Why is ATV Spark Plug Black? Easy fix!


A fouled or black spark plug is a real pain, symptoms vary by how badly the plug is fouled. But you can be sure the engine is struggling to perform and ATV is frustrating to ride. A black or fouled ATV spark plug is a very common ATV issue and now the good news, it’s usually a five-minute fix. I’m a mechanic and you are in the correct place, a few minutes from now she’ll be revving clean.

A fouled or black ATV spark plug is a sign the engine is running fuel-rich and that’s commonly caused by:

  • Blocked air filter
  • Incorrect choke use or fault
  • Ignition system fault
  • Fuel system fault
  • Mechanical fault

In this post you’ll learn the most common causes of a black or fouled ATV spark plug and what you can do to fix them right now.

ATV in woods

Black ATV Spark Plug Common Causes

A black spark plug means your engine is running rich. Rich means your engine is getting too much gas in relation to the amount of air (oxygen) it’s receiving. Your ATV engine likes to run with 14.7 parts air (oxygen) to one part gas, it’s called the AFR or Air Fuel Ratio.

Your ATV’s carburetor is responsible for mixing and maintaining the AFR. When either the airflow to the engine or the gas supply is out of spec, the engine won’t operate at its optimum or maybe won’t run at all.

AFR conditions may be described as:

Rich – An engine that receives too much gas or not enough air is said to be running rich.

Lean – Conversely, an engine that gets too much air or not enough gas is said to run lean.

Both conditions are bad, but since your ATV plug is black I suspect your engine is running rich. Rich as you know means your engine is either getting too much gas or not enough air.

A common cause of a rich running engine and consequently a black plug is a dirty air filter. And so that’s what we cover first in the list of common rich running causes below.

Blocked Air Filter

A blocked air filter as you know restricts airflow to the engine and that may cause a rich condition. An air filter is easy to check since most manufacturers employ toolless access. And since it’s a common cause and it’s so easy to check we’ll do that first.

The fix – Simple enough, right, clean or replace the filter. Foam filters are washable in detergent but sure to allow them to dry completely and if your filter is a performance filter it will need to be oiled. And yes you will need to use a special air filter, filter oil is sticky it’s formulated to catch and trap fine dust particles that would otherwise sandblast the interior of your motor.

Choke

Incorrect Choke Use – Riding with the choke partially “On” will create a rich condition and that may cause the plug to become black. I understand some ATVs run a little better with the choke “On” and that’s a sign of a possible carburetor blockage or the carburetor requires adjustment.

Although the engine runs better with the choke “On” it does run the risk of washing the protective oil coat from the cylinder walls and diluting the engine oil.

Choke Stuck “On” – A choke sticking in the “On” or partially “On” position will create a rich condition and as mentioned above, wash the cylinder and contaminate the engine oil. This is a real risk of seizing the engine, and a seized engine as you know is beyond economic repair.

To check choke operation you’ll need to remove the airbox pipework so as to gain access to the choke plate. Operating the choke and observing its “Off” open position. Some choke systems are operated by cable and may require adjustment.

Ignition System Fault

There are many components in the ignition system all of which could in theory cause a rich condition by intermittingly breaking down. However the most likely component in the ignition system to cause a black spark plug is the plug itself, and that’s what we’ll look at next.

Incorrect Plug Gap – A spark plug is a tough component and modern plugs tend to last a ton longer than previous generations. That said they do require a little maintenance and replacing. Most ATV engine makers recommend a plug replacement every two years and a plug clean and re-gap every three months.

Gapping the plug refers to the gap between the positive and negative electrodes. An excessive plug gap causes misfiring and a black spark plug, in addition, it places excessive stress on the ignition system and promotes excessive amp draw.

The fix – Check and gapping the spark plug takes just a few minutes once the plug is removed. Alternatively, replace the spark plug, they come already gapped.

To gap a plug, you’ll need a plug socket and ratchet, a feeler gauge, and pliers. Check your engine maker’s recommended gap spec, (usually about 0.7-0.8mm (0.028-0.031)) select the correct feeler gauge blade and place it in the plug gap. To close the gap, use the pliers to tap on the negative electrode.

To open the gap, grip the negative electrode with the pliers and open it slightly.

Re-adjust as necessary.

Wrong Spark Plug – This one is self-explanatory, well almost. Plugs are graded by heat range, meaning they have an optimum temperature of operation. A plug is designed to get hot enough to burn off plug contaminates that naturally form and contaminate the plug.

A plug that’s too hot will pre-ignite the fuel mixture and that’s damaging to the engine.

Conversely, a plug that’s too cold won’t ever get hot enough to burn off those carbon deposits and results in a black plug.

As a rough guide a brown plug insulator (discolored insulator around central electrode) suggests the plug is correct and all is well inside the combustion chamber.

An unmarked or dark colored insulator suggests there’s a problem.

The fix – Check the plug spec in your driver’s manual or check the spec online.

Ignition Electrical System Fault – An intermittent electrical system fault is a pain in the jacksie and is a contender for a rich condition. To be fair an intermittent electrical issue should make itself known, misfiring and loss of power and or stalling.

Fitting a Spark Plug

When fitting a spark plug it is important to tighten it correctly. Both too tight and too loose is bad. As a guide, tighten a used washered spark plug 1/8 to 1/4 turn after the plug seats. Tighten a new washered plug 1/2 turn after the plug seats.

Fuel System Fault

The system includes the following components – the gas itself, gas tank, gas filter, fuel pump (if fitted), and carburetor.

Bad Gas – The gas itself is a common cause of misfiring and a black plug. Using unapproved, low-quality gas, contaminated gas, or just stale gas will cause the engine to misfire, and that as you know causes a black plug. Draining the fuel system removing the gas and adding a fuel stabilizer will prevent the gas from going stale in the future.

Most are happy with 87 octane or 10% ethanol blend but when using ethanol use a gas stabilizer mix to help guard against moisture.

Incorrect Carburetor Mix – The carburetor employs an adjustment screw where the air to fuel ratio can be adjusted. It isn’t uncommon for carb screws to move out of adjustment. Adjusting isn’t difficult, but you will need to check the location of your mix screw(s).

Mechanical Fault

A mechanical fault has a place on this list also. Excessive oil inside the cylinder will result in a contaminated spark plug, however, this is usually accompanied by blue smoke.

Worn Oil Rings – Worn oil rings will allow oil to sneak past the rings and that will contaminate the spark plug.

Worn Valve Stem Oil Seals – Hard, worn, or cracked oil seals will allow oil into the cylinder and you guessed it contaminate the spark plug.

You may find the following posts helpful:

Air cooled ATV white smoke

Can I ride with choke on?

How to start an ATV in cold weather

ATV won’t start after washing

ATV won’t pull start

John Cunningham

John Cunningham is a certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty-five years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance & repair to troubleshooting.

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