How To Tell If A Lawn Mower Spark Plug Is Bad – And How To Fix It!

bad lawn mower spark plug

If you’re having trouble getting your lawnmower to start, it could be a bad spark plug that’s the problem. No matter if it’s a zero-turn mower, a riding mower, or a normal gas-powered lawn mower, they all require a well-functioning spark plug to run at peak efficiency. Read on to learn more about how to tell if a lawn mower spark plug is bad and what you can do to replace it.

What Makes a Spark Plug Go Bad?

There are plenty of reasons a spark plug might go bad sooner than expected. Overheating is a common issue in older lawnmowers, which can cause problems with the plug’s circuitry. Excessive carbon buildup and oil residue from surrounding dirt and debris can also lead to a failing spark plug.

Spark plugs may also suffer damage from regular use, especially for homeowners with a particularly challenging yard. Any contact that causes scratches, cracking, or chipping can cause a spark plug to fail.

How Long Does a Lawn Mower Spark Plug Last?

Even with proper care and maintenance, a lawn mower spark plug will inevitably go bad over time. All spark plugs need regular replacing throughout the course of a lawn mower’s life, depending on how often you cut your grass.

In general, you can expect your spark plug to last for around 25 hours of use. With thick or tough grass, however, you may find lawn mower parts wearing out more quickly.

Many homeowners simply replace their spark plugs once per season to keep their mower running at peak capacity.

Signs Your Spark Plug Needs Replacing

If your lawnmower is experiencing performance issues, it could indicate that your spark plug is bad. Here are some of the most common signs that you need to replace your spark plug, whether you have a push mower or a riding unit.

The Engine Turns But Won’t Start

If you hear your mower engine turning without starting, there’s a good chance it’s because your spark plug is worn or dirty. Thanks to all the grime coating the electrodes, it takes more effort to spark the combustion chamber and start the engine.

If you have a rewind start, you may notice that you have to tug especially hard to get your engine to start. Again, this is because your plug doesn’t spark as easily to start the combustion process.

gas usage up? could be a bad spark plug

Your Mower Guzzles More Gasoline

A mower engine with a failing spark plug will increase your fuel consumption, so you may notice yourself refilling your lawnmower tank more often than usual. A damaged or dirty plug will have a weak spark, allowing unused fuel to pass through the combustion chamber. Over time this can waste gallons of fuel, increasing both your energy bill and your carbon footprint.

The Engine Runs Poorly or Loses Power

You may notice performance issues during lawnmower operation with a bad spark plug. It may lose power or even die while mowing your lawn. A worn or damaged plug can’t spark consistently, disrupting the supply of energy to the engine.

What Does a Bad Spark Plug Look Like, Anyway?

The only way to be absolutely sure that a spark plug has gone bad is to open the hood of your lawnmower and take a look at the engine. You can spot a bad spark plug based on appearance alone, but you have to know what you’re looking for first.

To inspect the spark plug, you’ll first have to remove it from your mower. To do this, you’ll need a socket wrench or a spark plug wrench in the correct size. Connect the wrench to the base of the spark plug, loosen, and then unthread completely to remove it from your engine setup. It’s a good idea to clean it thoroughly to get a better look at the condition of your spark plug.

A new, functioning spark plug should have an intact plastic insulator and shiny, clean threads around the electrode. If you notice any dark oils or carbon deposits caked into the unit, it can indicate that the plug is bad. Burning or blistering along the electrode is also a clear sign of a spark plug that needs replacing.

A stained or cracked insulator may also point to a damaged spark plug. It could indicate that the plug is too tight or that the plastic is getting old. If you leave the issue unaddressed, it can lead to an engine misfire later on down the line.

When in doubt, you can test your spark plug by attaching it to a multimeter. A working spark plug should register somewhere in the range of 10K to 20K ohms. Otherwise, you may notice impaired performance in your lawnmower.

How to Clean and Reinstall a Spark Plug

The best way to prolong the lifespan of your lawn mower’s spark plug is through proper maintenance. It’s important to remove, clean, and reinstall your spark plug regularly. Doing so will prevent carbon buildup, oil deposits, and other issues.

used spark plugs - see the carbon?

To clean your spark plug, you can use car chemicals that you likely have on hand in your garage. If not, appropriate products are easy to find at any nearby auto shop. Brake cleaner works best for maintaining spark plugs, but starter fluid can also remove most unwanted buildup.

Allow dirty spark plugs to soak in brake cleaner or starter fluid for around ten to fifteen minutes. Once you’ve loosened all debris, you can wipe off excess oil and residue using a clean rag or microfiber cloth. For stubborn grime, use a soft-wire brush that won’t damage the metal underneath.

As long as you notice no obvious signs of wear on the threads, electrodes, or insulator, you can reinstall your spark plug and continue using your lawnmower as usual.

How to Replace a Spark Plug

If you notice any signs of deterioration on your spark plug, you should replace it with a new one. Before installing, however, it’s important to gap your plug to avoid misfiring.

Once you’ve properly gapped your spark plug, you can thread it into your lawnmower using a socket wrench. Make sure not to overtighten, or you may end up damaging the insulator. Finally, attach the spark plug lead.

What Does a Gap for a Spark Plug Mean?

Before installing your spark plug, you need to ensure that it has a proper gap between the center and side electrodes. This space is essential to creating the spark that ignites the fuel and starts combustion within your mower’s engine.

Most small engines, such as a lawnmower engine, require around a 0.030” gap between spark plug electrodes. This number can vary from model to model, so always check the specifications for your specific lawnmower.

You can use a spark plug gauge to measure the gap distance and ensure it meets your mower’s requirements. To adjust the gap size, gently bend the outer electrode either closer to or farther away from the inner electrode.

Where Can I Buy Replacement Lawn Mower Spark Plugs?

Unfortunately, spark plugs aren’t universal. You need to find the right size to fit your mower. You can find a range of common spark plug sizes available at most DIY and home goods stores. You may also be able to order spark plugs directly from your mower’s manufacturer for a perfect fit.

The best lawn mowers will include all of this information in your user manual.

In Conclusion

If your lawnmower is failing to perform, it could be the spark plug that’s at fault. Fortunately, it’s easy to spot and replace a faulty spark plug to get your mower back into peak condition. With a clean and functioning spark plug, you can make the most out of your lawnmower’s engine performance. 

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