Why the Spark Plug Won't Fire on a Lawn Mower

Oil

• Excessive amounts of oil cause spark plugs to foul and in turn fail to spark. Using the incorrect grade oil or putting too much oil in the crankcase causes excess oil to coat the spark plug. When the spark plug tip is coated, it cannot ignite or ignites improperly. Using 2-cycle oil in a 4-cycle engine brings excess oil to the spark plug, which occurs because 2-cycle oil is mixed with fuel that flows against the spark plug tip. Overfilled crankcases force oil into other areas of the engine because there is no room. The excess oil will drain into any available opening, such as the spark plug chamber, thus covering the spark plug tip with oil.

  Tilting the mower at a greater angle in the wrong direction causes spark plug failure. Angles greater than 15 degrees allow oil to escape the chamber and flow into the crankcase. Once in the crankcase, oil flows to the spark plug chamber and coats the spark plug tip.

Sparking

• Examine the spark plug and related items to ensure proper working order. Loose connections or fittings can cause a spark plug not to fire. The wire connecting the spark plug to the ignition could be loose or off, or the spark plug might need to be screwed in tighter or is screwed in at an angle. Debris could lodge between the plug and the ignition switch. Moisture causes complete failure of sparking. Using a fresh spark plug with the proper gap at the tip ensures a spark. If the spark plug sparks but the ignition still fails, you have either starter problems or combustion problems.

Dirt and Moisture

• You'll need to look for dirt around the tip or in the chamber. Debris and dust can work into the chamber and onto the tip. Feel the plug for moisture. If the tip is wet, it will not spark. Water can also enter the chamber when the mower is left uncovered or stored outside for extended periods. Corrosion occurs when spark plugs are allowed to remain in moisture.

Solutions

• Changing or cleaning the spark plug may solve the problem. Newer spark plugs can be cleaned with a fine-grit sandpaper. Examine the gap at the tip by inserting a credit card between the tip and the plug. If the card fits between the coil and magnet, the gap is good. Older plugs, over one season old, need replacing if they fail to spark. Clean the chamber and spark plug wire with a stiff bristle brush.