Short of an irrigation system, the mower is your largest lawn-care investment. This investment should be protected and properly maintained. This means changing the oil regularly and storing the mower to protect it from the elements. Sometimes, maintenance can fall to the wayside, and the engine will stick. This happens because the oil isn't changed, the weather rusts the inside of the cylinder, or old age. Your alternatives are to replace the mower or fix it.
Locate the sparkplug(s) on the engine. These will normally be on the top or front of the engine, depending on the orientation of the motor. Look for the plug wires; this will aid in the location.
Remove the plug wire(s). Use the ratchet and sparkplug socket to remove the plugs. Use care when removing to not break the plugs.
Attach the straw to the nozzle of the WD-40 spray can.
Insert the spray straw into the sparkplug hole and press the nozzle. While spraying, draw a circle with your hand. This will assure that the entire inside of the cylinder is covered. Repeat this step for each cylinder.
Wait 15 minutes for the WD-40 to soak in.
Turn the ignition key or pull the start rope. If the engine turns, the problem is resolved; continue to the next step. If the engine does not turn, the engine or mower must be replaced.
Reinstall the sparkplugs and wires. Check the oil level, and add oil if necessary.
Start the engine. Allow it to run for five minutes at an idle. It will idle very rough at first, but should smooth out after idling for some time. Some mowers may require longer idling to smooth out.