Why Is My Lawn Mower Spark Plug Wet? (And How Can I Fix It?)

Spark plugs on wooden top

So you are here because you are asking why is my lawn mower spark plug wet?

It is a valid concern, as these small pieces of equipment are what essentially make your mower’s engine produce power by supplying the spark to ignite the mix of air and fuel.

If the spark plug is wet then it makes doing this A LOT harder!

Let’s take a look at what can cause a wet spark plug and how to fix.

Why Is My Lawn Mower Spark Plug Wet?

If your lawn mower spark plug is wet, then it will nearly always be wet with oil or without petrol. If it is wet with petrol it is often because either you have flooded the engine when trying to start it, or too much fuel is being allowed in the carburetor. If it is wet with oil usually either the tank has been overfilled, oil has leaked onto the plugs when it has been tilted or the piston seals are worn.

What Are the Symptoms of Wet Spark Plugs?

Let’s ignore the most obvious symptom of a wet spark plug for now (it feels wet…).

Beyond physically looking at, and touching the spark plug, there are a few telltale signs that there is an issue with your lawn mower’s spark plug:

  • Your Mower Is Difficult to Start
    This is the most common symptom of a faulty spark plug. It isn’t properly producing the spark to ignite the air/fuel mixture and start the engine. It might take several pulls on the starter rope or turns of the engine to get your mower going.
  • Your Mower Isn’t Running Properly
    We can all immediately tell when any piece of mechanical machinery isn’t performing to its optimum levels. It might keep dying out, spluttering or faltering. This could be because a faulty spark plug is causing the engine to misfire.
  • Your Mower is Consuming a Lot of Fuel
    When a spark plug is faulty the fuel is not being burnt properly, thus the mower is less efficient and fuel consumption is higher. Often the fuel will emit an unusual odor as well.

If you notice your spark plug is wet, then it stands to reason that it will have been made wet by one of two substances…

Why Is My Spark Plug Wet With Petrol?

Petrol pumps

If you are finding the spark plug on your lawn mower is wet with petrol, there are two possible explanations.

Reason #1: You Have Flooded the Engine by Excessive Use of the Choke

Overly using the choke when starting your mower will flood the engine with too much fuel and not enough air. 

The plug will become fuel fouled as a result and not spark.

How to Fix This

If this is the problem you can either:

  • Remove the spark plug. Pull the starter cord a few times to get rid of any fuel from the cylinder. Reinstall the spark plug when it has dried out.
  • Remove the air filter and put some starter fluid into the mouth of the carburetor. The starter fluid should get the engine going, and then combustion process should dry the spark plug.

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Reason #2: There Could Be a Problem With the Carburetor

If the above doesn’t appear to be the issue, then there could be a problem with the carburetor.

Often the cause here is linked to the carburetor float needle and seat. They might be worn out, the fuel float could be leaky, or something (such as dirt) could be holding the needle off its seat.

What then happens is too much fuel enters the float chamber and is passed into the carburetor.

Air pressure forces excessive fuel into the engine flooding the spark plug.

How to Fix This

Check the air filter.

If it is severely clogged, then sufficient amounts of air cannot enter the carburetor, which is then flooded by too much fuel.

Also refer back to the owner’s manual. Often failing to follow the correct procedure when starting the mower can be to blame.

#2: Why Is My Spark Plug Wet With Oil?

The other substance your spark plug is likely to be wet with is oil.

Usually, this will be due to one of three reasons.

Reason #1: The Tank Has Been Overfilled With Oil

If too much oil is poured into the tank, the crankcase can overflow, and the oil will leak onto the spark plug.

How to Fix This

Simply remove the oil cap and drain out some of the oil. The best way to do this is to use something like a turkey baster and extract the excess oil this way.

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Reason #2: The Mower Has Been Tipped on its Side

Have you recently tilted your mower sideways to clean under the deck or sharpen the blades maybe?

If so, when you did this oil may have pooled up under the piston and either seeped past the piston rings or entered the carburetor via the air intake when the mower was turned on. From there this can cause the spark plug electrodes to become oily.

How to Fix This

You can remove the spark plug and allow it to dry, or clean it with carburetor or brake cleaner.

When you reinstall the plug the engine should dry out any remaining oil on the spark plug.

Reason #3: The Piston Ring Seals Are Worn

The seals on piston rings prevent oil from leaking into the combustion chamber.

If they become too worn, oil can spray onto the spark plug.

How to Fix This

Piston ring wear occurs when there has been excessive use of the engine. It isn’t an easy part to replace and will require an engine rebuild.

If you are mechanically minded you can try and replace the piston rings, but often it is easier and sometimes cheaper to buy a new mower.

Or if the mower is still running fine you can continue to use it.

Wet Spark Plugs and Two-Cycle Engines

Although most mowers these days are four-cycle, there are still some two-cycle mowers knocking about.

If you have a two-cycle engine, then a wet spark plug isn’t actually too much to worry about, in fact it is somewhat normal.

With two-stroke engines, there is always a bit of oil residue in the cylinder, and often this will make its way onto the spark plug.

The mix of fuel and oil that two-stroke engines burn is meant to leave a bit of oil behind as a lubricant.

Lots of excess oil in the cylinder of a two-stroke mower might indicate that you have added too much oil to your fuel.

How to Fix Wet Spark Plugs: An Overview

We’ve already touched on various scenarios that could cause your spark plug to be wet, and the best way to deal with each situation.

The first thing to do is establish whether your spark plug is wet with oil or petrol.

Then you need to work out whether the cause is down to:

  1. Human Error: ie you flooded the engine, overfilled the oil or caused oil to leak onto the spark plug when you tilted the mower, or;
  2. Mechanical Error: ie there is a problem with a part of the lawn mower, for instance the carburetor or piston rings.

If it is down to human error, then that can be alleviated going forward. The spark plug can be dried or replaced and everything should be ok.

But if there is a problem with the piston rings or the carburetor or any other part of the mower, cleaning or replacing the spark plug will be pointless as, before long, it will be wet with oil or petrol again.

In these cases follow the advice listed above or take your mower to a mechanic, who should be able to diagnose and fix the issue for you.

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Final Thoughts

Why is my Lawn Mower Spark Plug Wet infographic

If you are having trouble starting or keeping your lawn mower running then there could be an issue with the spark plugs.

If they are wet then they won’t ignite properly.

Whilst sometimes water in the tank or injector leaks can be the cause of a wet lawn mower spark plug, this is very rare.

It will nearly always be wet with oil or with petrol.

Most of the time the problem can be fixed without a huge amount of expense. The exception to this rule is if the cause is worn piston rings.

But even then most lawn mower engines are built to last at least five years, so if your piston rings are worn you should hopefully have got plenty of use out of them by this point.

Good luck!

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