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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
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IMG_20181014_104905866.jpg (1.75 MiB) Viewed 3607 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.

Re: High idle help

Posted: Sat Oct 20, 2018 10:13 am
by flatlander80
I would loosen the screws that hold the plates, and see if that gets rid of the binding problem. Let them fully close and retighten them. I had the same thing happen with my choke plates.


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Re: High idle help

Posted: Tue Nov 13, 2018 6:08 pm
by Hutch1
As an update, I made the following changes which by and large resolved the issues.

It seems that the high idle was due to a vacuum leak which was fixed with new carb boots. Check that off the list.

I corrected the problems with the butterflies by re-aligning the carbs. Problem fixed.

I had it running, but soon after it would not start. I found some very small debris when draining the tank which clogged slow speed jet, pilot screw opening, and accelerator pump check valve. Aside from rinsing the tank several times with gas, I did a thorough carb cleaning with chem-tool carb cleaner spray, e-string, and compressed air. It seamed to do the trick. I set the idle mixture screws at 1.5 turns out which I'm not sure is the right starting point for KEIHIN VB44c carbs. The mixture screws are on the air side.

Here's where I'm at today...

Starts cold with full choke, runs with no choke after 30 seconds or so. Idles at 900rpm, although somewhat labored, occasionally stalls with no throttle. There's a stutter or lag with initial throttle, but then it picks right up. All cylinders seem to be equally hot when checking the exhaust pipes.

I'll put a couple of miles riding in then check the plugs, right now its been ridden less than a mile. It seems to be running a bit rich, other than adjusting the mixture screws and a proper sync, I'm at a loss.

Re: High idle help

Posted: Wed Nov 14, 2018 5:12 am
by Volker_P
Sounds like time to remind to the most important link of this forum !