Engine revs high when clutch disengaged - 81 Honda CB750C

[gtwhiting]

Hi - nebie here so please be gentle!

I have been working on an '81 CB750C for a year now and have got it running, road safe and looking great. Upon getting it started this season (long winters here in Ontario), it has developed an issue.

The bike starts fine, and idles in the proper range. Starts out fine, but as soon as I disengage (pull in) the clutch the engine starts to rev very high. Back in gear with the engine regulating things the revs go back down to non deafening. Seems like the throttle is sticking open, or fuel is being delivered unchecked, or something like that. I can ride it a few blocks but the high revs on an already noisy bike are not particularly neighbour friendly!

I've checked and re-installed throttle cable, which is returning fine. I'm thinking something in the carb? I conditioned the fuel for winter storage, but it may still have fouled... seems fine in the tank. I've been advised to run a tank of new gas and Sea Foam or some engine cleaner on a good long ride, but I can't take it on a good long ride in it's current state. Any advice?

Thanks!

Graham
Waterloo, Ontario, Canada
'81 Honda CB750 Custom
'07 Vespa LX150

[fireguzzi]

What RPMs does it idle at when in neutral?

[gtwhiting]

around 4000
forgot to mention that if I rev higher while in gear, it tends to stick at that higher RPM and then come down a bit.

[fireguzzi]

Thats way too high. Have you tried adjusting the idle down? It should be around 1100 or there about. You probably want to pull the carbs off take them apart and clean them good. Might think about getting them synced too.

There will probably be someone with more experience then me come along and give you some ideas as well. Good Luck!

[gtwhiting]

Have already adjusted the idle down, it sits nicely at 1100 after starting, and only sticks at high revs after I've accelerated in gear, then pull in the clutch. Any lower and it stalls out after starting the engine.
I guess I have the sense I'm going to have to take off and scrub the carb again (did it last year), but would love a simpler solution if it's out there. Thanks!

[Wrider]

That sounds like you need your carbs balanced. Have you done that yet?

[CHP4JC]

sounds like there is a air leak between the carbs and the motor. check the rubber boot or what ever you have there for a crack or something not sealing up tight.

[gtwhiting]

Thanks all for the suggestions. Carbs are coming off this weekend. Nothing but rain in the forecast, so good days to spend in the garage, not on the road!