What do you think?
Update: Yesterday I tried again. I lightly seated the pilot needles, backed out to 2-1/2 turns, started and warmed the engine and the best idle was at 3-1/2 turns. Any more than that and the idle wasn’t as smooth. Any less than that it seemed to struggle to keep a smooth idle. Drove it ten miles and pulled all 4 plugs out. The plugs looked better, medium brown and whiter near the tip.
Next time you have them apart it would be best to bump pilot jets up one size.
Ok I will and I’ll check back in when I do. Thanks
Hi Matthew, i have a 98 Kawasaki Vulcan 1500 classic. Kurykyn hyper charger and Vance and Hines slash exhaust, baffles in. I had a very rich condition and replaced the stock needle and and jet from stock to a Vance and Hines stage one. Clip is in the 3rd position and put in the 132 jet. Air/fuel screw is out 3 turns. Great idle and throttle at the bottom end but getting top end sputter. Should i change to maybe a 136 jet?
What is the stock main? What did the instructions say to install?
Trying to get my 2001 Sportster 1200XL to run, I cleaned the carb out and replaced all gaskets and o rings as well as new float pin(may be issue need to know whats correct setting of pin), I can crank the bike over with choke open once I release it kills out and fuel begins to overflow from carb. I also replaced vaccum lines and fuel/res selector.
Thanks
Did you replace the float needle and seat? Did you set the float height or at least inspect it?
Hello Mathew,
I am new on here and have found it quite interesting.
I have a 250 kawasaki kle which is basically a zzr250 in semi trail form.
I have cleaned carbs etc and have a good idle at 1200 rpm and screw 2 1/2 turns out.a blip on throttle has a good return to 1200 (about 2sec or less) so all good there.However just above idle say 1600 the engine staggers then more throttle and is ok .I tried none shim under the needle but not much difference. On the road it is not noticeable. Would a worn needle and or needle jet give these symptoms. Its hard to tell if its a lean spot or a rich condition .Carbs are cvk30 keihin.Any suggestions Thanks .Phil.
Did you try lowering the needle? Maybe it’s rich?
Hi Mathew,
Came here from your Youtube video on Pilot Air/Fuel Screw Adjustment. Excellent video. I have read a few carb books over the years and it was always confusing. You did an excellent job explaining “if this happens then your mixture is too rich or too lean”. You really know your stuff!
Do you feel like trading a beer for a diagnosis? I have a 2019 DRZ400S. It’s bone stock (CV carb). Bought as a left over from a dealer this spring so it’s only had gas in it since June. I ride about 10-150 feet above sea level. We’ve just had a major heat wave over the last month so 95+ days. The bike has an annoying problem. Once warmed up, if I blip the throttle at idle, it kills the engine. Acceleration above idle to redline-ish is fine.
Based on your video, I would guess that the pilot screw is too rich. Factory setting was 2 – 3/8 turns out.
Any thoughts?
Hi Rick,
I would go through the adjustment procedure per the video. Factory setting is a good starting point, however should be tuned to what your engine needs. Make sure you are idling at 1600-1700 RPM which I believe is spec.
Get back to me where your fuel screw ends up.
And when you say blip….is it a small blip?
Thanks for replying Matthew,
I am waiting on an extended pilot screw. As soon as I get it I will follow up. I would say medium small to medium blip. I’ve got two other carbed bikes that I can blip the same without problem. I will check the idle speed first. Since there is no tach on the bike, that might be low and I would not know it.
OK keep me posted.
Matt,
would a overly-rich pilot FUEL screw offer the following symptoms:
-engine will not die/stumble with fuel screws seated?
-intermittent backfire during heavy deceleration?
What is the year make model?
Yes, the pilot jet is too rich if you can seat the screws and it won’t die. However, it is possible that it may be pulling fuel from somewhere else if this thing is not tuned properly.
Are you getting popping or backfiring?
