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Why Is My ATV Spark Plug Black? Reading Your Plug’s Color

Why Is My ATV Spark Plug Black? Reading Your Plug’s Color

Spark plugs play a vital role in the performance of your ATV’s engine. During the course of inspecting and performing maintenance on your ATV, you may have noticed that one or all of the spark plugs is black.

If your ATV spark plug is black, it is likely due to an overly rich fuel mixture, an oil leak, or because the engine has been flooded. Overly rich fuel mixtures are almost always a result of carbon buildup, and an oil leak can be caused by bad O-rings, gaskets, pistons, or shotty valve guides.

In this article, we will briefly explain how spark plugs work for anyone who is not sure. Next, we will go over what it means when your spark plug is black, as well as how to rectify the problem at its source. Finally, we will explain how to change a spark plug and how often this should be done.

How Do Spark Plugs Work?

For any combustion engine to function, the inclusion of working spark plugs is necessary.

The purpose of a spark plug is to create a spark and ignite the air/fuel mixture. It does this by creating what is known as a “spark gap” wherein electricity produced by the engine’s ignition system tries to seek a ground pathway, and in doing so, arcs across from the ignition coil to the combustion chamber, creating a spark.

This process creates a miniature explosion, which then powers the engine and allows it to run.

This is, of course, a very layman explanation of an incredibly complex process, but for the purpose of this article, a broad understanding will suffice.

For a more comprehensive explanation of a spark plug’s nuanced functions, see this article from

Here’s a quick animation showing how a spark plug works in action:

When there is a problem with your engine, the spark plug may give us some signs. Sometimes the spark plug may appear dry and white due to overheating, or it may appear corroded and rusted; but for this article, we want to know what has caused your spark plug to go black.

There is no one size fits all blanket explanation for why your spark plug is charred, but based on its look, we can make some very safe assumptions and narrow down the root cause.

Here we will discuss possible causes and what to look for.

Overly Rich Fuel Mixture

One of the things that happen in ATV engines is that the fuel mixture can become overly rich and lead to an excessive buildup of carbon. Carbon buildup is a buildup of crud that occurs in an engine’s air intake and exhaust systems.

Eventually, this buildup of carbon begins to inhibit the airflow of the engine, making it, so the fuel/air mixture in the engine’s combustion system is overly rich with fuel.

Because electricity always seeks the path to the ground of least resistance, it may begin to bypass the spark plug in favor of the carbon deposits, effectively negating the spark plug and reducing the ATV’s engine’s performance as a whole.

The leading cause of carbon buildup is more or less just a lack of upkeep and regular care. Always check and change the engine oil frequently and when necessary.

New and clean oil will begin to dissolve the built-up carbon, allowing it to be flushed through and trapped in the oil filter. Change your oil filter routinely as well.

Engine Flooding

Another common reason that an ATV’s spark plug may have been blackened is that it gets wet by way of engine flooding.

When we speak of a flooded engine, we refer to the process wherein unburned fuel sits in the engine. In turn, this unburned fuel soaks the spark plug and makes it either not start at all or increases the difficulty of doing so.

The most typical way this will occur is when the engine is started up and then immediately turned back off. The engine is not given enough time to burn off the fuel, and it floods the engine.

If necessary, release the carburetor’s drain screw and allow the fuel to be drained. You may be able to avoid draining the carburetor by simply replacing the wet, oily spark plug with a new dry one, however.

Often, ATV users who encounter a flooded engine that will not allow their vehicle to start will remove the spark plug and use a torch and other tools to clean and dry off the spark plug’s exterior.

This may be useful in a pinch, but we strongly recommend that you just replace the plug and save yourself the complications and hassle.

High-quality spark plugs are very inexpensive and last a long time, so acquiring a new one should not be much trouble.

Oil Leak

The third likely cause of a blackened spark plug is an oil leak that has made its way to the wells which house the spark plugs.

There are multiple potential causes of this, and you will need to search around for the root of the problem.

The most probable sources of oil leaking into the spark plug wells are bad washers, faulty gaskets, and worn out deteriorating O-rings.

You can also look for the source of an oil leak is the piston or the valve guides. Once you have pinpointed the source of the oil leak, replace whatever part is causing it.

Also, replace the oily spark plugs as they are now compromised. Recheck soon after repairing to ensure that the problem has been resolved.

How To Change an ATV Spark Plug

Replacing a spark plug is a relatively simple and easy process that can save you time and money instead of having it done by a professional mechanic.

Begin by removing the spark plug wire, which will usually (but not always) appear as a colored wire with a plastic attachment piece, sometimes bent at 90 degrees. The attachment should simply pull right off.

Once you have removed the wire, use a wrench to carefully unscrew and remove the old spark plug. Screw your new spark plug into the hole, ensuring that it is secure but not excessively tight.

How Often Should Your Spark Plugs Be Changed?

In general, it is recommended to replace your ATV’s spark plug following every 100 hours of ride time.However, this will depend on the age of the vehicle, the intensity of the riding, and the climate.The 100-hour recommendation is just a general guideline, and the ATV owner/rider should always be mindful of watching for signs of a bad spark plug.

The most common signs it’s time to replace your spark plug are:

  • Trouble getting the engine to start
  • Noticeable reduction in fuel efficiency
  • Reduced performance
  • Noisy unpleasant sounding idling and startups
  • Feeling that the engine is struggling and sluggish when accelerating

If you require a new spark plug for your ATV, we recommend the X-PRO A7TC Spark Plug (link to Amazon). This spark plug is designed specifically for all-terrain vehicles and comes at a very low price.


In this article, we sought to provide the reader with a comprehensive explanation of why their ATV’s spark plug has been rendered black and what they may do to fix it once the problem has been identified.

There are three primary reasons why this may have happened. Either the engine is leaking oil into the plug well, the fuel mixture is overly rich and lacks airflow, or the engine has been flooded, causing the spark plug to be wetted with oil and made unusable.

Ride safe and have fun!

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