Before adjusting the motor up or down- check to see the belt is seated properly on the motor pulley and there's no thread wrapped around the pulley. I was cleaning up a white 221 I'd purchased that had a rotted and stuck belt. Found the belt had come partially off the pulley and jammed many years ago. I had to remove the motor pulley (teeny set screw) and it was made of plastic. When the belt had jammed on some thread it had jumped partially out of the pulley groove and shattered the back of the pulley wheel.