The GY6 line of engines such as the GY6 150cc, were never intended to be used in anything but a scooter design.  As a scooter engine it was optimized for motivating a vehicle of mass between 350 to 400 lbs including driver.  Once these engines were placed in a mini buggy or go kart design the weights that the powerplant is being asked to push is far greater.  With driver and passenger, total vehicle weights of approximately 700 to 850 lbs are not uncommon.