If you closely inspect the top of the mower deck on your MTD riding mower you will see a pulley mounted to a square plate. Attached to the square plate is a spring. This is the belt tensioning system for your mower deck. It is called an idler pulley. As the belt wears and stretches, the spring on the idler pulley pulls the pulley against the belt and maintains pressure. However, the spring is only long enough to allow so much travel before it runs out of adjustment. Unfortunately, the only way to properly tension the belt is with the idler pulley. Therefore, when the pulley runs out of adjustment, the only way to adjust the belt further is to replace it with a new one.
Park the MTD mower on level ground, set the parking brake and turn off the mower.
Disengage the mower deck and lower the mower deck to the ground.
Disconnect the spark plug wire form the spark plug to ensure the mower does not come on while working on it.
Remove the guide pins that press into the PTO pulley bracket that rests just below the engine. Use pliers to remove the pins.
Disconnect the long spring that connects to the idler pulley with a spring puller to release tension on the belt.
Pull the belt off the PTO pulley mounted under the engine.
Remove the hairpins that hold the front bracket to the underside of the mower and then lower the front of the mower to the ground.
Pull the belt off the pulley system on the MTD mower deck. Follow the guide printed on the top of the deck and install the new belt per the routing on the guide.
Lift the front of the mower deck up and reattach it to the underside of the mower with the hairpins.
Place the belt onto the PTO pulley, then reconnect the spring to the idler pulley. Press the guide pins into the PTO mounting bracket.
Reconnect the spark plug wire to the spark plug.