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High idle help

Posted: Tue Oct 09, 2018 8:04 pm
by Hutch1
I got my bike started, 1982 cb650sc, but its idling at 4200 rpm.

I couldn't find anything in the manual, or online related to idle adjustment for CV carbs. Here are the changes made since removing the carbs.

complete carb cleaning and rebuild
replaced vacuum-assisted petcock with a regular petcock capped vacuum hole
bench synced
backed out master throttle control/throttle stop

Checked the slack in the throttle cables, that's not the problem.

Also sprayed carb cleaner around the carb boots with no change in rpm, so ruling out an air leak at the boots.

Re: High idle help

Posted: Wed Oct 10, 2018 5:26 am
by Volker_P
If you turn out the black idle knob completely and you have no air leak probably one closed throttle flap (CV carbs) or slider (piston carbs) holds open three others a bit which will cause your high idle.
So check your synchronization visually. The flaps/sliders should all be closed when the manual throttle adjust is turned out completely and open simultaneously to the same tiny gap when you turn the black knob in again.
Note that the choke has a mechanism to open the throttle a bit too. Something with a screw and a spring in the depths between the carbs. Check this function first. It is expected to do anything at all only just before the choke is completely pulled.

Re: High idle help

Posted: Wed Oct 10, 2018 6:15 am
by Hutch1
I ordered new carb boots, which I suspect is the problem. I'll check to ensure the slide is working properly when I remove the carbs, and the choke connection. I also found a video on bench testing slide lift with a bit of compressed air.
https://www.youtube.com/watch?v=lfQz4cxezAk

Re: High idle help

Posted: Wed Oct 10, 2018 6:47 am
by Folsoml
When I rebuilt mine, the idle was too high as well. Turns out one of the throttle butterflies had a burr on it, and was not allowing it to close completely. This held the others open as well.

Re: High idle help

Posted: Wed Oct 10, 2018 2:46 pm
by Hutch1
Thanks for that. I want to do a thorough check when I pull the carbs off, I'll add that to the list.

Re: High idle help

Posted: Sun Oct 14, 2018 10:27 am
by Hutch1
I removed the carbs while waiting for the new boots to arrive, found that the butterfly plates are all gaped differently when closed as you can see from the pics. #4 is completely closed, #1 & #3 have about a 1/32" gap while #2 has over a 1/16" gap.

Also, the throttle mechanism sticks after I snap it back, but I can't find anything binding.
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Re: High idle help

Posted: Sun Oct 14, 2018 10:30 am
by Hutch1
IMG_20181014_104905866.jpg
IMG_20181014_104905866.jpg (1.75 MiB) Viewed 180 times

Re: High idle help

Posted: Tue Oct 16, 2018 1:33 am
by Volker_P
That's tough, no chance for a reasonable idle that way.
If you have the black knob completely turned out I suppose flap #4 might cause the sticky throttle.
In this case when you turn the black knob in until contact to the throttle cam and another half turn in, it should not be sticky any more.