How to test a spark plug on a lawnmower

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Spark plugs need to be replaced if you’re experiencing troubles when starting the lawn mower or if the machine is constantly producing a sluggish performance.

But before changing the spark plugs, you want to be sure that they’re the actual guilty party.

This guide teaches you how to test a spark plug on a lawnmower when you suspect that you have bad spark plugs.

Before we go further, perhaps it’s necessary I tell you about what these little accessories do to ensure we are on the same page.

What does a spark plug do?

Spark plugs are crucial for your lawn mower’s starting.

They make powerful electrical sparks – they have electrodes- that help the mower ignite and the engine may fail to start if the spark plugs are in a bad state.

And if you’re lucky to crank it to life, the gas won’t burn efficiently leading to an extremely sluggish performance.

You’ll be facing loads of other issues if your spark plugs are damaged including increased fuel consumption, nasty engine misfires, oil dilution, and more.

For this reason, you should learn how to test spark plugs and read the signs that hint that it’s time to clean the part (sometimes it’s just dirt making your life hard) or replace it.

How to test a spark plug on a lawnmower

The following steps will teach you how to test a spark plug on a lawnmower:

Tools you need:

  • Spark plug removal tool such as a spark plug socket.
  • Pliers (optional).
  • Penetrating lubricant (optional).
  • Spark plug tester.
  • A wire brush (for cleaning)

Preparatory measures:

  • Drive the mower to a level ground.
  • Wait for the engine to cool.

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Stage 1- Remove the spark plug

Your first goal is to remove the plug and we take you through the procedure below.

How to test a spark plug on a lawnmower

Tip: These steps are general to most mowers and you may want to check what your owner’s manual says even as you follow them.

Step 1: Find the spark plug.

On most lawnmower engines, the plug is located at either the front or on the side of the mower.

Again refer to your operator’s manual if you can’t figure out the correct location.

Step 2: Remove the connected spark plug wire.

Pull the usually visible spark plug wire off the head (start by removing the cover to the engine).

Of course, there’s no way you’re going to get the spark plug out with the wire still attached.

Tip:  Sometimes the wire could be stuck and you might have to move up it and down (or from one side to the other) a bit to make it come off.

For the more stubborn wires, your pliers will suffice but you have to be careful not to harm it.

Step 3: Clear the spark plug surroundings

Now use the wire (any other suitable cleaner including a compressed air can) to clear any dirt and junk in the area surrounding the spark plug. 

You don’t want this to fall into the combustion chamber because it can mess the engine.

Step 4: Pull the spark plug out

Next, unscrew the spark plug from the mower engine.

A spark plug wrench should work fine- fit it over the plug then turn counterclockwise until you get the plug out.

Tip:  In case the plug feels seized, try to apply some WD40, CRC, or your favorite penetrating lubricant and see if it will free it.

Stage 2- Test the spark plug

You will use the spark plug tester in the first test.

As the name implies, the tool tests whether the spark plug has an electrical current powerful enough to trigger a spark.

Next we look at how to use the tool:

How to test a spark plug on a lawnmower with a spark plug tester

  • Insert the spark tester tool into the plug wire (in the mower).
  • Ground the tool. You can effectively do this by placing the tester directly against the motor’s housing (metallic).
  • Return the spark plug to its place and set the plug wire right against the motor.
  • Wrap a rope (not that long) around the mower’s motor flywheel (It’s a round disk-shaped component above the motor).
  • Pull this rope to rotate the flywheel while watching for a spark.
  • The test ends with you noting a spark (meaning its working fine) or having no spark (indicating that the spark plug is damaged).

Note: An inline spark plug tester like this is pretty easy to use and will brightly light up if sparks are present (it mirrors the spark).

Do a Visual check

The simplest test is actually visual.

Take a good look at the plug for signs of erosion, corrosion, being burned, and cracking (in the porcelain sheath).

These communicates that the part won’t work properly and must be changed.

The same applies if you observe some difficult deposits upon inspecting the spark plug.

Tip: If everything seems right, replace the plug. It’s important not to over-tighten it when fixing as that can stress the metal shell destroying the thread. It can make the insulator to fracture too.

Test with a working spark plug

Another very straightforward way of doing it is by installing a plug that you’re certain is in good shape then try to fire it.

Be sure to measure the gap accurately when mounting it as it is unlikely to spark if not set correctly (confirm the recommended gap measurement for your model from the owner’s manual).

You can be sure that the old plug is faulty if the mower starts without problems after putting the working spark plug and it’s time to use your spare spark plug.

On the other hand, you will have known that the complications lies elsewhere if the mower won’t turn over even with a functional spark plug in.

Like other small engines, a lawn mower’s engine is quite simplified and you should have minimal problems testing and replace the spark plug.

What is important is to recognize the signs and take action before the worn out spark plugs causes more issues.

How to change a spark plug in a lawn mower.

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