Gary, that is exactly what happens to me, I put the belt on and it pops right back off as soon as the mower engages. VERY poor design, and there is no adjustment in the pulleys on the blower unit. There are 'keeper' tabs I guess meant to help hold the belt in place on the pulleys, but these are useless. My cheap Craftsman mower/bagger combo I had before this did a better job than this, maybe I should try and get it back.