Reconsidering revving problem-thoughts on this?

About a month ago I posted about a problem after rebuilding the engine in my 1982 KZ750H 750-4. When I rev the engine up past about 2500 to 3000rpm's, it stays there, even though I can hear and see the throttle close when I let the grip go. I got a lot of good info here, mostly involving vacuum leaks. Since then I've installed new vacuum caps, hoses, head to carb boots, cleaned and rebuilt the carbs, etc. etc., about everything you folks and I could think of....problem still exists (and also existed once before the rebuild).

One poster (sorry, can't remember who) suggested a possible problem with the ignition advance mechanism...that it might be stuck or not returning. This advancer comes into play around the above rpm's...right? I checked it out and it is loose and does return, but it doesn't snap back as one might expect. Just slides back fairly quickly but not with the spring force expected.

So, I'm wondering if after 30 years, the springs might be streached or weak? Any opinions on this, or am I creating a problem in my head? Are these springs even still available? Thanks!

Would remove the advancer, clean, inspect and lube.
Reinstall and test run.

Springs strong enough to close it at idle rpm should be serviceable.

Would also assure pilot circuits aren't too lean.

And repeat testing for air leaks.

Good Fortune!

Would also double-check for existence and condition of 0-rings on the pilot adjuster screws.

Good Fortune!

I'll check on the advancer in the morning. The pilot O rings are new, and the screws are set to 2 turns out from closed. Should I open them up some more?

polkat wrote:

I'll check on the advancer in the morning. The pilot O rings are new, and the screws are set to 2 turns out from closed. Should I open them up some more?

Won't hurt to try. If it doesn't help calm the racing idle, return settings to default positions.

Seems most of the common vacuum leak sources have already been addressed. But might ultimately need to again resort to the WD40 spraying exercise.

Good Fortune!

What carb?

I just stole this from another forum, hope this will help as I have been following your project.

I think the pilot screws are air screws and you made the mixture leaner. I also think that the "cleaning" that was done was not sufficient. The air jets on the CV34 can easily clog and must be cleared. Note they are located under the slide diaphragms and continue on passageways down into the secondary main jet well and the pilot jet well.

So try to make sure that the Idle mixture screws are set correctly since turning them out sounds like adding air/ leaning the mixture, also double check the Air bleed orifice referred to by this other Poster.

The carbs are Keihin CV34's. The pilot screw, toward the engine, is a fuel screw, and turning it out adds more fuel (according to my manual). I pulled the pilot screws and jets when cleaning and found no dirt anywhere. These things are clean as a pin! The standard setting is 2 turns out (again according to the manual). It idles just fine, and there is no problem until I rev it higher then a certain rpm. I had rejetting it again and the plugs look good now.

I don't think it's a carb problem. In fact, it does act just like a cable hang-up problem, but there's no hang-up. I have noticed that if, when it's holding a high rev, if I turn the kill switch on for just a second, just long enough for the engine to drop a bit in rpm, then turn it on again, the engine drops to normal idle.

This is what is leading me to believe it might be a problem with the advancer. If, when revving the engine, the advancer gets stuck open, would that cause the rpm's to stick up high?

Got that info from another bike site on your carbs, as they are chasing the same problem. Found though a google search of your carb type and symptom. Sorry if it was no help.
I remember you saying it would hang in RPM with the cable disconnected, have you held a timing light on it to see how much advance at RPM and smooth advance/ return?
Also don't these still have a throttle valve on a shaft? If so have you checked the small seals that go on each out board shaft end? A buddy of mine had this problem with a set of carbs on a Yammer XJ.

polkat wrote:

...might be a problem with the advancer. If, when revving the engine, the advancer gets stuck open, would that cause the rpm's to stick up high?

Yes. Could watch it with a timing light. Advancer should quickly close when throttle closed.

Good Fortune!

Hummm...twice now people say that I mentioned it would rev even with the cable loose. No. That is not the case. It will not rev unless I rev it. It's as if every thing is fine unless I rev it past a certain rpm (I was thinking above maybe 2500rpm's but it's more like 1500rpms). Only then will it stick and not come back. Now, I can do this without the cable attached by blipping the throttle linkage at the carbs. The linkage still comes back fine as the rev stays, so at least it's not the cable.

I just came in from pulling the advancer. Didn't see a problem. It snaps back just fine from twisting it ahead with no spring slop. I took it apart anyway and cleaned it up and lubed it. Works fine, so I guess we're back to an air leak somewhere.

I rebuilt the top end recently. Could the head gasket need re-torquing? If that was letting air in wouldn't I hear blown gasket-like sounds? Runs strong otherwise. It's a mystery!!

Won't be the head gasket, you would have to have an vacuum leak somewhere from behind the intake valve and the jet in the carb. Did you consider the worn seal on the throttle shaft/ bushings in the throttle shaft? Don't these Carbs still have a butterfly and a slide valve?