what would cause a deck belt to slip off the idler pulley? I engaged my mower yesterday and my belt slipped off the idler pulley and tore the belt all to pieces. The mower only has 120 hrs. on it. it is a super z. the day before it slipped off i noticed the belt was twisted and riding way low on the idler pulley so i tightened the belt up 1 notch in the chain. any help would be appreciated.