81 Kz650 hangs on revs.

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.

I am running stock airbox, and a Mac 4into1 with 102.5 mains, and 17.5 pilots. Stock 1981 needles with the thick washer on top of eclip, and thin on bottom of eclip. My air screws are currently at 2.5 out each. I started at 1.5 and then read it should be 2.5. It does the same thing at each setting. Speaking of the air screws, I have the screw with a spring on it, no o ring. I am assuming That is the way it's supposed to be? Any help would be appreciated!

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.

When you tested for carb holder leaks did you first lower the idle as low as possible without stalling the engine? When performing the test, the idle should temporarily be set below 900rpm. The lower the better because with the idle very low the slides are nearly shut and the vacuum inside the carb holders is at its highest. Then spray carb cleaner around the carb holders, especially where they mate with the cylinder head and where the carbs attach to them. Carb cleaner works best for this test as it doesn't leave an oily mess with WD40 or evaporate so quickly that it is not drawn into the leaks as can happen with propane or starting fluid (ether). If the engine dies or reacts to the carb cleaner, you need to replace the carb holders. If you find you need to replace the carb holders soak the screws with Kroil for a couple days before attempting to remove them, and use JIS standard, NOT Phillips, screwdriver bits, and this will greatly improve the odds that you will not break or strip a carb holder fastener as so many have done. Let me know if you need info on JIS bits, I have a good link for making your own at no cost. Ed

The bike was idling at about 800 or 900 when I sprayed it down with carb cleaner. I will give it another go, but I have done it a few times now and no luck in finding a leak.

It sounds like you tested it correctly. Ed

The first thing I thought of was an air leak somewhere. I'm at a loss now. I'll pull the plugs and see what they tell me

Would also check tightness of the clamps holding carbs to their holders.
And assure all vacuum caps/plugs are present, fitting properly and in good condition.

Could also test for air leak in the vacuum hose to petcock -- remove the hose from its vacuum nipple, plug/cap the nipple, switch petcock to PRIme position and test run. This should discover or rule out a vacuum hose leak or possible air leak from an internally defective petcock.

Air may also leak through worn or defective pads on the choke plungers.

Question -- is the clean-air system still fitted? (It has various air-leak-prone hoses and components.)

Could also resort to spray testing with WD40, because it produces smoke from the exhaust when ingested through an air leak source.

Good Fortune!

I no longer have the vacuum Petcock. And the corresponding vacuum nipple on the carb is plugged. I also got rid of the clean air crap, branched the ports on the valve covers, and capped the 2 nipples on the carb boots. I never thought of the choke plungers. Will spraying them do any good?

Double post:
I do not have vent tubes on the 2 vents on the carbs. Just open to atmosphere. Is it a requirement to have tubes there?
Also. Right now I am only running a pull throttle cable, as my push one was broken. This should have no effect on the action of the assembly? It closes and opens as it should

Are all 4 of the synch ports on the carb holders tightly capped? Ed

Yes sir they are

How high do you have the idle set & does the choke function normally?

The air screws don't have grooves for o-rings, so you are fine without o-rings
I don't have them on my 81 650-CSR & it runs fine.

I don't know much about the jetting, but if you have the stock airbox, the stock pilot jets might be fine (#15). Not sure if it matters, though.

If your advancer snaps back & the springs look normal, then it's probably fine.
Rusty or stretched out springs would be cause for concern.

ShouterVW wrote: I no longer have the vacuum Petcock. And the corresponding vacuum nipple on the carb is plugged. I also got rid of the clean air crap, branched the ports on the valve covers, and capped the 2 nipples on the carb boots. I never thought of the choke plungers. Will spraying them do any good?

Would double-check the other possible air leak sources before investigating condition of the plunger pads. Worn or damaged pads are more likely to admit fuel rather than air when the plungers are fully down (choke OFF). In the case at hand, would just assure that all four choke plungers are closing fully down when the choke is off.



Good Fortune!