The most common engine found on residential lawn mowers is a Briggs & Stratton. While these engines provide years of dependable service, like all mechanical equipment, they are prone to intermittent problems. When a small air-cooled engine such as the Briggs & Stratton isn't running quite right, or fails to start, start with the symptoms to best troubleshoot and isolate the cause of the problem.
Check the fuel level and fuel quality in the mower's engine when the engine fails to start. Mowers that have been stored for long periods of time, or parked outdoors, can acquire either stale or contaminated fuel. If the fuel is cloudy, or looks dark or dirty, drain the fuel from the tank and replace it with fresh fuel.
Remove and clean the air-breather filter. Restriction in the air filter from the buildup of dirt will create both starting and idling problems. Clean the foam filter-element in warm soapy water. Rinse it well and allow the filter to air-dry before installing it back into the air-breather.
Inspect the spark plug if the engine is hard to start, or doesn't start at all. Using a spark plug socket and ratchet, remove the spark plug and inspect the tip for signs of heavy carbon buildup or burning. While a visual inspection is not foolproof in determining if a spark plug is bad, it is a good indication of the plug's overall condition. If in doubt about the spark plug's condition, replace it with a new one.
Ensure the primer bulb is pumped up and doesn't have a hole in it. Many pull-start engines employ a primer bulb to charge the fuel lines to the carburetor to assist with starting. If the bulb is punctured or cracked, it will not create a suction of the fuel line. Without fuel delivery to the carburetor, the engine will not start. Replace the bulb with a new one if it fails to prime the fuel line.
Check the oil level in the lawn mower's engine. Low engine oil causes the engine to run hotter than normal, and will eventually result in a damaged engine. Also, too much oil in the engine's crankcase builds up excessive oil pressure and results in oil leaks, or possible seal failure. Always maintain the oil level at the recommended fill-line on the oil level check-stick.