I replaced drive belt on 42in. Crafsman belt jumps off idler pully each time you start forward.

Hi,
A few things, I separated your post from the one you put it in from 2007, it is necessry to know if you refer to the belt that drive the machine or the deck and, lastly, it is always best to furnish the nine digit model number from under the seat usually which probably begins with either 536 or 917.
Peace,
Clarke