Great thnxxx
Hey Mathew, I have a 1998 Kawasaki 400 4 wheeler (actually 2 wheeler) I was not getting “high end” or fast acceleration so I rebuilt the entire engine including new carb. I was running perfect for 2 weeks and then I lost “high end” AGAIN. I an at a loss. Let me know how I can help $ you to help me and this machine! Thanks buddie. Donnie
Shoot me an email here: https://howtomotorcyclerepair.com/contact/
I have a 81 gs450l suzuki .Had carbs rebuilt,what a waste of money. So I never worked on bikes carb, cleaned carbs. then used del. acid. Changed pilot jet to 12.5 fuel screw 1/2 turn. Still can’t get it to idle more than 45 sec. No more black plugs.Factory air box. Can not find # 10 pilot jet for makuni carbs? Coils tested to factory spec. Still has good spark after dying. Adjusted floats the max both ways.Oh 115 main jet. Live on west coast. Any suggestions?
Sorry for late response. Why are you trying to lean it out so much? Are plugs black after some idling? Did you check the choke plungers to see that they seal?
Anyway here are #10 pilots: https://www.denniskirk.com/ebc/rd-vm22-210-pilot-jets.p173396.prd/17339610.sku
My Yamaha YZF600R will only run for 20-30 secs on choke and then dies, with blackened plugs (even new ones). Seems to be running very rich. I adjusted the pilot screws so does this mean air is getting in somewhere else?
Will it run with choke off? Is this a new problem?
I upgraded my TTR from 250 to 350 cc. Dellorto PHF carb, 52 pilot, air screw 2,5 turns out for ideal throttle response. My problem is cold starting. The motor starts but dies in a second. I have to use the choke for cold start for 5 seconds and then everything works great. What could be the issue?
Cold engines need choke, not sure what the problem is?
With the 250 displacement it didn’t need the choke for cold start, only when the temperature was very low, that is 8 degrees Celsius and below.
I purchase an 81 Suzuki gs450, I can get the motor to idle without full choke on the previous owner took off the stock exhaust and added an aftermarket system with out baffles. I am thinking I need to change my main jet and Pilot Jet. I have adjusted the air Fuel mixture screw in all the way with no changes to idle while running. Iam I going in the right direction?
It wants more fuel. Are the carbs (pilot jets) clean?
I am sorry I cannot get the motor to idle without being full choke
Hi I have a Honda CD200 motorcycle which has a piston type carb. It has a very high idle. I even fully tighten both screws but still idle does not drop. I even fitted a brand new carb but got the same issue. What should I try?
To lower the idle, the slide in carburetor need to move down. This is achieved by turning the idle screw CCW. Also, make sure that your throttle cable has proper slack adjustment, because that can cause the slide to stay up.
my 1977 cb 750k7 backfires thru the number 2 an3 cly. i rejetted the carbs put a 4 into 1 exaust on it and going to run pods have e. i. ingtion when it backfires it shoots fire out the carbs 1 and 4 will run ok what could be wrong
What was stock jetting? And jetting now? Did you snych the carbs? What style carbs?
Hi, I have a Honda 400ex with some issues. I have a 42 pilot and 165 main and the fuel screw is 5 turns out. I have a big bore kit, hot cams and slip on pipe with a drilled out air box lid. It Boggs in the low rpm’s until you really hit the throttle and get into the main jet part and doesn’t want to idle. What do I need to do?!
Sounds like it wants more fuel. Increase pilot jet to a 48 and retune the fuel screw and let me know if that helped.
Hello guys. I have problem with my suzuki GSX400f 82 (4 cylinder). Problem is that on idle, runs OK, when i add more throtle its still ok. .but when i ride and had over 5K (5 and 6th gear) RPM, its almost dead. No power, so lazy. I feel that bike twitches in my hand. Everything is clean (carbs, jets). Im on 3 turns from basis (tightened screw). And im desperate :/
At that RPM and throttle position, you needle and mains are flowing, so those are the circuits you need to pay attention to. Also, is the ignition system in good shape?
Thanks Matthew. Yes, ignition system is perfect (synced), all 4 plugs gives a spark, and bike starts at first try. Meanwhile when you replied, im at 4 and 3/4 turns and problem is still there.
Can’t edit previous post.
When problem occurs during ride, and i push choke on (more fuel) problem is gone. That means maybe replace main jet for bigger diameter (now im on 100), or is there different solution? I can add more turns for air/gas screw (cant rotate to infinity) but not sure if does it helps, because it looks it need lot of gas
Tomas,
What year make model? Also any modifications like exhaust or pods?
