125ccm loncin motor runs only a few seconds

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Dec 10, 2020
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Hi, I recently got a loncin pit bike with 125ccm engine. I set valve clearance to .10mm cleaned carb( actually it is right now additional in sonic cleaner) und checked for spark. The bike starts with choke but then stalls after a few seconds. Seems like there is no fuel but carb bowl is full when i pull it. When I screw in the air screw almost full in, the motor runs a bit longer. So I assume it is a carb problem. Flushed the tank and blown out all jets and channels with compressed air.
Now I am clueless. I think it must be a carb problem, but I cleaned it like 5 times, blown out the pilot jet and all passages. The carb is a kai hen chinese keihin nock off. Could this also be a cdi or electrical problem? Spark is good though.

Any tips?

Thanks and regards

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