I have an older model 2072 Cub Cadet (don't even know what year, but I'm thinking build in the mid to late 80's). Problem: When driving (especially uphill) the tractor has very little pulling power. Most often, I have to move the shifter in and out of forward the get it to pull the hill. Looking underneath, it appears that I need more tension on the drive belt, but I can't figure out how to increase the tension. All the fly-wheels associated with the drive belt look to be unmovable. Help