How-To: Synch Motorcycle Carbs
Anytime you degang/derack carburetors, you will need to synchronize them. Also, anytime you exhibit the symptoms listed below, you will need to synch the carbs. This procedure is also listed as a “check” every so often in maintenance charts. This procedure only applies if you have more than one carburetor.
What Does Synching Do?
The purpose of vacuum synching is to balance the carburetors and the combustion that occurs in every cylinder. If the vacuum levels are equal across all intake tracts, then the same amount of fuel/air will be drawn into each cylinder, therefore the combustion forces will be equal on each piston. The result is a smooth running engine.
Out of Synch Symptoms
- Engine will sound like it is knocking, or running rough
- White smoke can be generated from an exhaust pipe (I can’t explain why)
- Erratic idle speed. One minute it can idle high, the next really low. Small idle speed screw changes results in huge RPM swings.
Tools
I was able to try out the Prop-Tech Dsynch2 in this video. The cool thing about this tool is that it runs software on your PC. The software has some cool features and diagnostic tools that simple manometer tools can’t give you.
Before Synching Carbs
- Ignition timing must be set. Overall ignition system must be healthy.
- Valve adjustment should be performed. Valve timing events will effect vacuum readings.
- Compression withing service manual specifications.
- NO VACUUM LEAKS! Since we are reading vacuum in this procedure, any vacuum leaks will screw up the results.
Bench Synching Carbs
In this video, I show you how to get the synch close by adjusting the blades/slides with a drill bit.
Hello Matt,
First thanks for your videos they are very instructive and very well done.
Then, I rebuilt a CB550 and replaced the original air filter by cone filters. I went from 92 to 100 main jet.
I did all the steps (valve clearance, timing and synch) and now I wonder how I can know if my air screw are well set. I set them as original (1 and 3/4 turns) and I don’t know is this is good.
Bike runs great but when I take out the spark plugs they are really white (and I have in mind that they should be brown).
Can you help me with the way to check my air screws (they are also very hard to reach while the bike is running on a CB550) ?
Thanks !
Use a long screwdriver and turn the screws in 1/2 turn at a time until RPM’s drop. Then turn out 1/2 turn at a time until RPM’s increase and then go further until it drops. Note the min/max turns and the setting for peak and smoothest RPM should be in between those settings.
If screw setting is around 1 turn out then pilot jets are sized correctly.
Matt
Hey Matt, I have a CB550F I’m trying to dial-in. I’m running pod filters in a Yoshi style 4-into-1 header. Factory jets are 98-main……38-slow/pilot
I’m running 110- main…..40 or 42 slow ….I live in Florida 100 degrees/100% Humidity
My # 1 and #3 Spark plugs are black, 2 & 4 are clean. When I try to gauge/sync them using the #2 carb as the gauge…..any adjustment I make to carbs 1,3 or 4 throws #2 carb off so I’m always chasing # 2’s setting…….on the air-mix screw I’m 1-1/2–-1-3/4 turns out. Thoughts?
Those carbs with adjuster and locknut are a pain to adjust. By all means adjust #2 if you have to. Make small adjustment at a time and you will get there. Don’t read plugs until they are synched.
Thanks for the video, is this the same way i should check the carburetor of Yamaha FZS BS4 bike. My bike do not have fule injection system as the new bike have today.
Your bike is a single cylinder, no need to synch.
Great site with loads of good information. Thought I’d add that I love using The Digi Sync as my carb sync tool of choice. It is far simpler and has all the info I need on the LCD screen. No need to hookup a laptop and worry about it getting damaged in my messy garage. And, it’s dead-on accurate!