1977 XS 750 High idle

DonCamalot

New Member
My 77 experiences high acceleration when idling and when touching the throttle at times. Engaging the choke seems to help out the idle and random acceleration. I'm trying to get a synch tool from a mechanic. But he also suggested that the valves might need adjustment. Any advice what it might be?
Things i've done:
Check for air leaks (haven't found any as of now)
Check air filter ( brand new/stock)
tune idle and rpm needles ( got it very rich and then very lean, still same problem)
Check throttle cable (no obstructions)
That's all i have for now.
a crappy mechanic said he gave the carburetor a "service" however, based on the work he did and other lies he told me, I have no idea if he actually did or what he has done to it. So i didn't take it completely apart and cleaned as much as i could.
The major difference is that when it's cold, it idles and throttles well when it starts heating up and running the idle shoots up and hangs there (4-5k rpms)
Thanks in advance for your help.
 
I seem to remember this happening to me a while back. If I remember correctly, it was the carbs not.being sync`d. It`s also good practice to adjust valves, too. Just out of curosity, how did you check for air leaks?
 
Start back at square one.

From your post I got that it has the stock air filter/airbox setup. Good. Now you say RPM and IDLE needles....
http://www.boats.net/parts/search/Yamaha/Motorcycle/1977/XS750/CARBURETOR/parts.html

#45 is the pilot screw adjustment. Or mixture screw/fuel screw on this carb. They adjust fueling at idle. Once set, typically don't need playing around with unless you change intake or exhaust.
Sounds like about 2.5turns out should be OK for this bike. There's a cover over them unless the previous person (or you) drilled the cover out and removed it. You VERY LIGHTLY SEAT it (bottom it out) and then back out 2.5 turns for each carb. That's a general setting, some take a little less 2.25 turns or a little more. But for now 2.5 should get you to run decent enough or perfectly.

#53 is tougher to visualize on this diagram, but that's the throttle plate screw or commonly referred to as the SYNCHRONIZING screw. This set of carbs will have 2 screws with ONE carb being the master carb. What I can gather from this diagram is that you will synchronize the #2 and #3 carbs to cylinder #1. If you don't have a carb sync tool. Get one if you'll be working on bikes. Or make your own, there's tons of threads on making your own manometer for super cheap!

Stock exhaust?

Lastly, your fuel level in the carbs can be a factor. Too low can cause lean issues. Typically too little of fuel causes hanging idle and slow-to-come-down idle. So I'd still look for an air leak,usually at the manifold boots. And make sure the sync nipples have a good vacuum cap on them that's not cracked...or you'll get the same problem :)
 
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