Greenfield Ride on Mower Drive Belt Over tensioning?

I have a greenfield mower 12-30 say 10-15 years old at a guess. The drive belt keeps over tensioning causing the engine to cut out out. I have removed and replaced the belt a couple times which loosens it up to start but it quickly tightens up and cuts out engine say 30seconds. Anyone with an idea out there. cant find manuals online or from greenfield as they are currently in the process of making them available online.

Christian
June 2011