It’s 82 model. Yes, i have this K&N RC-1060 air filters -https://www.amazon.de/gp/product/B00062YO8G/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Thats all my modification (no exhaust modifications).
Do the following:
1. Increase pilot jet 3 sizes. Keep increasing until fuel screws net you 3 turns out for peak idle RPM.
2. Raise needles 1 clip position. Or find a #4 washer .040″ thick and shim needles up.
3. Increase mains by 20 (example: if mains are 100 go 120).
Report back after changes.
Matt
Thanks!
I will try to do that next week after come back home. If weather allows.
Hello Matthew.
I changed mains to 120 and it solved my problem. Bike runs pretty nice, with no problem, runs smooth in higher RPMS (5-8k) with no problems.
Unfortunately, other problems occurs. When i start the bike, runs in ideal RPM in idle (no ups and downs, no sudden fluctuations, no idle hunting) just stable RPMs. But wheni i pull throttle a little bit in idle to cca 2-4K RPMs, and release throttle, the RPMs are going slow to normal. They stuck at 2k for while (1 to max 3 sec).
When i hit the gas a bit more to 4-6K RPM, they they return very slowly back to normal. Good thing is that always go back to ideal idle RPM, no stuck at 2,3,4K RPM, but back to nicely 1100 RPM.
Could be a problem a bigger main jet? Or, just play with mixture screw? Thank you!
Your issue is a “lean hanging idle”. You should follow the rest of my suggestions and you will see that it will run great (1 & 3). All you did is changed the mains which is 3/4 to full throttle fueling. You need to address the other circuits of the carb.
Hello, I have a 2009 Yamaha TTR125 with a Mikuni VM26 carb. It bogs down and dies upon quick throttle. Main jet is a 95, pilot is a 20. Air fuel mixture screw is out 1.5 turns. Needle is set at the 2nd notch. We are at 7000′. Any suggestions to eliminate the bogging? The spark plug shows is running rich.
Is the carb clean? Any vacuum leaks? Any modification like air pod or exhaust?
Hello, I have a 2004 Yamaha Raptor 350 with a stock carb and stock everything. The idle is a bit high but its what I was able to settle at so it does not die out or get way too high. I have been watching a lot of your videos and have learned a ton. The issue I am having is that once I get to like 2 gear or above and try to go full throttle it bogs to the point where it wants to turn off. Hope you can help.
Have you removed the carb and verified that it is clean?
Hi, I have a 2002 Suzuki vinson 500 with Keihin CVK36 carb. The previous owner messed with the fuel screw and made it larger. Now the engine wouldn’t die when I fully close the screw. Where can I purchase a leaner or bigger pilot screw?
There is no leaner or richer “screw”. Maybe you are referring to the pilot jet? You will have to take apart carb and verify the number stamped on the jet and compare to stock.
Thanks for the reply! Sorry I’m pretty new with carburetor here. Is the “pilot jet” same as the “slow jet”? The service manual only shows the slow jet and it looks like the pilot jet to me compare to the Google search. The slow jet on my carb is #35.
and what size up or down will be a “leaner” pilot? I have a short video of my ATV condition. I’ve set the pilot screw to the factory default turns but it runs horrible. Can you see if is a rich mixture condition?
https://mega.nz/file/6c0Qza5S#1UcD9fP3szSm78XdOmEw1XITLjR8Wk8nNcdiT2J3VP4
Yes pilot jet is the slow jet, same thing.
I watched your video and it idles fine, so the pilot jet circuit seems fine. As you add throttle, then it starts to run bad. Remove carb again and make sure jet needle and main jet are clean.
What year make model is this?
Hi, it’s a 2002 suzuki Vinson 500 4×4 with Keihin CVK 36 carb. I replaced all the jets with the new kit, Clean the carb like 4 times with carb cleaner and compress air, made sure all passages are clear, adjusted the needle valve float to factory setting, but same problem still happen. I tried close the pilot screw all the way till it lightly sited and the engine still running. Is that normal?
What brand carb kit or parts did you use?
I bought the kit from Amazon, it’s made in Taiwan and quality looks pretty good.
https://www.amazon.com/dp/B07CY9D2W8?psc=1&ref=ppx_yo2_dt_b_product_details
If you have all the old jets, clean them up and reinstall them.
