I bought a used go kart with a 5.5 B&S. I do not think the engine was original. Two seater has roll cage and knobby tires, back are 15.5" tall. I had the motor serviced and had the the centrifuge clutch replaced. Runs pretty good on flat concrete, won't go fast on grass and slows to a stop on hills. Any advice? Engine too small? Need different clutch?