High Idle - KA 100

Folks, please comment if you dealt with KA motor wanting to idle high after a carb rebuild - I’d be real interested in causes and remedies.
I recently serviced top end on our motor and changed a piston. Also, changed out reed and rebuilt the carb. I did not mess with needle valve as in didn’t

It’s possible you have an air leak in your intake chain somewhere… I’d start there since things were recently disassembled.

Gaskets used in the carb rebuild, carb to reed block gasket, reed block to engine block gasket etc.

Agree with James, probably an air leak somewhere.

I assume you are verifying the butterfly is fully closing (hitting the stop) and the throttle cable is in the correct adjustment?


Like Robert said, I would check the needle valve inside the carb to make sure it is fully closing and not hitting the top and staying open some. Aside from making sure all screws are tight on the carb.


Thank you gents… will disassemble and check everything… need to find me a thin 9mm socket to get to the needle valve. Appreciate the help!

If you have access to a pop off tester you can test it without removing it.

If you have a 1/4” 9mm socket you can usually grind the outside enough to squeeze in there

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That’s what I did, sacrificed a socket that is seldom used went to work on the grinding wheel and now I leave it in the box with my pop-off gauge.

I can’t see how a malfunctioning needle valve in the fuel regulation system of the carb can lead to a high idle which cannot be reduced by closing the throttle/ butterfly in the carb bore.
Air leak: yes
Throttle not closing: yes
Throttle plate out of centre: yes
I thought the Tillotson needle valve needed an 8mm socket.??

I had a throttle shaft start to leak air where it passes through to the exterior and is held in place with the retaining plate. Replaced the shaft and made sure to grease it on re-assembly and haven’t seen the problem again. Might be worth checking.

If the air leak is external you should be able to spray (carb cleaner / starting fluid, etc) around possible causes while the motor is running and it will change the speed of the engine when you spray around where air is leaking

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