Idle races when I take the choke off

Ok, I need some help on this one. I am working on my '79 650 and seem to be having what I think may be a carb problem. The bike has had no modifications that I am aware of so I am assuming that the jets in the mikuni carbs are stock. The problem I am having is when I get the bike started and after it is warmed up when I start to back off of the choke even the slightest ammount, the idle immediatly races up to 6 or 7 k and sounds as if it would go higher if I continurled to lessen the choke. This seems opposite of everything I am used to and I am having a hard time figuring it out. I have already checked for air leaks around the carb boots with the wd-40 check and got no response. Any suggestions as to where to begin?

have you tried cleaning the carbs?
i would think something is plugged up causing it to starve for fuel without the choke,unless the choke plungers themselves are allowing a lot of air in around them? :blink:

I did a deep cleaning of the carbs last spring and the bIke has sat in my garage unused ever since (the bowls were empty while in storage) so I don't think is is a plugged passage. What would be the best way to tell if there was air getting past the choke plungers?

maybe try spraying some carb cleaner around the choke plungers and see what it does,if all else fails maybe throw a can of sea foam in the tank and try cruising around neighborhood to see if it clears something out of there.
if your advance weights on ignition were sticking i would think it would do it constantly

What happens if attempt to reduce idle speed by using the thumb screw?

Perhaps bench sync left throttle slides too high and failed to allow sufficient thread remaining on the idle screw to further lower the slides.

Good Fortune!

Stuck slide?

Re: Patton
Idle thumb screw is almost as low as it can be. That is what is so strange. There does not seem to be an obvious leak (that I can find) and yet the idle spikes when I back off the choke.
When I bench synced the carbs last year I set the slides at the level listed in the clymer manuel so I don't think that a slide is set too high.
I have noticed that the throttle does not spring back down as quickly as it used to, but this should not be a factor here as I am not even rolling the throttle. I am still just trying to get the idle squared away.

After the carbs were removed and re-installed did you run the bike and did it act normal? If not, when the carbs were removed the thumbscrew may not have been screwed into the correct position during re-assembly, so it no longer lets the slides drop down far enough. If that's the case the carbs will need to be removed and the thumb screw position corrected. Ed

Sorry for the delay in response, I have been away on business. The night before I left I ran a bunch of Seafoam through the bike and it seemed to do the trick. I can't say that it is running tip-top but at least it is not racing when I take the choke off. I still need to sync the carbs using the homemade manometer system I have seen described on the forum , but that will have to wait until the weekend. In the meantime, it looks like despite the cleaning I gave the carbs a year ago there was some lingering dirt because the seafoam cleared it right up. Thanks for.all of the suggestions and advice. Viva la Seafoam!