81 Kz650 hangs on revs.

Anything's possible, but the spring is fairly stout. You could take a look at it to see if it's stretched or mangled or something. Have you tried lowering the idle rpms using the big Idle adjusting thumb screw?

To determine whether there's a cable issue, could disconnect the cable at the carb, and see if the problem persists without the throttle cable attached. Is it then still necessary to manually push the carbs fully closed?

If not, could add some slack in the cable, assure the cable is in good condition, adequately lubed, free of kinks, and routed correctly with smooth curves.

If so, could inspect, clean and lube the carb throttle assembly mechanism.

Good Fortune!

New air screw O rings and screws set to 2 and 3/4 out finally fixed mine. Best £4 I ever spent.
 

Replying to an 8 y.o. thread ?

Well yes really, having tried everything suggested in all posts 8 years or not re hanging, the cure was one that hasn't been mentioned.
 

I'm always happy when someone replies with the solution to their problem - even after 8 years.