really high idle after throttling down

in general my bike runs pretty well, everything is stock, mostly. no muffler baffle, no headgasket, needle on leanest setting, holes in intake cover. when i first start the engine it idles properly, but when i give it gas and then release it back to idle, it spins way too fast. if i let it idle for 10-15 seconds it slows back down to a proper idle. i also noticed that more often than not this happens in the first five to ten minutes of running the engine, not as often as when its fully warmed up. any ideas?

I know that my motor runs pitiful with that needle setting. I'd be checking your throttle cable.

I could be a small air leak, or maybe the throttle it sticking?
I know that if you screw the top cap of the carb on too tight (the cap that holds the slider in), it can warp the carb body & cause the slider to stick.

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i checked the slide and i didnt see anything wronf with it, slide wasnt sticking and it opened up all the way like its supposed to

OK... but does it CLOSE all the way like it's supossed to?
Also, theres a washer under the spring with a slot in it; is the needle UNDER it? or above it??? (it's supossed to be under the washer)

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does the idle change at all if you adjust the idle screw in the carb? if the screw seems like it's not doing anything at all, check your throttle cable, it might be too tight.

mine did that when i first built my (third) bike, and the slide was just a little bit above the screw, causing the rpms to rev.

i just loosened the throttle cable a bit and it solved everything.

i checked the slide and it comes down too, the idle screw works, i had the engine running, then i loosened it until it died and tightened it back up a quarter turn.

could be a small air leak where the intake manifold bolts up to the jug. when it warms up, the metal would expand, sealing it.

What you describe is always a sing of leanness. Either an air leak or mixture.

Did you seal the carburetor to the intake manifold? Just relying on the clamping function will leave the way for an air leak. A good quality silicone gasket sealer should be smeared on the intake prior to sliding the carb on. I'm not impressed with the 'O' ring or gasket that some use inside the carburetor to seal against the manifold. The silicone makes a fool-proof seal. Be sure to fill the slots made for allowing the carb to be tightened on the intake with a little silicone before you install it.
Tom

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