ATV Keeps Fouling Spark Plugs – Common Causes and Symptoms

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When your ATV keeps fouling spark plugs, there is likely an underlying issue that should be addressed before installing new plugs. 

Fouled spark plugs can cause engine misfiring, reduced engine power, poorer fuel economy, rough idling, increased emissions, and in some cases, not being able to start the engine at all.

In this post, we’ll have a look at what could be causing an ATV to foul spark plugs and what you can do to prevent it from happening. While the post is written with ATVs in mind, the same basic principles apply to all gasoline engines.

Page Contents

What is spark plug fouling?

A spark plug is considered fouled when the ceramic insulator, center electrode, and a ground electrode at the firing tip becomes coated with a foreign substance such as fuel, oil, carbon, or coolant.

Several substances can foul a spark plug, but the most common are:

  • Contamination from gasoline
  • Contamination from engine oil
  • Contamination from coolant

Which substance is causing the fouling will usually determine the appearance of the plug.

On a clean and undamaged spark plug, electricity is sent from the coil through the plug’s center electrode, creating a spark when it bridges the gap over to the ground electrode. The ceramic insulator prevents the current from going straight to ground.

The coating on a fouled plug prevents the insulator tip of the plug from doing its job.

Electric current will always choose the path of least resistance, and the deposits covering the ceramic insulator have less resistance than the insulator itself. This causes the current to leach from the tip, past the insulator, and into the metal shell, where it will ground out without creating a spark. 

Without a strong and consistent spark, the plug can no longer ignite the fuel/air mixture properly, which causes misfiring and further fouling.

The old spark plug may tell you why it has fouled; how to read different types of spark plug fouling

It is said that a spark plug is like the canary in a coal mine. If you know how to read its appearance, it will not only tell you when something is wrong with the engine; it will even tell you what may be causing the problem.

  • Remove the spark plug wire boot from the spark plug by pulling it away from the engine.
  • Use a spark plug socket to remove the plug from the engine.
  • Bring the plug to a well-lit area and examine the firing end closely.
  • Take note of whether there is a coating on the insulating tip and electrodes, what color the coating has if it is evenly distributed, and whether it is wet or dry.

Normal condition: Brown or light grey appearance

Normal spark plug
A normal spark plug

Appearance: When a spark plug is operating at an optimal temperature in an engine that’s in good condition, it will get a brown, almost tan, or light grey appearance. This type of coloring on a spark plug is not considered fouling. 

Carbon fouling or dry fouling: Grey to matte black appearance 

Carbon fouled spark plug
A carbon fouled spark plug

Appearance: A carbon fouled plug has a grey to matte black fluffy appearance that is evenly distributed over the center electrode, ground electrode, and the porcelain tip. 

Symptoms: Hard starts, power loss, rough idle, slow acceleration, misfiring, sputtering, poorer fuel economy, increased emissions.

How does it happen? Carbon fouling occurs when carbon residue, for some reason, is adhering to the firing end of the spark plug.

As long as the fuel-to-air ratio is correct and the spark plug runs at the right temperature, the plug is self-cleaning. Meaning, it will burn off any carbon deposits and thereby avoid fouling. 

However, when an issue is causing incomplete combustion, if the firing temperature for some reason drops too low, or if the fuel-to-air ratio becomes too rich, the plug may no longer be able to burn off all the carbon. 

Carbon residue will gradually begin accumulating to the firing end of the plug until it eventually fouls completely. 

Common causes of spark plug carbon fouling or dry fouling

1. The Air-fuel mixture is too rich

A gasoline engine needs a near-perfect balance between fuel and air for optimal combustion. 

Many components come into play to deliver the right air-fuel mixture to the combustion chamber, and any one of them can throw the ratio off-balance.

The air-fuel mix can become too rich either when an issue is causing excess fuel flow or when there is too little air at disposal in proportion with the gasoline. 

Too much fuel can decrease the plug tip temperatures to a point where it is no longer self-cleaning, allowing carbon deposits to accumulate. 

Too much fuel will also cause incomplete combustion, where not all of the fuel gets ignited. The hydrocarbons from the unburnt fuel tend to stick to hot portions of the plug, such as the center electrode, ground electrode, and insulator tip, forming a layer of carbon deposits. 

