I rebuilt both carbs on my CB77 about 700 miles ago. Both have worked fine, except that the left carb was running slightly rich, and I was going to drop the needle lower to compensate. Then about a week ago, a new problem suddenly developed: the engine continues to rev high during shifts and takes several seconds to rev down when I return the throttle to the "idle" position. Below about 2500 rpms there's no problem; only when the revs go higher does the engine resist returning to idle. The only way I've been able to have any effect on this is to close the left idle air mixture screw completely. With the left idle air screw closed completely, it compensates enough that the engine runs fine at idle and doesn't race too badly between shifts. I'm now riding it with the left idle air screw completely closed.
I've disassembled and checked the carb several times and can't figure this one out. As I said, it ran fine for 700 miles following a rebuild. Because closing the idle air screw helps, I suspect that air is leaking past the throttle slide, which might be caused by a worn slide bore. The slide itself looks OK. Anyone else experience this problem? PS--this is NOT a stuck slide.
engine racesThanks, Buck. I have checked the insulator and carb flange and O-rings, which I replaced with new ones a few months ago, and I don't see any evidence of leakage there. But you are confirming my suspicion of an air leak somewhere...
When I disassembled the carb for about the 4th time trying to figure this out, I checked the fit of the slide in the bore. With the slide down in "idle" position, I shined a flashlight through the carb from the back side. Looking at the slide from the front, I could see light leaking through around the slide, on the side with the long slot in it. So I'm thinking if light can leak through, so can air. But I don't know whether or not that gap is large enough to be the source of this problem. Maybe it's within tolerance, maybe not. Anybody?....
jesmed, I have found that when you set the idle speed screw to far in (to raise the idle speed) these engines tend to not want to return to idle but do as you are describing. 1. are the slides in in the correct position with the relief facing the air inlet side? 2. Check your idle mixture screws,is the point still there? when overtightening them you can break off the point,you need that! 3. Have you checked the idle air bleed? for that matter look over the entire low speed system. 4. start frome scratch,set the needle at one cut below the center,idle mixture screws at 1 1/8 turn. check to make sure the throttle cable isn't binding and that the slides arn't binding and that the slides are moving together(very critical) and back out the idle screws to a point they only lift the slide a little bit. all this is your starting point with the carbs. after you are sure you dont have an ignition or other mechanical problem! I'm guessing slides are backwards or,,idle circut has a prob. Good Luck, Steve Greer
Steve, thanks, but it's still a mystery...idle air mixture screw is intact (even has a new O-ring) and the slide is facing the right way, with no binding of the slide or cable. Ignition and other mechanicals are fine. You may be right that the problem is in the idle circuit, but if the idle air mixture screw is OK, I don't know where the extra air could be getting in.
My best guess is still that air is leaking past the slide. If that's the case, I may just have to bite the bullet and get another used carb to rebuild. ---Jesse
Did you verify the integrity of your intake manifolds by spraying them with contact cleaner while the engine is running? A change in engine speed will tell all when the spray is sucked in through a leak. It's very common for an air leak to occur when the intake studs are over tightened, causing the flange to deform. Many people assume that these studs need to be super tight, but nice fresh o-rings are more than capable of sealing with only moderate pressure.
Joe
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