Hi, it’s a 2002 suzuki Vinson 500 4×4 with Keihin CVK 36 carb. I replaced all the jets with the new kit, Clean the carb like 4 times with carb cleaner and compress air, made sure all passages are clear, adjusted the needle valve float to factory setting, but same problem still happen. I tried close the pilot screw all the way till it lightly sited and the engine still running. Is that normal?
Hi. My atv also wouldn’t die when the pilot screw turns all the way in, it only makes the engine run rough. Luckily my friend has the same ATV as mine, so I compared my carb with his. I found that my pilot screw needle hole is bigger. His pilot needle only sticks out about 0.5 mm when close all the way in, but mine is about 2mm out of the hole. I guess the previous owner probably over tighten the pilot screw and messed up the hole. If this is the case, how should I adjust the pilot screw?
Just adjust to achieve peak idle RPM and smoothest idle.
Hi I have a 1966 Yamaha 250cc model YDS3 that I am restoring to running condition the needle and seats are bad and leaks fuel when not running do you know where I could get replacement needle and seat or a way to reconditioned the ones I have I am also looking for other items maybe you know someone that might have some parts or could help in my search
You should be able to order OEM parts: https://youtu.be/eqe59cINk7A
Hi , just wondering if you make these adjustments to the Pilot Air/Fuel screw with the engine idle screw backed off.
Leave the idle screw alone until after the fuel/air screw adjustment is done.
That’s what I thought , Thanks Matthew
Hi Matthew, thanks for the information, it helps me understand jetting just that little bit more.
But I am stuck with my 2014 yz250 and hoping you could help me out!
Fouling a heap of plugs usually on cold start up a couple days after a fresh plug (runs fantastic)
I am usually 600-750ft above sea level and at the moment its around 14-22 degrees Celsius (59-70F)
I had a 180 MJ, 52 PJ, and N3EW needle in centre notch. Running 40:1, and the Air screw was about just over 2 3/4 out, so it’s a jetted a little bit rich. (I think) Bike pulled well, no real issues on performance just a fair amount of spooge and plugs always coming out wet (if not fouled) and fouled.
I did drop the needle one notch (to 2nd notch from top) the plug seemed to come out better, a bit of carbon but still wet
Also.. It’s hard to admit this but many of my probelms could have come from my air filter being too sticky, I may have noticed a couple drips coming from the airbox.. so thicker filter=less air=rich condition=fouling..(I assume) BUT I’ve since properly oiled my filter, and still have had one foul since (first start up since cleaning)
Anyway sorry for the novel, the more info the better, right? so I ordered a JD jetting kit and it came with 175, 172, 170, 168 main jets, a 45 PJ and 2 needles, one red (leaner) and one green (richer)
I have the air screw at 2 turns out, any further and there is a bog when WOT, spluttering and sounds like its out of fuel and any further in and the low end bogs. The “instructions” pointed me to the 45 PJ, 172 MJ and the red needle in middle notch (3) I figured I would work my way down so I’ve got the 175 MJ In now, with obviously the 45 PJ because there is only the one size (I assumed the kit would come with a few choices for $190 AUD, I should’ve properly read what I was buying.. duhh) bike runs great, plug came out with browning on the ring and insulator tip is greyish brown, threads a little wet but barely
I ride a lot of single track, tight between trees, steep/rocky climbs, I usually cruise around in second gear, now when I corner I feel as though I should be able to get the back end around corners quite easily, before the bandkicks in, but it kind of struggles to pull hard, like there’s a flat spot from idle- 1/4 throttle (indicating pilot needs to change, would that be a lean or rich condition?) obviously there’s not much power before powerband but it just doesn’t feel right having to pop the clutch around most corners if I’m not on the pipe. Is this normal? I know there’s first gear but second should be more than enough, right? Before I went leaner in the jets, I definitely had more power low in second gear than I do now. Richer does make more power but I really just want to eliminate spooge and plug fouling..
Cheers Matthew 🤙
Bow,
Seems like you are close with jetting. I’d try 1.5 out on the AS with the 45 PJ. Otherwise go to a 48 PJ, because it sounds like it needs more fuel down low.
I originally had that but was sluggish in the lower half of throttle, but I did move to notch #2 from top, so I’ll reset to middle notch and screw 1.5 out.