Carburetor causing excess fuel flow

  • Make sure the carb is adjusted according to factory spec, with the right float setting inside the fuel bowl. Also, check for a leaky float. Make sure the jets are not too large. Note that your jetting needs to be correct for your riding altitude. As altitude increases, the engine gets less air as atmospheric pressure decreases. Consequently, it needs less fuel to maintain the proper air/fuel ratio.
  • A leaky fuel inlet needle valve may cause a rich condition as well.

Fuel-injectors causing a rich condition

If only one plug is affected, the issue may be with the corresponding fuel injector. Make sure one or more of the fuel injectors are not stuck open or leaking. 

Too much fuel pressure on a fuel-injected engine

If all plugs are equally fouled, the fuel pressure may be too high, causing a rich condition. Make sure the fuel pressure regulator is not defective and that the fuel return line is not plugged.

A defective oxygen sensor that reads a false lean condition

If the oxygen sensor is defective and reads a lean condition, it will do what it can to make the mix richer. 

Ask a mechanic to use a scan tool to check your engine’s fuel trim readings. Negative fuel trim readings of minus 8 to minus 10 or more indicate a rich condition. 

The choke is out of adjustment or not closing

The job of the choke valve is to restrict airflow to the carburetor to enrich the fuel-air mixture. This makes it easier to start the engine in low-temperature conditions but may lead to carbon fouling if left on too long. 

Make sure that the choke valve is adjusted to spec and is opening and closing as it should. 

The air filter is dirty, or the air intake is obstructed

If dirt or any object is restricting airflow, there won’t be enough air to ignite all fuel that enters the combustion chamber. 

Clean or replace the air filter if necessary, and make sure there are no obstructions in or in front of the air intake that may restrict airflow. 

There is a vacuum leak

A vacuum leak may send the fuel mix out of balance into a rich condition. Make sure all vacuum hoses are connected, undamaged, and without any kinks that may obstruct airflow. 

High Ambient Air Temperature or Humidity require a leaner fuel mixture

When air temperature and humidity increases, air density will decrease. This can cause the fuel mix to become too rich since the amount of air gets reduced. 

2. The spark plug heat rating is too cold

It’s a common misconception that the heat range or heat rating of a spark plug is related to the spark’s temperature or intensity, but this is false. 

The combustion process heats the spark plug’s firing end, and this heat needs to go somewhere to prevent overheating. Most of the heat is directly transferred to the engine via the cylinder head.

The plug’s heat range tells us at what rate the plug can dissipate its firing head heat to the cylinder head. This property is also known as the plug’s “thermal performance.”

The temperature at the firing end of a spark plug needs to be hot enough so that it burns off deposits and prevents fouling. Simultaneously, it must be low enough to avoid overheating the firing end, causing pre-ignition that may lead to severe engine damage. 

  • The optimal firing end temperature is approximately 500°C (932°F) to 800°C (1472°F). When in this temperature range, carbon deposits will burn off the spark plug tip, making it self-cleaning. 
  • Carbon fouling happens when the temperature drops below 450°C (842°F)
  • Overheating happens when the temperature exceeds 800°C (1472°F)

You achieve optimal thermal performance by using plugs with the correct heat rating for your engine. Please refer to your service manual or ask a dealer to find the right plug. 

Check out eManual Online to get factory workshop service and repair manuals for your ATV.

The factory-recommended plugs are sufficient for most ATVs. If your bike is modified to increase engine performance or run under extreme light or heavy loads, you may consider choosing an alternative heat range. 

3. Continuous low-speed driving, driving short distances, or prolonged idling

The engine must run under some load for a while before the spark plugs reach the self-cleaning temperature of a minimum of 450°C (842°F).

Certain riding applications and behavior may prevent the plugs from heating up properly, eventually leading to spark plug fouling.

