Tighten your exhaust. Mine did that for a while. Get a Delorto 15/15 carb and a full set of jets Does it have a variator? If it doesn't, that is your problem. There are two kind of 50Vs, variated (fast), and Dimoby (slow) I have that carb set up on mine, I weigh 210, and I've clocked it at 37. With the jets, it cost me around $150.