High idle help

If it's broken or just needs tweaked

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Hutch1
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Location: Ohio

High idle help

Postby Hutch1 » Tue Oct 09, 2018 8:04 pm

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.
1982 CB650SC

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Volker_P
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Re: High idle help

Postby Volker_P » Wed Oct 10, 2018 5:26 am

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.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

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Hutch1
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Joined: Sat Jan 25, 2014 7:09 am
Location: Ohio

Re: High idle help

Postby Hutch1 » Wed Oct 10, 2018 6:15 am

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
1982 CB650SC

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Folsoml
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Re: High idle help

Postby Folsoml » Wed Oct 10, 2018 6:47 am

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.
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Hutch1
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Location: Ohio

Re: High idle help

Postby Hutch1 » Wed Oct 10, 2018 2:46 pm

Thanks for that. I want to do a thorough check when I pull the carbs off, I'll add that to the list.
1982 CB650SC

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Hutch1
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Location: Ohio

Re: High idle help

Postby Hutch1 » Sun Oct 14, 2018 10:27 am

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.
IMG_20181014_104901722.jpg
IMG_20181014_104901722.jpg (1.87 MiB) Viewed 186 times
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IMG_20181014_104855568.jpg (2.07 MiB) Viewed 186 times
IMG_20181014_104845245.jpg
IMG_20181014_104845245.jpg (1.38 MiB) Viewed 186 times
1982 CB650SC

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Hutch1
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Location: Ohio

Re: High idle help

Postby Hutch1 » Sun Oct 14, 2018 10:30 am

IMG_20181014_104905866.jpg
IMG_20181014_104905866.jpg (1.75 MiB) Viewed 186 times
1982 CB650SC

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Volker_P
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Re: High idle help

Postby Volker_P » Tue Oct 16, 2018 1:33 am

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.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues


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