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How Long Do Lawn Mower Spark Plugs Last? (And How to Replace Them)

lawn mower spark plug

How Long Do Lawn Mower Spark Plugs Last?

It can always be a little scary starting the lawn mower for the first time each Spring. Did I store it correctly over the Winter? Did mice chew all my wiring? Am I going to need costly repairs to get this thing up and running?

Sometimes you get lucky and the mower fires right up. Other times a little TLC is needed to break it out of hibernation. The good thing is that more often than not, the only ‘costly’ repairs you’ll need to get your mower ready for the newest cutting season is simply replacing the spark plugs.

What do Lawn Mower Spark Plugs Do? How Do They Go Bad?

Basically a spark plug creates the fire that helps your engine start up. Pulling the starter rope on the mower creates an ignition from the spark plug, which in turn lights the fuel and air mixture (delivered from the lawn mower primer) that makes the engine start.

You need every part of this process to be in working order for the mower to start up. The starter rope has a tendency to fail just because there’s so much force used in every pull. Spark plugs are under a lot of pressure and wear too though, just internally.

Spark plugs have repeated exposure to heat from the engine, as well as fuel and other fluids. Spark plugs have a small gap between a center electrode and a ground probe. When this gap becomes too big (wear) or the electrode or probe damaged (bad fuel to air ratio, foul oil, old age) the performance starts to wane and then ultimately the mower won’t start.

Signs That Spark Plugs are Starting to Fail

If your lawn mower won’t start, one of the easiest things to check is the spark plugs. Even if the mower starts, but sputters or rattles while it runs, bad spark plugs could be the culprit.

It’s usually better just to replace the spark plug, as they can be found for just a couple of bucks at a hardware store and it doesn’t hurt to change them even every season if so inclined. There is also a way to visually inspect the plugs to see if they may be past their prime.

The spark plugs center electrode should have a flat top for the broadest striking surface. It that electrode is starting to get rounded that’s a sign of wear. Any cracks in either the porcelain shell or the firing electrode are going to have an impact on performance as well.

Spark plugs can also be cleaned and replaced if so desired. If the spark plug is black it’s a sign of carbon buildup and if it’s wet there’s gas that could be impeding the ignition. Taking a wire brush and cleaning off the debris may be all that’s needed to restore easy start-up and better engine performance for the mower.

How To Replace Lawn Mower Spark Plugs

Some people will own a push mower for 10 years and never change their spark plugs. It may never affect performance, but it’s also not an ideal strategy either and one that may be costing them much more gas than needed every Spring and Summer.

Either way, a person should at least be familiar with the process for changing the plugs. The plugs are pretty easy to locate on the mower, as they are inserted into the engine and have a wire, also known as a lead, attached. Here is the method for changing them: 

  • Remove the lead
  • Use a 15/16”, 13/16”, 5/8, or 3/4” socket to remove the old plug. Special spark plug sockets have inserts that won’t damage the unit when tightening and loosening.
  • Check the size of the old spark plug so you know the correct replacement. It will likely be 13/16” or 3/4”, but all engines vary.
  • A tool called a spark plug gauge can help ensure you have the proper gap between the center electrode and the ground probe. A.030” gap is the most common, but once again varies by engine type.
  • Tighten the new spark plug by hand first.
  • Tighten the plug the rest of the way with the socket wrench, ¼ turn after it clicks as tight. Overtightening can damage the plug or make it hard to remove.

Overall changing spark plugs is a pretty simple process, even for the most novice do-it-yourselfer. It’s a very good project to know how to perform, even if you never have to.

Will a Lawn Mower Run With a Bad Spark Plug?

A faulty spark plug may cause the engine to misfire, resulting in uneven power delivery, reduced fuel efficiency, and increased emissions. In some cases, the engine may stall or be difficult to start. A lawn mower with a bad spark plug might also experience reduced power or struggle to maintain a consistent cutting speed, impacting the overall quality of the mowing job.

If you suspect your lawn mower has a bad spark plug, it is essential to inspect and replace it promptly. Spark plugs are relatively inexpensive, and replacing a damaged one can significantly improve the performance and longevity of your lawn mower. Regular maintenance, including cleaning or replacing spark plugs as needed, will help ensure that your lawn mower runs smoothly and efficiently throughout the cutting season.

Final Thoughts

Like a lot of home equipment, the lifespan of a spark plug varies depending on how frequently it is used, how well it is maintained, and sometimes based on just some good old fashioned good luck. 

Lawn mower spark plugs may have as little as a 90-day warranty up to a year. They may need to be replaced after the first season, or may work perfectly for a decade. You just never know.

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