I have found a diagram of this machines belt fitting, check that you have the belt on the correct way and check the tension springs for the belt, (http://dl.owneriq.net/2/2ec4d400-707b-42db-b1ab-a486fcfb3d03.pdf) then check the gearbox is still mounted correctly on the mower. These machines have a sealed gearbox on them so if there is a problem there it needs to be taken to a dealer to be replaced or repaired if possible. I would check everything from the font of the machine back to the gearbox to rule it out as these gearbox's are expensive.
Maggott, February 2011
