Uncontrollable revving

Hi all. I'm new to this forum, and new to owning/working on bikes.

I have a 1980 KZ750 Twin which I got a few weeks ago.

At first, after it got warm, it wouldn't idle back down to normal, but would stay at 3k-4k when the clutch was put in.

I've done a bunch of adjustments, etc. I've had the carbs off for cleaning twice (the second time very thoroughly, minus the "air needle" which I've heard referred to and which I didn't see.)

The butterfly valves are perfectly synchronized.

The throttle cable is not too tight, and the throttle body snaps back to fully shut. The diaphram/slider dealy's move smoothly, and hold vacuum.

I've got the air mixture screws/idle screw adjusted to get it to idle perfectly.

NOW, it's started not even having the warmup phase. When I start it, it revs instantly to 5500-6k or so and won't come down. (I shut it off immediately, obviously).

I've seen this problem described across multiple forums, but never a really successful answer. Generally just: "it's an air leak"... followed by "But I've checked and replaced everything!"

The ONE unique answer that I've seen was: "I had that exact problem, and checked carbs, valves, timing etc.... but then I found a three pronged electrical connector behind the headlight that had the ground intermittantly disconnected. I fixed that wire, and the problem went away."

Now... I really have no idea why an electrical problem like this would have ANY effect on the revving of the engine.

But anyway.......... ANY suggestions? All the rubber connections around the carbs appear to be new, but I'm going to try the "spray WD-40 around to find an air leak" trick.

Any help is very very appreciated. Since I'm new (~3 weeks) to riding, I'm dying to get out there! It was 85 today! Aggghhh!

Have you tried to spray some carb cleaner at all the joints between the carbs and the engine? If so did the idle/revs change?

Pretty good way to diagnose an intake leak.


KZCSI

If the charging system is disconnected or drops the electrical load from the alternator, it is possible to suddenly rev for no reason. If an air leak search has revealed none, I'd look at the electrical connectors inspecting for loose pins,bad crimps, or broken wires.

Could check ignition timing with strobe-type timimg light to watch and assure advancer unit is quickly moving back and forth as rpm's are varied by blipping the throttle.

Uncertain whether applicable to the bike in question, but on some models the advancer unit has been known to stick in the advanced position so as to disallow a fast return to the retarded position necessary for slow idle.

Perhaps cleaning and lubing the advancer (with moly grease) is indicated.

Good Luck!

sounds simple but did you turn your idle screw out?
if so, i would think you have a vacuum leak.

Hey all. Thanks for all your help so quickly.

I've done a pretty thorough air leak check now: nothing.

I checked the advancer, and it seems to be moving freely.

I'm going to begin a thorough check of every wire and connection... electrical is definitely something I'm not real familiar with, but I guess I'll look for connectors loose, etc.

Thanks again, and I'll keep you posted as to the results!

don't mean to bring up the obvious any time i have had a set of carbs hang up like that it has been in the throttle linkage or cable somewhere if you have to try forcing the linkage closed with your hand and back the idle adjuster right out first sounds like something is bent to cause it to hang like that.