Alright I will definitely do that when my 20 set of jets arrive, thanks a heap mate, really appreciate it 👍👍
Cool. Keep me posted.
Hello I have a 2009 honda vt750c2 with cobra exhaust baffled and a open air carb I’m have issues with start up spits sputters and will start for a min then deis I’m running a 150 piolet and a 148 main Jett and having trouble eveytime I start it I have to kick the idle upwith plunger chocked open I live 501 altitude in texas any info would help
Don,
You are running lean. 150 pilot jet number does not make sense. If you want to make it easy on yourself, buy a “high performance” air intake setup and a jet kit. It takes the guess work out of proper jetting. Otherwise expect to make major jet changes and spend a lot of time tuning.
Matt
hello!! i have a cb 500 twin year 2000. after adding a sport exhaust and free flow filter, it behaves oddly. runs well in all situation, except when i go WOT in 6th gear, (only 6th, if downshift and wot, it will redline no prob) like for passing, at around 5000-6000 rpm, it bogs and wont increase revs, but if i only slightly back up on throttle, revs start to climb again.
i tried to add shims to the needle, from 1 to 3, but no difference, maybe even worse.
pilot screw 3 turns out.
opened the carbs and are perfectly clean. main jet is 122, pilot 38. dunno where to go… smaller main or bigger? i have a 115 and a 135 at home.
Riccardo,
By closing the throttle at WOT and it sped up tells me that it is lean. Increase main size 2-3 sizes up to start.
To verify that it is lean, wrap some masking tape around air filter to restrict air. Go do a WOT test. If it runs better then lean for sure.
thanks for the reply! do you think that going from 122 to 135 will be too enriching? i only have those handy at home, or better aim to 127?
forgot…hile increasing main,
would you also add extra washers to the needle, or leave it alone ( 1 washer as stock)
Sure try the 135 since you have them.
Yes, at least one .040″ shim on needle.
Needle is 1/2 to 3/4 throttle position.
Main is 3/4 to FULL throttle position.
update: i installed 135 main jets, and 1 shim (stock) on needle.
result: starts and idle well, but while before bogged when wot in 6th, now , in every gear, between 5000-7000 rpm sputters, but adjusting the throttle, get past 7000 and rushes to 10500 limiter without problem.
also releasing throttle there are some crackling sounds, absent before.
clues?
thanks
5-7k is all the needle. What was the setting before this current one shim only, and how did it run?
Cracking/popping on decel is lean pilot circuit. You can probably go up one size on pilot jet size since you are at 3 turns out.
update: i have added two extra shims to both needles ( 3 total ) and popping/sputter midrange was WORSE.
i then removed ALL three shims, and it helped lessening the symptoms, but not cured.
as before, past 7000ish engine revs freely to redline.
i also carefully checked the membranes with a lens for micro-tears and there was none.
meanwhile i ordered a main jet kit with sizes 125 and 130, in case 135 (from 122 stock) was too much.
so i drove it for 80 km. if i only peel the first 1/3 of gas all is fine, beyond that, sputters through 7000-ish rpms. also gas mileage dropped from 20km/l to 15 km/l, that is dramatic worsening,
so 135 main jet is a no-go .
Sound like you need to work on the needle. Do some simple tests to see if it lean or rich. Turn choke on, and test the problem area. Better or worse? Tape up some of the air intake and perform same test. Better or worse?
The main jet only supplies fuel during 3/4 – FULL throttle, so I’m guessing that is not your increased fuel consumption issue.
so i tried masking air filter and deffo got WORSE and earlier sputtering. same with open choke. so that is an unmistakeable sign of too rich fuel mix, right? but if i increase shimming makes it worse, and no shims is no cure. if i go one size more pilot jet, from 38 to 40 it would again make a richer mix… im at loss here 🤔😏
i also tried completely removing air filter, and marginally less intense popping, still in 5-7k range.
so i finally made it!! it took me probably 8 times with carburator-off, but at the end i found the balance. it was the main jet TOO BIG, i tried 135 main, 130, 125…nothing… also changed the pilot jet, from 38 to 40… no difference. all the while adding/removing shims on the needle. the turnaround was when i went SMALLER main jet, from 122 to 120, and also REMOVE all the shims i had on the needle. that cured all the popping and hesitations. perseverance pays.:D thanks for the help!!
