I use kevlar belt 95” x ½” for both lawn tractor drive belt and for the deck/mowing belt. On mine, there is no way to tighten the deck/mowing belt. There are springs that should take up any slack. I've found on mine that the pulley on the right side of deck (looking forward from the rear) can get dirt/debris caught up in the slot and not be able to slide into the "tight" position when one moves the lever up and to the right to engage it. So make sure that pulley assembly is clean and lube, and can move in its slot.
