The belt could be too tight as you suggested. The engine may be idling too high/fast. Make sure the throttle cable is adjusted properly. You can loosen the throttle locking screw on the carb/govenor and make sure the butterfly is closing all the way. You could also have a clutch not opening or closing all the way (depending on the clutch) but it is more likely the belt adjustment or the idle speed. Make sure the throttle cable moves freely, is not sticky or sticking and has full range of motion.
