Have a look at the main jet it may be blocked,if it is a dirt bike I would suspect dirty jet,muck or water in fuel bowl,either way your carb sounds like it needs a clean.
Rich, March 2013
Not enough fuel/air mixture will cause this. Does it have an air filter? A fuel filter? When the motor on my garden tiller died like this last week it was the air filter - I washed it in spirit and we were back in action.
