An uneven mowing cut, often called step-cutting, is a common problem for most lawn mowers and can be caused by a malfunction in or misuse of machinery. It's easy to diagnose an uneven cut, once you know what to look for. Checking a lawn mower is best done on a flat, level surface, like your driveway. Before doing any work on a lawn mower, make sure that the engine is off and the blades aren't rotating.
The wheels of many walk-behind lawn mowers are used to adjust the height of the cut, so it's a natural first place to begin looking for problems. A wheel may have been mistakenly set to a different height than the others. If it's a riding lawn mower with inflatable tires, check the pressure in each wheel. One may be low and causing the lawn mower to tilt.
Decks are the part of the lawn mower that houses the blade. In some mowers, the blade and the deck are connected and a tilted deck could cause the blade to cut unevenly. Decks can loosen with age or bend with normal wear. Occasionally mowers may have been manufactured with a tilted deck. Using a tape measure, compare the height of the deck from the level surface on both the right and left sides. If the measurement is significantly different, a tilted deck could be the cause of uneven mowing.
The blade in most lawn mowers cuts by rotating with a shaft that is either belt driven or attached directly to an engine. A blade that is just bent will not cause step-cutting, but will instead just cut the entire lawn lower as it rotates. However, a tilted blade, caused by a bent shaft or misaligned engine can cause an uneven cut. To check for a tilted blade, manually rotate it until each end is pointing directly to the left or right of the mower. Using a tape measure again, compare the distance from the tip of each blade to the level surface. If the two sides are different, the blade is misaligned. Fixing it may require a trip to the local lawn mower repair shop.
An uneven mowing cut can be caused by other issues as well. Uneven cuts can also be caused by mowing wet lawns or by never varying the mowing direction. Self-propelled mowers that are operated at excessive speeds for the terrain can cause a poor cut. Also, riding lawn mowers that have an uneven weight distribution could be the cause.