  • Continuous low-speed riding, such as putting around the farm, may not add enough load to the engine for the plugs to heat up properly.
  • A lot of short-distance riding, such as daily rides up and down the driveway to get the mail, won’t allow enough time for the plugs to heat up.
  • Prolonged idling. Idling does not generate enough heat to keep the plugs clean. Some idling is ok as the carbon will burn off as soon as you ride, but prolonged idling may produce more residue than the plugs can burn off. 
  • Riding in icy conditions. When air temperatures are low, the engine needs to work harder to spark plugs to reach self-cleaning temperature. 

ATVs with a fuel-air mixture that is too rich or that has a spark plug with a heat range that is too cold are particularly prone to fouling under conditions as listed above. 

To prevent fouling, you should allow the engine to heat up properly regularly by going for a long and active ride (30 mins +). Alternatively, you can add load by attaching a trailer or bring some cargo to make the engine work a bit harder. 

4. The spark plug gap is too large

The spark plug gap is the distance measured between the firing electrode and the grounded electrode. 

Although most spark plugs come pre-gapped from the factory, the gapping may not be right for your particular engine. 

Riding with an incorrect gap may lead to a higher misfire rate, power loss, poorer fuel economy, increased plug wear, and spark plug fouling. 

Use a wire-style gap tool to check if the gap meets manufacturer specifications. Wire-style gap tools are more gentle than the older coin-style tools. 

If the gap needs adjustment, it’s only the grounded electrode that should be moved. Be careful not to pry against the center electrode or insulator, as they may quickly get damaged. 

5. The fuel injectors are clogged

When fuel injectors get clogged by carbon deposits, it may alter the fuel pattern from a mist into a stream. 

The poorer fuel dispersion will cause less efficient or incomplete combustion inside the combustion chamber. 

In turn, poor combustion causes spark plug carbon fouling because of excess unburnt fuel in conjunction with the spark plug not reaching a high enough temp for it to self-clean. 

6. The electrical system is weak or defective, causing a weak spark

A weak spark will lead to incomplete combustion and prevent the spark plug tips from reaching a self-cleaning temperature. 

A weak spark can simulate a rich air-fuel mixture, where not all of the fuel gets ignited. Over time, this will cause the plug to carbon foul.  

The symptoms of a weak spark are most visible at startup. The engine will flood very quickly, and built-up, unburned fuel will cause the occasional backfire, mainly if you apply too much throttle when trying to start the engine. 

  • Components that may lead to a weak spark when not working correctly or defective are the stator, coil, CDI/ECU, or the spark plug wires.
  • Ensuring the ignition timing is set correctly, as overly retarded timing can also cause a weak spark.
  • Ensure the battery terminals are not loose, as this could cause sporadic misfiring, which can cause plugs to foul.

7. Cold weather riding on fuel-injected ATVs

In sub-zero temperatures, you may find that the gasoline cannot form a fine mist and, therefore, won’t mix as well with the air.  Liquid gasoline that enters the combustion chamber will lead to incomplete combustion.

Fuel fouling / wet fouling: Wet and dark/black appearance 

wet fouled fuel fouled spark plug
A Wet fouled healthy spark plug.
Wet fouled carbon fouled spark plug
A wet fouled, carbon fouled spark plug.

Appearance: A fuel fouled plug will be wet with gasoline immediately after removing it from the engine but dries off quickly when out in the open. The gas may be covering a plug that is otherwise in good condition, or it is soaking a carbon fouled plug into a wet, dark finish. 

Symptoms: Hard starts and flooding. Suppose you can start the engine: sputtering and misfiring. A machine with wet foul sputtering will experience poor gas mileage, power loss, and hard cold starts.

How does it happen? 

The fuel that enters the combustion chamber is not being ignited, leaving the spark plug soaked in unburned gasoline. 

  • Fuel pre-delivery or an excessive amount of fuel covers the plug before it can ignite.
  • The fuel allows the current to short circuit to ground instead of jumping between the center electrode and the ground electrode to create a spark as it usually would. 
  • The center electrode on a fuel soaked plug is too cold to reach a high enough temperature to ignite the fuel/air mixture.
  • Wet fouling is more common on carbureted engines than fuel injected engines but could happen with both. 

Common causes of spark plug fuel fouling / wet fouling 

1. The engine is flooded from multiple failed starting attempts

Flooding happens when you repeatedly try to start a carbureted engine without it firing up.

