Troubleshoot a lawnmower by checking for fuel, spark and air. Often, the reason a lawnmower does not start is simply that the gas tank is empty. Fill the tank, press the primer bulb three times and try starting again.
If it has been several months since the last use of the mower and there is old fuel in the tank, starting is often difficult. Drain the fuel from the tank and refill with fresh gasoline.
The fuel filter keeps dust, dirt and debris out of the carburetor by trapping them. Over time, these trapped materials reduce airflow, resulting in an improper fuel to air ratio. On mowers with a foam type filter, clean the foam with mild dishwashing liquid and water. Rinse the detergent from the foam and squeeze as much water as possible from the filter with an old towel. After the foam air dries, apply a small amount of oil and squeeze it through the filter. Folded paper filters cannot be cleaned and require replacement.
A loose, dirty or disconnected spark plug also prevents a mower from starting. Remove the plug wire from the plug and the plug from the mower. Check the electrode for wear and contamination. If the plug is dirty, clean it, or replace a worn plug with a new one. Tighten the plug according to the manufacturer's direction, and attach the wire.