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accelerator pump timing?

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My 08 yz250f stumbles when I blip the throttle. I have tried rejetting. It doesn't seem to help. I have an adjustable leak jet and an adjustable pilot screw. I did buy a James Dean jetting kit last year and it seemed to work great. Not only did the kit have a different main jet and needle, but it also contained a rubber o ring that slipped around the accelerator pump timing screw and the lever. The claim was more snap off the bottom. It worked great for a while, but the bike seems to have developed this hiccup when the throttle is opened quickly. Maybe a rich stumble or maybe it's a lean stumble. I can't tell. Any advice please?

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Make sure the accelerator pump is squirting. The orifice can easily become clogged and/or the diaphragm can go bad. If it does have a squirt then make sure it is timed correctly and is strong enough.

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