WHY your motorcycle only runs on CHOKE
With spring right around the corner, you may be itching to start up your motorcycle and get some riding in. However, you start it up on choke and everything is fine until you turn the choke off, and the engine dies. What gives?
This is a classic symptom of a partially or fully clogged pilot jet. Fuel goes bad after time, and the first circuit/jet to clog with varnished up fuel is the pilot jet. This circuit delivers fuel for idle, and has the smallest passages found in carburetors.
Another sign of a pilot jet starting to get clogged up is a longer time for engine to warm up on choke.
Can I just pour some magic stuff in the gas to help clean it out? Nope. You are going to have to take the carbs all apart and clean them.
Carb Cleaning Methods
Regular carb clean spray is the easiest to start with. I like Berryman Chemtool.
If you need something stronger, try Berryman Chemdip. This is for soaking parts.
Lastly my favorite, soda blast and ultrasonic cleaning.
Carb Tuning
Once you have the motorcycle up and running, you will need to tune the idle mixture. Every carburetor is equipped with an AIR or FUEL screw to tune the air fuel mixture at idle.
Fuel/Air screw tuning explained
Thanks for the free info! I am trying to become a motorcycle mechanic, and these are helpful!:)
Your welcome!
Hey Matt,
I have an 81 Harley my friend is letting me use. Last night, all the gas flowed out from the tube coming from the bottom of the float bowl. Any suggestions as to what I should/can do would be appreciated.
Carb has a dirty or worn float needle. You need to take the carb apart.
I just purchased a 2006 Hyosung GT650. Starts up only on full choke and sits at1k rpm’s. If I touch the choke or throttle it dies. If i let it idle for 30 seconds it dies. Exhaust is blowing out light smoke and sounds like it’s flooded. Ideas?
You have to fully disassemble carb and clean it.
Hey Matthew, I just bought a 79 cb650 and I rebuilt the carbs 3 times, soaked every part in carb cleaner 3 times, used a rebuild kit for the seals and stuff, air hoses all the small holes, but it still only runs on choke. I want to try everything before I spend 400 and a month of waiting to get a new one. Do you have any suggestions?
Did you check for vacuum leaks?
https://howtomotorcyclerepair.com/how-to-diagnose-motorcycle-vaccum-leaks/
Is the bike stock?
Just completed a carb re-build on
My Honda cb350f using your video. ( great video, by the way!)
I used the FH-5196N Nencho Kit & upped the Pilot, Main, and Needle jet one size. The bike has
Pods & 4 into 1 exhaust. I have the carbs open about 1/16 a inch.
Air screw at 1-1/2. The bike idles
With the choke on but reves up
With the choke open. I can change the RPM’s by opening &
Closing the choke. Ideas?
Yes, the choke will change idle RPM, don’t worry about it once warm.
Did you vacuum synch the carbs? Adjust the idle air screws for highest and smoothest idle RPM?
I have a Honda Shadow 2003 VT 750CC . Recently, it started to me some torubles.. it starts only on Choke and also has difficulty in pick up. I showed it to a mehcanic and he says it’s a carb issue. Currently they have a backlog of 30 days and also the service is a shopping $400 + parts.. I am wondering If I can give it a try with your video. I usually do only air filter and Oil filter changes..
Go for it. It will save you money.
Hi
Have a 2013 Lexmoto arizona
can go start her up anytime first time
However when going up the gears have to run on a bit of choke otherwise bogs down
Help
Sounds lean. Check for vacuum leaks and clean the carb.
I just got an 81 Suzuki GS550T. Cleaned the carbs twice, new fuel, no vaccume leaks. IT will only run on choke. Set float hight to factory settings, all carbs are synced, petcock functions properly, blah blah. What am I missing here?
Did you remove pilot jets? Did you remove welch plugs and fuel screws? All those passages clear?
Matt
Yup. All is clear. This is my first bike. I got it to learn on. I had it out this afternoon. Shes wanting to stall going into first. I have to give it alot more throttle than I’m comfortable with to keep it from stalling. I’m guessing I need a complete rebuild on the carbs. Bigger jets maybe? Float compromised? Trying to apply what I know about cars to bike. But that dog dont hunt. Thanks.
Did you synch carbs? Did you adjust the fuel screws? How many turns out?
Help with choke problems for yamaha blaster it runs great with choke half way out
Did you clean carb? Check for vacuum leaks? Perform pressure test?
I was in a panic several hours ago as I sold my 1990 KX60 on Craigslist and the guy is coming to pick it up in the morning. It would only run with the choke on like you said and I followed your instructions and had the bike running perfectly in an hour. Look me up if you’re ever in Sacramento and I’ll buy you a beer! Thanks
Nice job!
I have an Xr 250 And it only runs on choke but when I take the choke off the RPMs skyrocket
Sounds like you need to tune the idle speed and fuel screw once off choke.
Read this:
https://howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/
Thanks Matt, Your info has been really helpful and I’m def happy to support you and your site.
