I bought a used troy bilt 12av839n711 and the guy said it needed a belt.bought the right belt and it will not go forward.when I lift the mower while running I cant hold the rear wheels.when I put it down it wants to move but don't.if I try to hold the rear wheels I cant stop them.when I put the mower down it don't go.it has the spur gears on the right side and new rear wheels with the gears.the cable is also all the way out.i don't understand what could be causing this