Riding Mowers & Tractors: Why is my riding mower not cutting evenly?

The mandrel assembly attaches to the deck and spins the blade to cut grass. A blade won't move if the mandrel locks up. A damaged mandrel causes mower deck vibration and uneven cutting. Examine the mandrels regularly and replace any damaged mandrels.

A tilted mower deck can cause uneven cuts. Park the lawn tractor on a flat surface and inflate all 4 tires evenly. Lower the deck to the cutting position and position the blades sideways. Measure the height of the cutting blade outer edges. Adjust the mower deck hangers to level the mower blades for an even cut. Watch our How to level a riding lawn mower deck for an even cut video for detailed mower deck leveling instructions.

A bent mower blade won't cut grass evenly and may cause mower deck vibration. Dull blades leave ragged edges on cut grass. Inspect mower blades regularly and replace a riding mower blade if it's damaged, bent or dull.

The blade belt spins the blades to cut the grass. A worn or broken drive belt won't spin the cutting blades so your mower won't cut the grass. A worn blade belt can also cause mower deck vibration or uneven cuts. Examine the blade belt regularly and replace it if it's worn or broken.

The engine tune-up includes routine maintenance for a riding lawn mower or tractor. Engine performance suffers if you don't regularly tune up the engine. A poor-running engine may stall when you try to drive the mower. You may notice uneven cuts if the engine doesn't run smoothly when you're mowing grass. Routinely tune up the engine by replacing the engine oil, oil filter, air filter, fuel filter and spark plug. The tune-up also includes checking the ignition, carburetor, battery, throttle and choke controls, as well as cleaning the engine cooling fins.

The mower deck houses the cutting blades, mandrel pulleys and blade drive belts. You may see uneven cutting if the mower deck gets damaged because the deck. Severe rust can corrode the deck, causing holes in the deck. Replace the mower deck if it's damaged or there's a lot of rust.

The deck idler pulley tightens the blade belt against the blade pulleys to spin the blades when you shift the blade engagement lever to engage the blades. You might need to replace the riding mower deck idler pulley if the blades don't spin. A worn idler pulley can also cause uneven cuts and mower deck vibration, so check the idler pulley if you notice these symptoms. Replace the deck idler pulley if it's worn or broken.