Just a thought, but might want to check your drive shaft bearing on that side. The belt tension should pull shaft away under load but if you let off and the shaft relaxes, it may come in contact with rail tip. Just a thought.
Brainfart. The tension would be on the chain side under load and causing the clutch side to pull back towards the rail under load. Does your speedo still work. That's one of the first signs of the bearing going out or already gone.
I havent had a speedo for a long time on that sled. So I dont know.
I had the same thought about the bearing, so it is going to get a new one just because of age.
