How To Tell If Your Chainsaw Spark Plug Is Bad – Easy Method

The spark plug of a chainsaw is critical to the operation of a chainsaw. If your chainsaw is running erratically, hard to start, or won’t start at all you may have a fouled spark plug. How can you tell if your chainsaw’s spark plug is fouled?

You can tell a chainsaw spark plug is fouled by inspecting the central electrode of the spark plug. The spark plug is fouled if the central electrode is covered in a sooty black, substance known as carbon. Carbon is a conductive material and can cause the spark plug to misfire.

In some cases, the spark plug’s central electrode can be cleaned with a wire brush to restore functionality to the spark plug. However, in most cases, a chainsaw’s fouled spark plug will need to be replaced for the chainsaw to operate correctly.

What Causes A Chainsaw Spark Plug To Go Bad?

When inspecting your chainsaw’s spark plug and you notice the central electrode is covered in a sooty substance, known as carbon, your chainsaw’s spark plug is fouled.

You may be able to clean your spark plug’s central electrode with a wire brush to regain the spark plug’s function. However, you will need to replace your chainsaw’s spark plug to regain optimum chainsaw performance in most cases.

What causes a chainsaw’s spark plug to foul or go bad?

Chainsaw spark plugs foul for 3 primary reasons which are as follows:

  1. Chainsaw Fuel Oil Ratio Too Rich
  2. Dirty Air Filter of Chainsaw
  3. Excessive Idling of Chainsaw

1. Chainsaw Fuel Ratio Too Rich

Gasoline chainsaws run on fuel that consists of gasoline and oil mixed in a specific ratio.

If the fuel mix contains more oil than the chainsaw requires, the mixture is considered rich. If fuel is overly rich the chainsaw will not be able to properly combust the fuel.

The fuel leftover residue and deposits accumulate on the spark plug central and ground electrodes causing the spark plug to misfire or not fire at all.

Run only properly mixed fuel in your chainsaw. Fuel that is too rich fuel equals fouled spark plugs.

2. Dirty Air Filter of Chainsaw

Chainsaws need air, fuel, and spark for engine combustion. If any of these three components are not within spec, the chainsaw engine will not run properly.

The air filter’s function is to filter the dirt from the air as it is pulled into the chainsaw’s air intake. If the air filter becomes partially blocked with dirt, the engine will not receive the correct amount of air to mix with the fuel and the chainsaw will run rich.

If a chainsaw has too rich of an air-fuel ratio the spark plug will foul.

3. Excessive Idling of Chainsaw

Chainsaws are designed to be run at high RPMs and not idle for extended periods.

When a chainsaw idles for too long the spark plug’s central and ground electrode can become coated in carbon, fouling the spark plug.

An idling chainsaw engine does not operate at peak efficiency and does not get hot enough to burn 100% of the fuel entering the combustion chamber. The unburned fuel leads to engine deposits and build-up.

Letting your chainsaw idle for short periods of time is fine but not for extended time periods.

What Are The Symptoms Of A Bad Spark Plug In A Chainsaw?

When a spark plug is fouled or is in the process of fouling a chainsaw will exhibit some specific behaviors.

The symptoms of fouled or partially fouled spark plugs include:

  • Engine Will Not Start
  • Hard To Start Engine
  • Engine Throttle Hesitation
  • Rough Idling Engine

How Long Should A Chainsaw Spark Plug Last?

Chainsaw spark plugs can last for long periods of time depending on certain factors like fuel quality, chainsaw maintenance, and operator habits.

According to Stihl, a spark plug should be changed after 100 hours of operation regardless of condition. In instances of fouled spark plugs, they should be changed at the time of failure.

What Color Should A Chainsaw Spark Plug Be?

The color of a spark plug’s central and ground electrodes can tell you a lot about the health of your spark plug.

Here is a list of spark plug colors and their meanings.

  • Tan and Gray means spark plug is good and should function proplery
  • Black and Sooty means spark plug is fouled due to being run too rich
  • White and Chauky Insulator means spark plug was too hot due to being run lean

How Do You Clean A Chainsaw Spark Plug?

A fouled spark plug can sometimes be cleaned to restore much of its functionality. However, in most cases, the spark plug must be replaced in order for the chainsaw to achieve peak performance.

I don’t really recommend cleaning a spark plug over replacing one but in some instances, it may be needed.

How do you clean a spark plug?

Spark Plug Cleaning Steps:

  1. Remove spark plug from engine
  2. Use a wire brush to clean electrode end of spark plug to loosen deposits
  3. Spray electrode end of spark plug with brake cleaner and wait 10 to 15 mins (Wear eye protection)
  4. Use compressed air to blow away loose deposits
  5. Repeat if necessary
  6. Check spark plug gap with feeler guage
  7. Reinstall spark plug into engine

Wrapping Up

A poorly running chainsaw could be the result of many different factors.

One of the most common factors of a poorly running chainsaw is a fouled spark plug.

A fouled spark plug can be identified by examining the central and ground electrodes of the spark plug. If there is carbon buildup coating the electrodes of the spark plug it is fouled.

Carbon is a conductive material that can cause a spark plug to fire at the wrong time which leads to non-starting engines, hard-to-start engines, throttle hesitation, and rough engine idle.

Now that you know how to identify a fouled spark plug maybe you can pinpoint why your chainsaw is running rough and hard to start. If your spark plug is indeed fouled you just solved your problem.

Be safe out there and happy cutting.

Steven R

I have been part of the chainsaw and outdoor power equipment business in one way or the other for over 35 years. There are not many things that I have not seen in the business. From repairs, sales, equipment operation, and safety I can help you with your questions.

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