Have a Craftsman mower model 917.376572 drive belt to loose. When pull to activate self propel pulley raises a 1/2 inch but does nothing to tighten up the belt. appears gear box needs rotate to put tension on the belt but it is tight and won't move. Inst manual says to push down on pulley to remove belt, their is no movement when push down on pulley. appears gear box is wedged under the mower case. I guess the gear case should rotate when engage self propel lever to put tension on belt.
You need to replace drive belt item # 33 part number 196857. Follow instructions in manual and roll belt off pulley, be careful and put some muscle into it. Inspect underside of mower while belt is off.
