What to Know About Lawn Mower Spark Plugs

When it’s time to cut the grass, you want your lawn mower to start easily and run well. Maintaining your mower’s spark plug is an important and simple part of responsible gas-powered lawn mower ownership.

Lawn Mower Spark Plug Function

Spark plugs are an essential lawn mower component that ignite the fuel-and-air mixture inside the cylinder of the mower engine. High-voltage electricity jumps the gap at the tip of every spark plug at just the right moment to trigger fuel combustion.

When spark plugs get dirty or begin to fail electrically, the quality of spark decreases, triggering incomplete fuel combustion and lower engine performance and efficiency. Taking care of spark plugs is one of the most important maintenance tasks for optimal gas-powered lawn mower performance.

Checking Spark Plugs

Begin by removing your mower’s spark plug to check it. Spark plugs are typically located on the back or side of a lawn mower engine. Look for a short wire that’s about 1/4-in.-thick leading to the top of a short, cylindrical protrusion from the engine. If you’re not sure where the spark plug is, consult your owner’s manual.

Use your fingers to pull the wire off the top of the spark plug, then use a spark plug wrench to turn the plug counterclockwise to loosen and remove it.

How does the tip look? A spark plug in good condition from an engine that’s running well will have a light gray or tan color. Any oily deposits, soot or baked-on carbon indicates that the spark plug needs to be cleaned or replaced.

Cleaning Spark Plugs

Once upon a time, spark plugs were expensive enough and small engines ran dirty enough that cleaning and reusing spark plugs was typical. I’ve cleaned many spark plugs during my motorcycling and go-karting days in the 1970s and 80s.

But even though I have dozens of engines in my life now, I almost never clean spark plugs. For $5 or $6 you can install a new plug that will work well. Still, knowing how to clean spark plugs is a valuable skill because you might not always have a new plug on hand.

Oil or carbon on the tip of any spark plug gives electricity an alternative pathway, preventing the fuel-igniting spark at the gap. That’s a problem. Cleaning usually eliminates this alternative path.

Flood the tip of the plug with rubbing alcohol to cut through any oil that might be present. Scrub the area with an old toothbrush and rinse the plug again with rubbing alcohol. Then repeat the scrubbing. If the tip of the plug doesn’t look clean after drying, repeat the process.

Should I Reuse My Lawnmower Spark Plugs?

You can, but there’s a good reason not to. Besides oil and carbon deposits creating that alternative electrical path, spark plugs can also fail electrically. This is why a used plug you’ve cleaned perfectly can still malfunction under load, failing to deliver an optimal spark inside the engine.

Changing Spark Plugs

Every reputable spark plug has a code number on the side, making it easy to buy a replacement. Different manufacturers have different codes, but all companies offer plugs that can be used interchangeably with different brands.

Place a small amount of anti-seize compound on the threads of the new plug to make the plug easier to remove next time, thread it into the engine with your fingers, then tighten an additional half or three-quarters of a turn using a spark plug wrench.