I just finished building a go-kart from scratch with a 6.5 horsepower gas engine,so what it is doing is upon takeoff I get real heavy vibration in the rear end so I have to let off the gas to smooth it out.I thought it was the engine loose that's not it. So could it be the drive sprocket ENGINE CLUTCH SPROCKET( max torque clutch #35 chain from BMI kart supplies)and the drive sprocket for the rear was custom made with 45 teeth .
Clutch part #40293512

It's likely related to the centrifugal clutch, engine, and weight combination. It's best to use 2-cycle engines for go carts, since they develop greater torque and rpm than 4-cycle engines. Emission regulations may have changed the equation, however, against 2-cycle engines in many areas.