Too many lawn darts maybe Kev? lol. The whole belt tunnel might be mis-aligned. I know my belt barely rides on the tensioner but that might have been due to the extra washer i was running behind the slipper pad.
You are running the belt covers right? I know you have an underbody. inspect the covers if they are being used and make sure there not cut up badlt from the belt.
Yes maybe I need some stiffening. I use a zip tie to hold the belt tunnel in place from buckling to the right, and it's actually gotten worse since.
The belt covers are a little jacked but that's because of the belt coming off not the other way around. I'll put my other belt on there and try it out. It lost enough rubber that it was loading up in the spur gear.
I might also throw a shim behind the tensioner so it lines up better with the edge of the pulley. I want the pulley to center the belt, not the tensioner with that sharp edge that seems to easily snags the belt and lets it climb over.