81 Kz650 hangs on revs.

Each choke plunger individually will not go down any further than they are already. I'm assuming that is "all the way down, and sealed"

ShouterVW wrote: I finally got my bike together and it fires right up, idles fantastic, and transitions into throttle great. The thing that has me stumped is when I rev it, it hangs on the rev after I release the throttle. The throttle is returning to idle, I checked that. I also checked for air leaks around the airbox boots, and carb holders and found nothing. It just revs and stays there for awhile. Makes it hard to shift when the revs won't come down right away...Also, my valve clearances check in spec, and my ignition advancer is free and springs back. The spring tension is not very strong. Not sure how stiff it is supposed to be....

Weaker than normal timing advancer weight arm return springs might be failing to immediately return the arms to idle position.

The manual likely provides an rpm range during which the arms are supposed to move back and forth. But I don't recall seeing a specification about spring tension strength.

Is it possible that the weight arm pivot points need additional lube? Or that one of the springs is "stretched" beyond normal length?

Is it for certain that the pilot air screws are not turned out too far?
Does turning each pilot air screw produce a change in rpm (as it's supposed to)?

Good Fortune!

When checking the throttle for full return to idle, are the handlebars in the straight ahead position?

When the engine isn't running, and the throttle is opened and closed by twisting the throttle grip, is there an audible click when the throttle is gently closed by hand (not snapped shut by the carb return spring), which indicates that the throttle mechanism at the carb is contacting its stop?

Good Fortune!

The carb parts diagram at kawasaki.com indicates that some 1981 KZ650-H1 model have carbs with o'rings on the pilot air screws, while other models have carb pilot air screws without o'rings.

The o'ring part number is 16038-022 and still an "active" part, meaning available through Kawasaki dealerships.

The non-o'ring pilot air screws are shown with non-tamper caps/plugs to prevent access by us tree-hating reprobates. Am wondering if the cap/plug may have helped seal against air leakage where there's no o'ring?

Is it known how long the racing idle issue has persisted?

Good Fortune!

There is an audible click for sure from the carburetors. The duration of the hanging idle is random. Seems to hang longer at a higher rpm.

When mine was hanging up I fixed it by cleaning and lubing the advancing mechanism. I used heavy duty silicone spray as it.goes on wet but dries and does not attract debris.

Check your mechanical advancer

I am going to check the mechanical advance when I get the chance. There really is only 1 way to put it on right? I am just thinking when I took it off a few years ago there may be a chance I didn't get it set right? I don't know, just thinking out loud

Figured out my problem and man do I feel stupid! Turns out the slides were not seating all the way closed upon release of the throttle. I mean maybe half a mm they were staying open. I used my finger as more closing force, then ran the throttle. Perfect! Runs great. That's what I get for not ordering a push throttle cable right away when I found it was frayed to almost broken. I ordered a push cable on z1 today, so early next week it should be up and running like a champ!

That doesn't sound normal. I think the closing throttle cable is primarily for the unlikely event in which the throttle is stuck open, such as a broken or disconnected spring.
Normally, it should snap closed no problem. I would do some checking, starting w the throttle grip sleeve, then the cables, then the carb-cable linkage, then the throttle slide stuff.

With only the single "pull to open" throttle cable installed and properly adjusted, the carb return spring is supposed to fully and completely close down the throttle slides without any assistance from the other "pull to close" throttle cable.

If running without a carb return spring, the "pull to close" throttle cable is required to lower the carb throttle slides.

The return spring and the "pull to close" throttle cable are redundant components serving the same function, which is to lower the carb throttle slides.

Good Fortune!

The throttle seems to snap close no problem at all. But I need to add more pressure to get it to close that barely visible extra to get down to idle. I wonder if my spring is getting weak?