I have a 42" lawn tractor (Mod. #247.28901). I was mowing and heard a loud pop. The deck belt cover (metal) on one of the pulleys had broken. I replaced it, but now I can not get the blade to engage. When I move the lever forward, it pulls the pulley correctly to give tension, but it is too loose to engage. I have compared the belt to a brand new one, and it has not been stretched. I can't see any other damage to the deck or the pulleys. Can you suggest where to look, or what to do? Is there a sheer pin somewhere? Could something have been bent? The tractor will roll fine, but I have no blade movement.