Why Does My Lawnmower Spark Plug Keep Fouling?

The engine of a lawnmower is only as powerful as the components that keep it running. If you have a spark plug that fouls after only a few uses, you have an annoying problem that keeps you from getting your job done. Luckily, there are specific reasons why a lawn mower spark plug may become fouled and fixes for each of these problems that you can do yourself.

The most common reasons why a lawn mower spark plug fouls are listed below: 

  • Air Intake blockage
  • Clogged fuel injector
  • Problems with the crankcase breather (PCV)
  • Oil fouling from piston rings

Even though spark plugs are relatively inexpensive to purchase, they are annoying to keep replacing on your lawnmower. Read on to find out more about the most common reason why a lawn mower spark plug becomes fouled and how to fix them yourself.

What Causes a Lawnmower Spark Plug to Keep Fouling?

A spark plug that fouls constantly could mean that there are more severe issues with the engine of your lawnmower. You must diagnose the problem and fix it before installing any new spark plugs.

Below are the most common reasons that can cause this, as well as their solutions.

Cause: Air Intake Blockage

If the spark plug is fouling because of a carbon build-up, the reason is likely due to the combustion system of the lawn mower’s engine. The most common reason for the fuel combustion system to cause carbon build-up on the spark plug is a failed mixture of fuel and oxygen. (Source: Auto Success Online)

All internal combustion engines are air-breathing and deal with an atmospheric pressure of air being forced into the mixture. Since this is the case, the incorrect mixture of fuel and oxygen in your lawnmower could be because of a blockage in the air intake, creating a fuel-rich mixture that could cause smoke and increased carbon build-up on spark plugs.


The air intake on the engine of your lawnmower is the carburetor. It can be checked by looking at the air filter and the connections. When the air intake is blocked, you should notice considerably less power from the engine. When the blockage is removed, you should notice a louder, more efficient sounding engine with more power. 

To check for and fix a clogged air intake on your lawnmower, do the following:

  • On top of the carburetor is the air filter intake, which can become clogged with grass clippings and dust. To fix this, either brush away the debris or use a screwdriver to remove the casing that the air intake screen filter is held in and remove as much of the blockage as you can.
  • The connection of the carburetor is at the throttle and choke plates. These connections can become sticky and blocked and may need to be cleaned out.
  • The screws that hold the carburetor connections can also be jostled free by the engine’s vibrations and may need to be tightened. (Source: Gold Eagle)

Cause: Clogged Fuel Injector

Another reason that spark plugs foul is that the fuel injector is clogged. This creates an oxygen-rich mixture that will not burn hot enough and will leave fuel and carbon residue on the spark plugs and other essential components in the lawnmower engine. 

Fuel injectors are more specialized components of the engine than air intake valves. They may require opening up the engine or using solutions that dissolve clogs that can’t be seen on the surface.


Fuel injection levels affect the fuel and oxygen mixture. Anything out of the normal range can create a carbon build-up on the spark plugs. The steps for checking for and fixing a clogged fuel injector on your lawnmower are:

  • Turn the fuel valve to the ‘off’ position to remove the fuel line and inspect it for clogs. If there are any clogs, clean the fuel line out.
  • Turn the fuel valve to the ‘off’ position or clamp the line to the fuel pump and remove the fuel pump. Look for cracks in the housing or other damage and blockage. Remove clogs or replace the pump if it is damaged. 

Cause: Problems with the Crankcase Breather (PCV)

The PCV is the valve that allows exhaust from the fuel out of the combustion chamber and into a valve that releases it in the crankcase. Not all lawnmowers have this PCV tubing, but for those that do, if it becomes loose or broken, it can cause oil leakage into the combustion chamber, causing oil fouling of the spark plugs.


In most lawnmowers, the crankcase breather is located just above the valves. If it is dirty with oil, there is a leakage or other problem that needs to be addressed. 

To determine if your PCV is the problem, follow these steps:

  • Remove the muffler to get access to the crankcase breather tubing. If the tubing is dirty with oil, there is a leak somewhere that needs to be fixed before replacing the spark plugs.
  • Using a screwdriver, remove the breather from the engine and see if the holes are open or not. If the holes are open, test the diameter to see if the gap is more extensive than .045 inches.  If they are stuck shut, or the gap is too broad, you will need to replace the breather since it cannot be fixed. 

Cause: Oil Fouling Piston Rings

Oil fouling will look like a translucent coating over the spark plug. The oil that leaks into the combustion chamber can also create build-up and fouling on the porcelain of the spark plug as well as the precious metal tip.

Seals keep the engine oil separate from the combustion chamber. Over time, these seals may become warped, out of place, or even go missing. When this happens, oil can leak into the combustion chamber and all over the spark plugs, causing significant damage and fouling. 


Oil found on the spark plug can also come from the piston rings being broken or damaged. The steps for checking for and fixing problems with the piston rings and oil fouling on your lawnmower are detailed below: 

  • Remove the spark plug and wiring away from the opening. 
  • Crank the engine to clear liquid from the cylinder. 
  • Reinstall the spark plug and then loosen the lug nut at the bottom of the carburetor until you can see clear gas flow through it then retighten the nut. This should remove the oil from the carburetor.
  • Check your oil for levels. You may need to add some more since there has been a leak.
  • Start the engine and let it run until the smoke clears. Then, reinstall the air filter. (Source: Repair FAQ)

If All Else Fails, Consider Getting a Repair Manual

If you really aren’t sure what the problem is, you can try reading a repair manual for your mower. Repair manuals are often released by the manufacturer itself to guide repair professionals in troubleshooting and fixing their equipment. As you can imagine, they are pretty helpful when you aren’t sure what the problem is or how to perform a specific task. 

Also, they are more detailed than the info I can provide here because they are model specific. 

So if you want to fix your equipment yourself, getting one of these can be a good option especially if you intend to use your tool for a long time as these guides can help you troubleshoot the most common malfunctions, not just the one you are dealing with right now. 

A good site to look for these is emanualonline.com – they have over 2,5 million repair manuals available, so there is a good chance they have one for yours too. Just make sure you get the one for your specific model!

In Conclusion

If your lawnmower spark plug is fouled, it is a sign that there is a combustion chamber or oil leakage problem that needs to be checked and fixed.

Using the steps in this guide, you should be able to address the most common problems that cause a spark plug to keep fouling. 

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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