snapper 331416bve rear engine rider belt coming off

I have replaced belt, and am sure I have the correct belt. Tried running before I put plastic cover over belt deck, and when I engage the idler pulley, the belt will slip off. Is there an adjustment I need to make? one of the pulleys is slightly bent, should I replace that? Will that solve the problem?

Perhaps I need to level the deck? but it was not a problem before?

Suggestions. Help. All appreciated

Hi this might help

http://www.searspartsdirect.com/par...er-tractor-Parts/Model-331416BVE/3290/1506000

Be careful not to over tighten the belt you can bend the crank if the belt is too tight and you overload the blade.
Change the bent pulley as it will almost certainly flick the belt off every time.