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I have an issue with very low power at mid to high RPM on a CRF50 I bought for my son. The bike is a 2012 CRF50. It starts great, idles fine and for a few seconds, when cold, it will seem to have power throughout the power band. However, once it warms up there is no power from mid to high rpm. The bike sputters like there is a governor on it and barely has power to move. It will rev up ok then quickly settle into this sputtering low power mode. So far I have cleaned and switched carbs, adjusted the air screw mixture, switched the CDI, switched plugs, checked and double checked the ground, checked for shorts, checked the timing, none of these have any affect. Anyone have any ideas?

Has the bike run good previously?
You are turning off the choke once warmed up, has been done before so that is why I ask.
Check the aircleaner not over oiled.
Breather on fuel tank not blocked, ensure good flow of fuel to carby.

Float could also be stuck, try bumbing the carbuator with a wrench but if it doesnt help try and clean the carb out.