The trouble stems from the drive being reversed on overrun. The belt will track to the other side each time you take your foot on or off the pedal. I think the aluminium pulley, as it wears out makes this worse. I used to take the thing out for a test drive each time I adjusted the back wheel spindle till it was about even but eventually I gave up as life is too short. The steel pulley or a super max overlay will make it much better I am sure.
