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.
Prevent debris from falling into the hole by using a small brush to sweep it away before removing 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.
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.
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.
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.