Hustler never incorporated a provision for adjusting the belt on it's machines. I'm not sure what they were thinking, but it's frustrating. The only cure is to modify your motor mounts to allow adjustment (or keep buying new belts). The main drive chains may have the wrong spacing on the adjuster bolts. If the bolt doesn't bottom out in the kendorf channel or have a really heavy washer to pinch against the channel on the sprocket side, it doesn't tighten up enough to hold the idler in place. Good luck!
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far