How to Tell if it’s Time to Replace a Lawnmower Spark Plug

A spark plug is one of the most critical parts of your lawn mower because it acts as the ignition source for the engine that powers your lawn mower. When the spark plug is malfunctioned, you’ll notice that it’s more challenging to start the lawn mower, and when you finally start it, it stops almost immediately. But how can you be sure the spark plug is the issue?

How do you know when it is time to replace a lawn mower spark plug? There are several tell-tale signs that you can look for to know when it’s time to replace your lawn mower spark plug. These include:

  • Difficulty starting the lawn mower
  • Increased consumption of fuel
  • Abnormal engine performance
  • Altered physical appearance

Mowing with a lawn mower that has a malfunctioned spark plug can be almost impossible. Therefore, it’s crucial to know which signs to look for to know when it’s time to replace the spark plug. In this article, you’ll learn about the signs of a bad spark plug and find a guide on how to replace it.

When to Replace a Lawn Mower Spark Plug

Before you learn how to troubleshoot your lawn mower spark plug, it’s crucial to understand what a spark plug does in your lawn mower. A spark plug is an electrical device that‘s fitted in the engine’s cylinder head. Its purpose is to spark ignition within the lawn mower engine to create combustion. 

You can tell if the spark plug is in bad condition or damaged if you notice any of the following things happening with your lawn mower.

Difficulty Starting the Lawn Mower

One of the obvious signs that the spark plug may be damaged is difficulty starting the lawn mower. If you have a push mower, you’ll notice that you have to pull the starter rope a couple of times before the engine starts.

A lawn tractor, on the other hand, will fail to start when you turn the key. Remember, the spark plug’s job is to spark ignition within the lawn mower engine. If it’s too weak or damaged, it won’t be able to ignite the air and fuel mixture that’s transported to the cylinder by the carburetor. 

For this reason, the engine will flood as more fuel and air mixture gets into the cylinder. Therefore, if you’ve been having trouble starting your lawn mower, the spark plug may need replacement.

Increased Consumption of Fuel

Do you have to refill gasoline while mowing or in between sessions? Are you doing it more often than usual? This is another sign that the spark plug is damaged. When there’s a weak spark, the result is incomplete combustion, which results in more fuel combustion, and less fuel efficiency.

You may also notice an odor of gasoline while mowing, which is a sign that there’s insufficient combustion of fuel.

Abnormal Engine Performance

Another sign that you have a damaged spark plug is poor engine performance. If the spark plug is malfunctioning, you’ll not only have a difficult time starting the lawn mower, but you will also have trouble keeping it on. 

Sometimes, the engine tends to die off almost immediately after getting it on, and other times, the engine will run for a short period then suddenly go off while you’re mowing. When this happens, you’ll have to restart the engine all over again, and you’ll notice that it gets more and more difficult to start the lawn mower as the engine warms up.

This happens as a result of expansion from the heat. When the spark plug gets too hot, the gap between its center and the firing electrodes increases. As a result, the ignition spark is weakened. The only solution when you’re experiencing this will be to allow the lawn mower to cool down before attempting to start it again. 

Altered Physical Appearance

You can also troubleshoot the spark plug by examining its physical appearance. A normal spark plug should have a center electrode with a flat top. 

  • If you notice that the top is rounded, it’s time to replace the spark plug.
  • Another sign that it’s time to replace the spark plug is cracks and chips on its porcelain sheath. 
  • Pitting on the firing electrode is also not a good sign and calls for replacement.

However, if there’s no physical damage but you notice that the spark plug is wet with gasoline or black with carbon, you may need to clean it with a wire brush and adjust the gap between the electrodes to get it functioning normally again.

How to Replace a Spark Plug in a Lawn Mower 

Once you’re sure that your spark plug needs replacement, the next step is to replace it. Fortunately, you can do it yourself in a few simple steps. 

First, gather these items:

  1. A new spark plug – make sure it is the same size as your previous spark plug.

  1. Spark plug wrench. You can remove the spark plug with other tools too, but a wrench designed for this task specifically makes things a lot easier. If you don’t have one, you can buy one from Amazon – just make sure it’s the right size!
  2. Spark plug gap gauge – It can be hard to tell if a spark plug has the right gap size just by looking at it, and this is where a gap gauge comes into play. It really isn’t a huge expense, and you can buy one from Amazon.

Then, follow these steps:

  1. Locate the spark plug: Depending on the brand of the lawn mower, the spark plug should be located either at the front or on the side. 
  2. Disconnect the spark plug: To do this, all you need to do is to pull off the spark plug wire. However, do it gently to avoid damaging the wire. If you notice that the wires are rusted, you may have to replace them, too. 
  3. Remove the spark plug from the mower: Using the spark plug wrench, loosen the spark plug by turning it in a counter-clockwise direction and remove it. 
  4. Analyze the spark plug gap: The majority of spark plugs come with a preset gap. However, it would help if you made sure the gap is what’s recommended on your lawn mower manual. To do this, you’ll need to use the spark plug gap gauge. If the gap is too big, tap the spark plug on a solid surface slightly to reduce it. If it’s too small, bend it slightly to increase the gap.  
  5. Install the new spark plug: Insert the spark plug in the hole and turn it in a clockwise direction using your hands. It should be easy to screw in without forcing it. If you notice it’s not screwing easily, take it out and try it again. Once you’ve managed to screw the spark plug with your hands, use the wrench to achieve a snug fit. However, do not over tighten the plug because doing so can cause threads or the ceramic plug to break. 
  6. Reconnect the spark plug wire: Connect the spark plug wire onto the new spark plug, and you’re good to go! Now you can use your lawn mower without any troubles.

Wrapping Up

A damaged spark plug can make mowing your lawn a real nightmare. You will not only have trouble starting the lawn mower but also keeping it on. Not to mention, you’ll have to spend a lot of money on fuel due to its increased fuel consumption. You don’t have to deal with this.

With these easy tips listed above, you can tell when to replace a lawn mower spark plug.

Don’t forget to find out the right spark plug to get for your lawn mower. You can get this information on the user manual or contact your lawn mower manufacturer for the precise details.

This way, you can rest assured the lawn mower will function without any difficulties after you’ve replaced the spark plug.


Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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