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Idle runaway when I blip the throttle


Honda CB Fanboy
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I recently finished rebuilding a 1982 CB750C; body, electrical, engine, carb, etc. I fired it up for the first time the other day and boom - she came to life. That was cool. Everything seems to be okay except for a problem with the idle/throttle. I can start the bike and it idles okay, but when I put a little throttle to it the bike revs high and stays high. I can bring the revs back down by putting it in gear and using the brake, but it will head north again with throttle, sometimes even without, and it seems to get worse with temp. I was thinking lean condition/vacuum leak or perhaps somehow related to the spark advance. I did the spray test around the boots (which are new), vacuum lines, etc. with no luck. I double checked the jetting and pilot screw setting on the carbs (stock settings). I did a strobe test on the spark advance and it's good. So at this point I seem to be at a loss and could really use some guru advice. Note that the throttle cables and butterfly valves seem to be working fine.
It's almost always a vacuum leak or throttle held too far open to be able to begin to lift a slide. Carb sync to be able to lower idle speed to 1000-1200 should help.

Most of the time it's the forward rubber carb manifolds.
I've been searching high and low for the vacuum leak but no luck. I put new rubber manifolds between the carb and block and those pass the spray test. The throttle snaps back cleanly when I roll it on and off with a little slack in the cable, but can you tell me more carb sync. I haven't synced the carbs yet, just a visual lineup during carb assembly at this point. Could a bad sync be enough to create this behavior? If so, could I just disconnect spark plugs and run just 1 cylinder to test that? Also, I have not checked out the gas side. It's not a fuel leak anywhere but could it be a failing vacuum valve that is not giving the carbs enough gas?
you really need to read up on carb sync, take it somewhere or get a decent set of gauges. Yes carb sync very much affects how it runs especially at idle.
Not the vacuum valve, you can easily bypass it to check anyway.

You can't sync carbs using one cylinder only running, as one really worn will be way too far open on carb to make your exact issue. The weaker a cylinder gets the wider the carb for it has to be open at idle, that is what hangs the idle to race up. If you tried to 'sync' doing that then much of your issue.

If you didn't break the carbs apart to get at the aircuts then they can make that act up too.
Thanks for the advice guys, I really appreciate it. I bypassed the vacuum valve just to rule that out - it's not that as you suspected. She behaved a little better today so I took her out for a little test ride and under load the idle problem was much better. But I did hear some knocking on the clutch when sitting at the light so I think you're right about the carb sync. I ordered up a set of gauges so that I can sync the carbs. I'm hoping that's it. Other than this issue, the whole rebuild turned out great so I'm pretty happy. I'll report back after I get a chance to sync the carbs. Thanks again, this old girl is pretty sweet.
Okay, reporting back on this one. I received my vacuum kit and went to work on syncing the carbs. I was surprised at how sensitive the throttle and idle is to the carb sync adjustment. In any case, I took my time and got the 4 carbs to align pretty nicely on the gauges, with the vacuum consistent across the carbs when I applied a little throttle. I then got the idle where I wanted it. It now idles and runs much better with no more runaway throttle. The rest of the rebuild is looking good with the exception of the clutch rattle at idle. I will probably take a shot at replacing the clutch dampers to see if I can quiet that down a little. Outside of that, I'm a happy camper and the old girl is ready to ride again! Thanks for the help.
One of the clutch discs (the one with rivets in two plates to make them one) is a shock absorber plate, they get really loose and can make that noise too. You can replace the shock plate with two normal steels back to back, they equal the same thickness but check me on that, it's been a while since I did it. When the shock plate gets old the rivets come loose and run through the motor. Not good.

If you are trying to idle under 1000 rpm the slow speed can rattle them too, 1000 is just enough to bring it slightly above that harmonic that induces the rattle.
Thanks for the tip. Since I don't feel like cracking open the clutch multiple times, I think I will go ahead and replace that plate as well when I swap out the dampers. I have the idle set right around 1100. The rattle goes away at somewhere around 1500 but I don't feel comfortable running the idle that high, so hopefully this these fixes do the trick.
I've got 10W-40 in there now while I break in the new rings. I assume that oil circulation and shifting is still acceptable with the 20W-50? I will try the clutch mods first as I'm curious to see how much of a difference it makes. If I still have rattling issues, I'll give the heavier weight black gold a try. Thanks.
Reporting back on the clutch job. I put in the new clutch damper kit and B plate and it made a big difference. It didn't totally knock out the rattle, but I would say it is 75% better. I noticed that on the old clutch, I could turn the clutch gear back and forth and feel the slop which went away with the mod kit. I think the remainder of the problem with the rattle comes from the main shaft of the transmission which I didn't realize has 8 rubbers in it as well. Next time I crack the case I will go ahead and replace those.