If your lawnmower is running rough or is hard to start, it is likely that the spark plug is bad. Use a spark plug wrench to replace the plug in a few minutes.
Remove the plug wire
Most mowers use plug wires that attach through a round metal cylinder that fits over the metal end of the plug. Pull the wire straight out from the engine to remove the electrode.
Clean around the plug
Prevent debris from falling into the hole by using a small brush to sweep it away before removing the plug.
Remove the plug
Generally, plugs require a 13/16-inch or a 3/4-inch wrench. Place the wrench over the plug, and turn counterclockwise to loosen and remove it.
Inspect the plug
Replace the plug if it has broken porcelain or a burned electrode. Change the plug at the beginning of each mowing season or after every 100 hours of use.
Check the gap
Use a wire feeler gauge to check the plug gap. Most small engines require a 0.3-inch gap. You should feel a slight drag as you pull the wire through the plug gap. Use the prongs on the feeler gauge to adjust the electrode if necessary.
Replace the plug
Insert the new plug into the hole and turn clockwise to tighten. Once the plug becomes snug, tighten another one-quarter turn. Push the plug wire onto the plug.