A lawn mower spark plug is used to ignite the air and fuel mixture in the same manner as the spark plugs in an automobile. Made of ceramic surrounding an iron core, a lawn mower spark plug is situated in a steel base. The spark gap in a lawn mower spark plug is set in the same manner as an automobile spark plug. After checking with the lawn mower's service manual, the spark gap can be set by grasping the electrode's tang with a set of pliers and pulling it open or pushing it closed until it meets the specified gap when a feeler gauge is inserted into the opening.
As the lawn mower engine is turned over, the ignition points send a charge to the lawn mower spark plug. The steel fixture of the spark plug forms the ground as it is threaded into the engine. With the spark plug grounded, the points can send the electrical charge through the spark plug's metal core and to the electrode situated at the end of the spark plug. As the spark jumps from the electrode to the electrode tang, the fuel mixture is ignited and the engine starts and runs.
With advances in engine oil, gasoline and engine design, the typical lawn mower spark plug needs to be serviced and changed no more than once every year. This service can be accomplished at oil change intervals. In the event that the lawn mower is one of the few that operates on a two-cycle engine design where the fuel is pre-mixed with oil, the spark plug may require changing twice a year. It is wise for owners to remove the spark plug periodically and examine it. A wet or oily spark plug indicates it needs replacing.
Due to the exposed spark plug design in push-type lawn mowers, the danger of striking an object with the front of the engine is always present. If the engine makes even the slightest contact with an object, there is a real danger of cracking the spark plug's ceramic body. A cracked spark plug may be able to fire and, thereby, keep the engine running; however, the engine may run poorly and have a notable decrease in power. If contact with an object is felt, it is wise for users to check the spark plug for cracks to the ceramic body. The spark plug wire should also be checked to ensure a tight fit.