Mowers that start and die are often suffering from unstabilized fuel that has broken down. Leaving fuel in the tank for more than a month allows the more volatile compounds to evaporate from the fuel, causing varnish to form in the tank. Other causes of this problem include leaving the packing plug in the fuel cap or a dirty air filter.
Lawn mower manufacturers recommend draining the fuel from the tank of a mower before leaving it idle for more than a few days. The mower must be started to burn any remaining fuel from the carburetor after the tank has been emptied. The other option is adding a fuel stabilizer, which maintains the fuel in the tank for six months.
Lawn mowers must have air to operate correctly. After several hours of use, especially in dusty conditions, the air filter becomes dirty, blocking air from passing to the carburetor. The manufacturer's manual usually provides directions for cleaning or replacing the air filter.
The fuel cap of a new mower should be checked for a packing plug. In order for the engine to move fuel to the carburetor, air must be able to enter the fuel tank through the small holes in the fuel cap. The machine can be started again after the packing plug is removed.