Awesome nice job. That is how jetting goes sometimes. Very time consuming.
Hello from the UK, I have a 2000 Kawasaki GTR 1000 trike conversion with the 4 carburettors framed as one. The airbox has been changed by the previous owner to 4 individual k&n style filters and the engine running is not reliable and hard to start. Would this be a pilot air/fuel issue or are the air filters to blame? I don’t have access to the original airbox to test and upon removing the petcock from the fuel tank the reserve filter screen has split not sure if this has added to the issue? I’m assuming that the best way forward is a complete carburettor strip and rebuild and an in line fuel filter fitted but wanted your advice first if possible.
Individual pods flow more air than an airbox. You will need to increase pilot jet size, shim or move needles, and increase main jet size. I would drain tank and see if any junk comes out. If so, flush tank. I an in favor of replacing petcock over those inline filters.
Hi Matthew, I have recently purchased a 2001 Yamaha Kodiak 400 4×4 that needed some wire harness repairs. After completing those repairs and getting the unit running I had to rebuild the carb. The unit runs great on top end but hesitates and bogs when going from a slow creep to around half throttle. Also at idle if I blip the throttle it hangs and doesn’t immediately come back to idle. Current settings are as follows, main jet 132.5, pilot 17.5, fuel screw 2 3/4 turns out from closed. Unit has a uni pod filter and a Dg slip on muffler. Does it sound like I need a richer pilot jet? Also what size would you recommend starting with? Thanks
Your mod from stock airbox to pod filter is allowing more air in, and you are getting a lean bog.
Adjust the fuel screw and if you hit 3 turns or more per the video, then go up in size.
Next, adjust your needle clip down or shim it up .040″ and see it that helps. It would help to go to a smaller diameter needle, however I don’t know how to find one for your model.
Matt
Hi don’t know if you are active, but i will try…. i bought a new set of mikuni rs 36 (flat slide) on my bike, with standard setting, on the road the bike runs good but if give gas suddently from idle it boggs sometimes, i tryed a bigger pilot( from 17,5 to 20 and to 22,5) but is the same i have also problem when riding, because rpm takes some times to go back to idle and the bike remains a bit accelerated, i moved the needle clip 1 step down (rich) but same problem. All let me think that is lean but the sparks are black the fuel screw are 1/2 turn out (if i try to close all them the bike keeps running) manual say the is the max out turn raccomended.Any idea? Tks!
Sounds like a lean condition. Have you tried to adjust the accelerator pump to shoot earlier? Watch this video, you want the pump shot to come in as early as possible (different carb, but you get the idea):
https://howtomotorcyclerepair.com/fcraptiming/
Also, your pilot jet is too rich based on fuel screw setting. Get the pilot jet sized so fuel screw is 2.5 out:
https://howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/
Slow return to idle can be a lean hanging idle, however your pilots are super rich so this leads me to think that the carbs are not synched. Did you vacuum synch the carbs?
Matt
Hi Matt, actually i need to play a bit with the fuel screw, but with only the fuel pump adjustment to start earlier and the needle clip back to third position the bike runs good, just the idle when the bike is very hot is not steady after pulling the bike.
Thanks for your help!
I would tune the fuel screws when the bike is “very hot” and see what fuel screw setting you are at. Keep me posted.
Hi Mathew, I’m working with a ’92 XT350, 5k miles. Bike sitting close to 10 years, finally have carbs torn down. They say this bike is extremely lean from factory. Have read so many posts from one extreme to the other. Just trying to give it a little more power. Primary is 125 main with 42 pilot. Removed plug and idle FUEL screw (manual calls it air screw) is just under 3 turns out. Any help on a good combination? Was thinking of 130 jet, finally found a 47.5 pilot (if needed). My needles only have 1 groove where some have 5. Not sure to lift needles? Secondary main is 106. Is a YDIS cv I think it’s called that. I figure I can fine tune like your video once at idle, but don’t want to tear into carb again If I can help it. Is there a good combo to do while I’m inside? Any help appreciated. Thanks
Patrick
I don’t have jetting specs for your model. Jetting is a trial and error process and sometimes you have to remove the carbs several times to get it right. Are there forums for that model that might recommend jetting so that you can get it right the first time?
I size larger on pilot is always a good start.