Getting the deck level front to back should fix the problem. It uses a 3/8"x81" belt. When I got my 100 from my dad, he complained that he could never keep the belt on. The problem was that he was using an "A" width belt which is closer to 1/2". Since I started using the right belt, it has never jumped off. I also added a simple belt guide to the slack side of the deck pulley so that it can never jump off.
