A dull mower still removes grass, but it tears rather than slices. The lawn looks pale or fuzzy as thousands of frayed tips dry to tan. Cutting quality is a better signal than an arbitrary number of mowing hours.
Inspect after mowing
Look at several grass blades a day later. A clean cut has a narrow, straight edge. A ragged end with separated fibers indicates a dull or damaged blade. Uneven rows can also come from deck buildup, a bent blade, tire tracks, or moving too fast.
Prepare safely
Follow the manual. Let a gasoline mower cool, close its fuel valve if equipped, and disconnect the spark-plug lead. Remove the battery and safety key from cordless equipment; unplug corded equipment. Never rely only on the control lever. Tip the machine only in the approved direction and block the blade so hands stay clear.
Remove and inspect
Mark the grass-facing side and photograph washer and hardware order. Replace a blade with cracks, deep gouges, severe thinning, a bend, or enlarged mounting holes. Use the exact blade type and fastener procedure specified for the mower.
Sharpen and balance
Maintain the original bevel and remove only enough metal for a clean edge. A mower blade should not be as thin as a kitchen knife. Take similar amounts from both ends and avoid overheating the steel. Check balance with a blade balancer; remove a little from the heavy side and test again.
Reinstall correctly
Fit the blade in the marked orientation and tighten to the manual’s torque specification with the required hardware. Stop immediately if vibration remains. A bent crankshaft, damaged adapter, loose hardware, or hidden blade damage needs inspection.