drive belt?

every time i disengage my clutch the drive belt jumps off. ridder number 917.273080 changed belts same thing. futher inspection is belt to long .84.75" went to and 83.5" still to much belt is there a belt a belt small enough that will work. oh by the way i was sitting still doing a preflight on the ridder and operating the clutch to see if it wood stay on .i wasted 35.00 on a useless belt