I dont know the yr., make, or model, but POSSIBLY it is the " needle& seat" inside the carburetor. There is a screw, in a drain-hole on the lower part of the carb.. Remove that screw, turn the gas on, and using an air hose, w/ small nozzle, blow air up in there, and it might blow out the "grit" that may be causing the problem. If not, there are 4 small phillips screws on the carb. "bowl" , remove the bowl, and raise the floats up & down several times. I am a certified motorcycle mechanic ! E-mail me if you need to ! REX