High RPM's not dropping / engine racing on it's own.

Getting back to my '81 XJ 650 I've been restoring for the last two years, I've had the carbs rebuilt by a technician, but since I got them back the bike has a problem with high RPM's. After warming up, the RPM's will not drop, and the engine can accelerate on its own. I have to kill the engine, or while riding drag the engine down while in gear. In neutral, if I crank the throttle, the RPMs will rise, then keep rising past 4000+.
I have new manifolds from the carbs to the block, checked the gaskets and the clamps around the carbs, the vacuum hose from the one manifold to the tank fuel petcock, and sprayed carb cleaner around the manifolds while the engine is running looking for air inlet leaks which would cause the RPMs to rise.
It's been suggested the CDI or TCI module which controls the spark might be at fault, but when I talked to someone who repairs those modules, he said it didn't sound like that was the source of the problem. He suggested I look for inlet air leaks again, but I am 99% certain this is not the problem. There just aren't that many places for an air inlet leak, and as far as I can tell I've checked them all.
I'm really stuck here. I can't ride a bike that accelerates out of control and won't slow down when I release the throttle, and I can't find the source of the problem. Any suggestions would be appreciated.
Tony

 

rebuild the carbs yourself. replace trottle shaft seals and try again

 

have you done a running sync?

 

have you tried the obvious first, do you have free play on the throttle cable?
is the idle screw backed off?
have you tried pressing down on the throttle shaft linkage, to see if its sticking open a little ?

 

It can't rev up without air and fuel. And it could be one carb doing this. Do a running synch. If your "technician" had done a decent bench synch it wouldn't be doing what you're describing, assuming you have no other air leaks. And, in my experience, the tolerances in these carbs are so close that even with original old throttle seals it wouldnt pull enough air in to race up to 4000...

 

mine did, not 4000+ but high 3's
happened on the first ride when i got it.
i got all technical on it, and my remedy
was the T.S. seals. I've since had other vac leaks
[ intake gask] that didn't produce the same severe runaway trottle.
as you say- in my experience.

 

have you tried the obvious first, do you have free play on the throttle cable?
is the idle screw backed off?
have you tried pressing down on the throttle shaft linkage, to see if its sticking open a little ?

The throttle cable and linkage is not sticking. In fact, it can accelerate when I have my hand off the throttle completely.
Tony

 

I don't have any sync equipment, and I have never done a running sync. I'll have to see if I can find someone around here who can do it for me.

Tony

 

And when you stripped them to replace seals did you then do a bench synch, or perhaps a running synch? if so you did two things, or maybe three, so can you then be sure it was the seals that sorted the issue, or a better synch?

 

yes , but , is it completely closing?

 

Yes.

 

Yes you can be sure it was the seals or yes you redid the synch so it could have been this?
I don't see how you could change the seals without a resynch (at least a bench synch), so you did at least two jobs on them.

 

Finally resolved this. Took the carbs to a small bike shop where he fixed a few problems, including adjusting the floats and syncing the carbs properly. Also found a minor intake boot leaking gasket and sealed it properly.
The main problem was that I wasn't using the choke, and I had adjusted the idle so that the bike ran smoothly when cold, but when it warmed up the idle was set far too high causing the racing engine problem. Using the choke the bike starts normally now and once it warms up and high idles I just back the choke off.
Live and learn.

 

OK, good news. You definitely can't be afraid to use the "Choke" on these engines. Especially in the more northern climates. You will get good at starting "full choke" and then easing it down over a few miles (depending on temperature) as it warms up.