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A well-maintained spark plug is critical to a properly running lawn mower engine. That’s why you must inspect your spark plug during your yearly lawn mower maintenance. In addition, it’s important to know the symptoms of a failing spark plug so you can tell if it wears out earlier.

How often should you change your lawn mower’s spark plug?

The Tractor Supply Company (TSC) recommends you always change your spark plug as part of your yearly lawn mower maintenance. Some folks like to do this maintenance in the fall, others do it in the spring. Either way, this annual service includes a new air filter, fuel filter, oil filter, and spark plug. You can often buy kits that contain all of the above components.

Black lawn tractor parked on a yard by a red shed wall.
Lawn Mower | Eric McLean via Unsplash

TSC adds that you might want to change some of these items earlier if you use your mower often enough. Once you hit 100 hours of mowing in one season, it’s time to do an oil change and change the filter. At the same time, you should swap the two other major filters: air and fuel.

That said, TSC does not suggest replacing your spark plug just because you hit 100 hours. You can almost always get through your season with a single spark plug. After enough mowing hours, you might want to do some spark plug maintenance–a cleaning and adjustment–but more on that later.

What are the signs of a bad spark plug?

Product photo of a Champion spark plug on a white background.
Spark plug | Champion

It is possible for your spark plug to go bad before the end of the season. If you have waited several seasons to swap your plug, it is even more likely that your spark plug is causing problems.

Here are possible signs of a bad spark plug, according to Champion: You suffer reduced gas mileage, acceleration or power. Or, you hear a rough idle or misfiring. Or, the mower is difficult to start or impossible to start.

Unfortunately, the symptoms of a bad spark plug are the same as the symptoms of several other problems. But pulling out your spark plug and examining it will give you valuable information on what else might be going wrong in your engine.

Remove your spark plug and note whether it is covered in burn deposits that are black, white, red, or even yellow. Also note if any liquid on the plug smells like gasoline or oil. Then service or replace the plug. If your problems persist, you can consult our guide to reading spark plugs or share the information you have gathered with your local mechanic.

Can you maintain your lawn mower spark plug?

Closeup of a spark jumping between a spark plug's electrodes.
Spark plug electrodes firing | Champion

You can often service a spark plug instead of replacing it altogether. There are two common reasons that your spark plugs begin to fail. Firstly, soot and other deposits from the combustion chamber build up on the spark plug electrodes and weaken its effectiveness. Secondly, the electrodes wear out with use and the gap between them changes slightly.

You can repair both a sooty and poorly-gapped spark plug at home, with simple tools. To clean a spark plug, buy a soft wire brush to clean the electrodes. In a pinch, use some sandpaper to clean just the surfaces of the electrodes that face one another to see if that lessens the problem.

You can re-gap a spark plug with a proper, spark plug gapping tool. The gapping tool is a small metal disk, twice the size of a coin. Its edge varies in width, so you can use it to double-check each spark-plug’s gap. Then you can simply press the electrodes closer together or pry them further apart.

Next, read our article on the proper procedure for changing or re-gapping a spark plug, or check out our ultimate guide on how to service your lawnmower.


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