I bought a 2013 Yamaha TW200 to modify to a naked/scrambler and it looks wicked (IMHO). However when I swapped the airbox for a small cone pod filter and the muffler with a shorty pipe with just a baffle, the fuel/air mix got all screwed up. I’ve increased the main jet to a #130, put three shims under the needle and increased the fuel adjuster screw to 2.5 turns out. But still, I need at least half choke to keep it going and after 2-3 minutes, the stainless pipe at the head runs glowing red hot. What am I missing? Any thoughts?
Bruce,
Thanks for the beers!
Sounds like you need to increase the pilot from stock 31 to say 35 or 37.5. Here is a place to order jets:
https://www.jetsrus.com/a_jet_kit_dirt/yamaha_200_TW200.html
Also, give this a read:
https://howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/
Matt
OK, Doh! I’m hoping you find this strangely amusing. regarding the racing carb issue… After all the jet swaps and needle shimmings, etc., it turns out I simply failed to reconnect the vacuum hose to the top of the intake manifold. So I guess it was sucking in massive amounts of air causing the ridiculously lean mix. Lesson learned and Problem solved.
Thanks again for your support of this rookie moto mechanic.
LOL….yeah that will do it! Glad you figured it out.
Thanks Matt, I’ll give that a try and let you know.
So I am working on my buddy’s 2017 suzuki 200s and it sat for about a year. I jumped the battery I drained out the old gas. I drained some from the carb and the gas was green. Anyways after i put new fuel in and jump started it, it will idle fine on full choke, but if I give it any gas or turn the choke off it dies and then I have to jump it again to start it. Itll run with no jumper cables but needs a jump again to start. I’m wondering if it’s just the battery or just the carb or both
If the gas was green, you need to pull the carbs and clean them. No other way around it.
Hi Mathew,
I have a 50cc turkish motorcycle called Kuba Dragon.
I installed direct filter to the carburator. I adjusted the idle and it perfect. When i my RPM is Between 4500 and 5000 the bike start buffing and as if it is taking too much air then it continues normally after passing 5500 rpm.
Can i have an advice from your side. My bike is model 2018. I removed the carburator and cleaned it but it was clean anyway.
Will the spark plug as such rpm do such an error although i tested it and it is igniting normally?
Thank you in advance.
Is this a 2 stroke or 4 stroke?
My 95 yzf 600r keeps stalling out won’t stay running
Has it been sitting a while?
Hi Matthew,
I just bought a 1982 Kawasaki KZ650 CSR the other day. It only starts with the choke all the way out, at which point it revs up to 4,000 RPM unless I adjust the choke (I can go down to 3,000 but stall on anything else), and takes about 5-7 minutes to warm up. Only then can I close the choke and it will idle around 1,000. If I close it anytime before that, it will stall.
It sounds exactly like what you said but the only thing that’s making me write the question is that the owner gave me documentation of the carb being completely rebuilt in the Summer of 2019. Is it possible that it just runs this way (which is what the owner said the mechanic told him after carb rebuild)? Or could it be that the mechanic made a mistake and I need to clean it? Or do I need new carbs?
It is just running lean. Shouldn’t need that much time on choke.
Check for vacuum leaks:
https://howtomotorcyclerepair.com/how-to-diagnose-motorcycle-vaccum-leaks/
Try adjusting the fuel screws:
https://howtomotorcyclerepair.com/how-to-size-your-pilot-jet-pilot-screw-explained/
Straight to the point, thanks for sharing this explanation.
Thanks!
Hi Matt, not every carburetor is equipped with an AIR or FUEL screw to tune the air fuel mixture at idle, the factory carbs on my 1986 TZR 250 dont have any, how do l adjust the air fuel mixture for idle? My bike will only idle with the choke on , Andrew
It is probably hidden from you under a welch plug: https://youtu.be/uAP2gfAqSII
I have a 2006 Yamaha yz 85 it has been siting for a long time it cranks up on choke then dies I haven’t took the carb apart but cleaned the pilot jet any suggestions
You need to completely disassemble the carb and clean it if it has been sitting for some time.
I have a 2000 Harley FLSTS, I am able to start it fine, it’s been sitting a long time due to a neck injury I wasn’t able to ride, but now that I have it started again, once I put the choke back in it stalls. I have been spraying carb cleaner into it, trying to get it cleaned, and it really doesn’t look to be that dirty, from what I can see anyhow. So, any info for this novice green mechanic, would be greatly appreciated. Thanks in advance.
Your pilot jet is clogged. You have to disassemble the carb and clean it.
I had this problem after I purchased a bike. Spoke to Scott at yuasa batteries who also used to own a bike service shop for over 30 years and he told me to run a tank full of fresh fuel in it with k100 additive. I put for every 1 gallon of gas, 1 oz of k100 (tank capacity 3.3 gallons = 3oz of k100) and after sitting and running from time to time during the course of a month the problem cleared itself – I was amazed. Haven’t had that problem since thank God!
You probably just has a thin film of varnish in the pilot jet. If the jet is completely plugged up, you have no choice but to take it apart and clean it.