[cart around]

Your belt is good.

Your clutches sound ok, but may need to be lubed with graphite.

Before you put the belt back on, push the cart to see if it rolls fairly easily. That will check to see if the brakes are dragging.

I'm going to bed for work. Maybe someone else can think of other things to check before you buy any parts.

Is your governor connected?

Is your throttle opening all the way?

Maybe the clutch spring is weak, I don't know. Maybe the engine really is weak. Don't buy anything until we know for sure.