All lawn mower engines require three things for the proper operation of the machine, fuel, fire and air. Fuel comes in the form of only using fresh gasoline in the gas tank. Fuel that has set around for longer than a year is probably bad, and must be discard according to local disposal regulations. Fire for the engine is created by the sparkplug. A new sparkplug should be installed at the beginning of every mowing season. Air is the last crucial ingredient to the mixture. With not enough air, the engine cannot operate at all.
Use only fresh gasoline in the fuel tank of the lawn mower. Old gasoline will only fowl the sparkplug, and may have water in the gas. Remove all old gasoline from the fuel tank. Dispose of the old gas in accordance with all local regulations.
Remove the sparkplug from the engine. Typically the sparkplug is located to the side and rear of the engine that faces the push handles. A large rubber wire connector will conceal the end of the sparkplug. Pull the rubber boot from the plug.
Place the correct size end wrench over the metal nut portion of the sparkplug. Turn the wrench in a counter clockwise direction. Remove the plug from the engine cavity.
Inspect the end of the sparkplug. It may be black and covered in soot. Install a new sparkplug with the correct replacement size.
Locate the air filter for the engine. After a mowing season, the air filter maybe covered and filled with all types of debris. Clean the filter following the engine specifications. Replace the air filter with a new one if it cannot be cleaned. The engine manufacturer will have recommendations of those air filters that can be cleaned, and those types of air filters that must be replaced.
Allow the mower to set for 30 minutes if it still will not start. The engine may be flooded with gasoline. Once the fuel has evaporated internally, it will start. Do not choke an engine while starting it, if it has been flooded.