Excessive pumping of the throttle to start the engine will only add to the problem by inducing more fuel to the combustion chamber. 

2. Carburetor related issues that cause wet fouling

The carburetor has an internal defect

The carburetor may have internal problems that allow too much gas into the engine. Typical curl pits to look in are a bad float or a leaky inlet needle valve or a choke valve stuck shut.

The carb is not adjusted properly

A carb is adjusted way off or is fitted with far too large jets; it will pump too much fuel into the combustion chamber. 

Ignition related issues that cause wet fouling

A failed or partly failed ignition will prevent the gas from igniting. The list of possible causes of a failed ignition is long, but here are the most common.

Damaged ignition wires due to low compression

Low engine compression can lead to misfiring. Misfiring can, in turn, damage the plug wire or coil boot. A damaged ignition wire can lead to wet fouling as it won’t provide a spark to ignite the fuel. 

Before replacing the ignition wires, it’s a good idea to trace the root cause of what started this bad cycle in the first place. 

The ignition coil(s) have failed

The spark plug gets its current from the ignition coil. When the coil goes bad, there is no longer a current to ignite the fuel-air mix.

The spark plug is dirty or wet on the outside

Dirt or moisture outside the spark plug may create a conductive path that shorts the ignition current to ground without making a spark.

The spark plug insulator has cracked

Cracks in the spark plug ceramic insulator can be internal, making them a bit hard to identify. When the insulator cracks, the plug may short to ground, failing to provide a spark.

The spark plug is too narrow or closed completely

A too-large spark plug gap may lead to carbon fouling, but a gap that is too narrow or completely closed may leave the plug enabled to provide a strong enough spark to ignite the fuel.

A bad crankshaft position sensor

The crankshaft position sensor generates an electrical signal that is used to control fuel injection and ignition timing. When the sensor fails and no longer sends a signal, the ignition system will no longer provide a spark.

3. Fuel injector related issues that cause wet fouling

When a fuel-injected engine wet foul spark plugs, it may be one or more fuel injectors leaking, there may be excessive fuel pressure, or the cold start injector valve may be stuck open. 

Oil fouling: A blotchy and inconsistent black or shiny black finish

Appearance: An oil fouled plug has a black, oily film covering the plug’s firing end. The coating is not evenly distributed as on a carbon fouled plug but rather blotchy and inconsistent as oil tends to draw against the plug’s colder parts. 

An oil fouled plug may also be covered with white, ashy deposits caused by burnt oil additives. 

Symptoms: Hard starts, misfiring, grey or white exhaust smoke (particularly after initial startup, may clear up as the engine heats up), and loss of oil. 

How does it happen? Oil fouling occurs when engine oil, for some reason, enters the combustion chamber where it disrupts the ignition process by contaminating the fuel-air mixture.

Common causes of spark plug oil fouling

Oil fouled spark plugs may be a sign of internal engine problems. A compression test, as well as a leak down test, may help you identify what is causing your plugs to foul.

The valve guides or valve stem seals are worn or damaged

When the valve guides or valve guide seals wear, they may allow some oil to dribble down the valve stems and into the combustion chamber. 

Simple test: Start the engine in neutral and allow it to idle for about 5 minutes. Then apply a short burst of full throttle. If you notice a puff of smoke, leaky valve guides or valve stem guides is likely your problem.

The piston rings or cylinder walls are worn or damaged

The piston rings’ main job is to create a tight seal between the piston and cylinder walls. This will prevent oil fumes and drops in the crankcase from entering the combustion chamber.

Piston rings may wear, break or crack and the cylinder walls may develop grooves or scoring. All of which may allow some oil to get into the combustion chamber to foul the spark plugs. 

Simple test: Start the engine in neutral and apply about ⅓ throttle for about 5 minutes. If smoke clouds start building up, you know that your issue is likely with the piston rings or cylinder walls. 

The PCV valve, breather or breather valve is clogged

The breather and breather valve allows fresh air to enter the crankcase so that the PCV valve can extract oil fumes. 

