Motorcycle Repair: Carb Synchronization Problems, vacuum leak, throttle plate

Question
Mark,

I am having a problem synchronizing the carbs on my 1986 Yamaha FZ600 (air cooled, 4-cyl, 2-valve/cyl 600cc motor).  I recently disassembled the carbs and replaced all the o-rings that join the carbs together, and in order to take the carbs apart, I had to back off all the synchro screws to disassemble the carbs.

I re-assembled the carbs, put them back on the bike, and now, no matter what I do, I can only get three of the four cylinders running.  The carbs are so far out of synch (the plugs are ok, compression is ok, ignition, etc. is okay), that the number three cylinder carb butterfly can't get synch'd with the other cylinders, not matter what I try to do.

When I try to adjust it, the idle goes way up, and then other carbs get out of synch.  Is there a base line opening that I can set all the carb butterflies at so that I can get them close, and then use my carb sticks to fine tune it?

I have been working with motorcycles for 15 years, have all the factory manuals, etc., but these carbs are so far out of synch that I don't know the best approach to fix it.

Thanks in advance for your help.

Tim

Answer
Hi Tim,

There's no baseline setting for synchronizing the carburetors.

You can try bench synchronizing the carbs using a length of wire or other suitable means.

I suspect a vacuum leak may be contributing to the difficulty of synchronizing the carbs.

Send me your email address as requested. I'll return numerous tech files that may be helpful.

Respectfully,
Mark Shively


(sample tech file)


Bench Synchronizing:  

If you look in the bore of the carbs, you see some small holes that the throttle plate passes as it's opening.  I am not talking about the large hole midway from the carb mouth to the throttle plate.  I'm talking about the hole right at the base of the throttle plate (you'll need to look at the blowup of the picture to see it).  Adjust carb #3 with the adjuster knob until the first hole is just fully visible.  Adjust the adjustment screws on the remaining carbs until their plates are in the same position.  Open and close the throttle a few times and check the adjustment.  Back out the adjuster knob the whole way and carefully open and close the throttle plates so they bottom out.  Readjust the knob so you can just see the hole in carb 3 and check the position of the other plates.   If it looks good, move the main adjuster knob out until the plates are just open and you're done.  There should be no dragging or binding when you open and close the throttle plates.  A real syncronizing will have to be done once the bike is running, but this should get it pretty close.