check your needles...

also check the idle set screw... when the throttle is closed, there should only be a gap opening of about 1 to 2 mm.

Also make sure that the flywheel is tight on the crank shaft. You check this by holding the flywheel and the then start to pull the pullstart. You should not be able to stop the flywheel from turning. If you can, then that is your problem.