If one of them gets clogged, the pressure will build up in the crankcase, forcing oil up past the piston rings or up the overflow tube and into the inlet manifold. This is also known as positive crankcase ventilation. 

Vacuum in the cylinder may cause similar behavior by drawing oil fumes past the piston rings. 

Coolant fouling – chalky appearance

Appearance: Coolant fouling leads to ashy or chalky white deposits on the spark plug’s firing end. The appearance may change a little, depending on what type of coolant is being used. 

In some cases, the plug may appear suspiciously clean due to the steam from the boiling coolant. 

Symptoms: Hard starts, misfiring, coolant level dropping, and white exhaust smoke.

How does it happen?

Coolant fouling happens when coolant/antifreeze, for some reason, enters the combustion chamber where it contaminates the fuel-air mixture.

Note that modern coolants do not cause fouling as quickly due to a reduction of zinc, phosphate, and other additives that can leave deposits on the firing end of the plug. This may lead to running with an undiagnosed coolant leak for an extended period while the plugs slowly become fouled. 

Common causes of coolant fouling spark plugs

Coolant fouled spark plugs is bad news as it usually means expensive engine repairs. 

The head gasket or intake manifold is leaking

Coolant is not supposed to enter the combustion chamber under any circumstance. Components such as the head gasket, intake manifold seal, and others make sure this does not happen. 

A coolant fouled spark plug can be an early sign that any of these are starting to leak.

As a temporary fix, you may try adding some cooling system sealer to the coolant. In the long run, you may be looking at an engine overhaul.

Make sure to pressure test the cooling system to diagnose your potential leak before installing new spark plugs.

Can you fix a fouled spark plug?

After identifying and fixing the underlying issue that was causing your spark plugs to foul in the first place, it may be tempting to save a few dollars by repairing and reusing the old plugs, but can you?

A light carbon fouled spark plug may become clean by going for a long ride to get the engine properly warm or pulling a heavy load, as this will burn off excess carbon.

A heavy carbon fouled spark plug that is causing the engine not to run correctly cannot be repaired and needs to be replaced. 

Some recommend sandblasting the firing end of the plug, but that is considered a short term fix at best. When the porous ceramic insulator becomes properly carbon fouled, there is no way of removing all of the carbon.

Wet fouled spark plugs with no carbon buildup can be reused after allowing it to vent dry. You can leave them in the engine to dry or remove them to speed up the process. 

As an alternative, you may try spraying some starter fluid into the throttle body while cranking the engine, as the plugs will dry if you can get the engine going. 

How to prevent your ATV spark plugs from fouling

After identifying and correcting any underlying mechanical or adjustment issues that may cause spark plug fouling, here are a few other tips on what you can do to minimize the risk of fouling. 

Can you install a plug with a higher heat rating to prevent fouling?

Some people use a hotter range plug to compensate for fuel or oil fouling, but this is generally not recommended, as it will reduce the pre-ignition safety margin.

A much better approach is to identify and correct the underlying issue that is causing the plug to foul in the first place. 

Make sure you don’t leave the choke on too long

Leaving the choke on for too long is a common cause of carbon fouling. 

Avoid idling and short distance riding

If avoiding riding situations that don’t heat the engine appropriately altogether isn’t an option, it is recommended to go for longer rides regularly to allow the plug to heat up enough to where it self cleans.  

Install a high flow air filter

A high flow air filter will cause a slightly leaner condition. In some cases, this may be all that you need to increase fouling resistance by making the plugs run a bit hotter. 

Use a spark plug with an iridium or platinum ground electrode 

Some manufacturers offer spark plugs that are designed to be more resistant than standard plugs. They achieve this by using specific alloys of precious metals in the electrodes, such as Iridium and platinum.

Related questions

Why is my ATV spark plug black?

Blach spark plugs are a sign of carbon-fouling or oil-fouling. This happens when the plug cannot ignite all of the fuel or when oil enters the combustion chamber. 


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Haavard Krislok
Haavard Krislok
I'm an ATV and offroad-enthusiast, an engineer, a farmer, and an avid home-mechanic. I'm also the owner and editor of If you have any questions or suggestions regarding this article, please feel free to contact me.

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