What Causes A Lawn mower Spark Plug To Turn Black? How to Clean?


Mowing a garden lawn is not an easy job, especially for new gardeners. You have most probably faced at least a single issue while using a lawnmower for maintaining this machine. You may see different types of problems in your lawnmower, and a black spark plug could be one of them.

What causes a lawn mower spark plug to turn black?

A lawnmower spark plug may turn black for several reasons, such as

  • Long-term use of the same oil
  • Clogged air filter
  • A malfunction occurs for the carburetor
  • Selection of incorrect heat rating
  • Too old lawn mower engine
  • An inaccurate mixture of oil and fuel for 2-stroke engines

If your lawnmower spark plug has any of the above indications, it has a probability of becoming black gradually. It is best to take the precaution as early as possible before ending up damaging the spark plug.

What Causes A Lawn Mower Spark Plug To Turn Black?

What causes a lawn mower spark plug to turn black depends on several factors, which are given below:

Your Parts Might Be Dirty

 What Causes A Lawn Mower Spark Plug To Turn Black - Your Parts Might Be Dirty

Many of the engine parts become dry gradually, as such carburetor, air filter. So keeping the spark plug always clean will ensure clog-free movement.

The best course of action is to clean the spark plug regularly to prevent it from being black. You can use a regular soft brush or emery paper to clean it. Don’t forget to let the machine cool if you used it a while ago.

Using Same Oil For A Long Time

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Are you using the same oil for your spark plug for a long time? Or, keeping your lawn mower idle for a long time with the old oil?

In this case, you want to pour out the old oil from the tank and add fresh new oil. Moreover, keeping the old oil in the tank for a long time may make the spark plug black. You may even struggle to start the engine with one attempt.

Malfunction Occurs For The Carburetor

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When the ratio of air and fuel is inappropriate, imperfect combustion can result from it. The malfunction of the carburetor mainly occurs if there is an excessive amount of fuel inside the combustion chamber.

The smoke generated from this problem will be stored on the spark plug eventually. Thus, the quick black transformation of the spark plug may be associated with carburetor malfunction.

In this case, you want to configure the carburetor properly. If you don’t know how to do it, take help from the instruction manual or a mechanic.

Selection Of Incorrect Heat Rating

You always want to follow the correct heat rating for your spark plug. Following wrong heat rating not only turns the spark plug into black, but it may also damage it.

Inaccurate Mixture of Oil And Fuel

Though the latest lawn mowers have 4-stroke engines, old lawnmowers have 2-stroke engines. For a 2-stroke engine, you need to mix an exact amount of oil and fuel.

How much oil and fuel you need to mix depends on your mower model. You want to check out the instruction manual to the correct ratio.

You should avoid mixing an excessive amount of oil as it could lead to too much smoke and turn your spark plug black.

Therefore, following the correct ratio is essential to prevent this issue. However, if you don’t want to take the hassle of mixing it, you can purchase a readymade premix oil/fuel ratio.

Too Old Lawnmower Engine

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How many years have you used your lawnmower? Have you ever changed the engine? Many auto mechanics stated that spark plug might turn into black due to too old engine.

The old engine may not fire sparks as smoothly as it used to. Also, it may stop automatically in the middle of the mowing session.

Clogged Air Filter

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Make sure the engine air filter doesn’t have any blockage issues. If the air filter in the system is clogged, it may turn the spark plug into black. If it is blocked, clean it or replace it when cleaning doesn’t eliminate the issue.

So, these are the possible reasons you want to know what causes a lawn mower spark plug to turn black. So, if you are wondering why does my lawn mower spark plug turns black, don’t take it lightly if you see any of them.

Black Sooting On Spark Plugs

The primary reason behind black sooting on spark plugs is either overheating or oil dirt. In addition, ignition systems may be negatively affected due to this.

You will notice electrodes failure due to the overheating issue. But, more importantly, spark plugs will lose the probability of recovering due because of it.

Use a new spark and see whether the new spark plug is also becoming black. If any black sooting even on the new spark plugs, the issue might be related to the cylinder or other but not with the spark plug.

How Do You Fix Black Spark Plugs?

Fixing your lawn mower spark plug is essential, and the earlier you do it, the better it is. The question of how do you fix black spark plugs depends on what is the reason behind the black issue of your spark plug. As being said, you need to implement different solutions for different reasons.

  1. If your lawnmower spark plug became black due to dirt, you want to clean the spark plug to fix the issue.
  2. If your lawnmower spark plug became black due to the old oil of the engine, you want to replace the oil.
  3. If your lawnmower spark plug became black due to a malfunctioning carburetor, you want to fix repair the carburetor or replace it.
  4. If your lawnmower spark plug became black due to an old engine, you want to replace it with a new engine.
  5. If your lawnmower spark plug became black due to an inaccurate mixture of oil and fuel, you want to add the correct ratio for these two liquids.
  6. If your lawnmower spark plug became black due to an incorrect heat rating, you want to follow the accurate heat rating as manufacturer instruction manuals.

You need to follow a particular solution for the type of problem your lawn mower spark plug has. Hopefully, the question of how do you fix black spark plugs has gotten cleared.

How Do You Clean Black Spark Plugs?

One of the most common solutions for black spark plugs is cleaning them properly. Even if the cause of the black spark plug is another, there is no harm in cleaning it.

Cleaning your lawn mower spark is essential, and you want to do it regularly. According to experts, it is best to clean them for every 25 hours of use.

To cut off some efforts and, you can do it when you replace the old oil. However, the necessity arises, you should do it earlier.

Caution: Never use any hard sandpaper to clean the electrodes of the spark plug. You may end up damaging the protective coating of it. It is especially essential the protective layer is too thin.

Before knowing how you clean black spark plugs, let’s know things; you will need to clean your lawn mower spark plug.

Things You Will Need

Serial No Things You Will Need
1 Detergent
2 Single-use plastic cups
3 A regular brush
4 Towel

Let’s dive into the cleaning process of black spark plugs!

Step 1 —Immersing The Black Spark Plug

First of all, make a hard detergent solution in a glass and dip the black spark plug inside the glass. Avoid immersing the insulator part, but other portions could be fully submerged.

Step 2 —Keep Them For 30-40 Minutes

You want to keep the spark plug inside the glass in this condition for 30-40 minutes. After that, you will notice a chemical cleaning reaction happens by attacking the dirt.

Step 3 —Take Out Spark Plug

After passing a reasonable time, you can now take out the spark plug from the glass solution and remove the cover too from the plug. Now, use a toothbrush or any regular brush to clean the surface gently.

Step 4 —Wash Through Running Water

After that, place the spark plug on the running water and eliminate any grease or direct build-up from the surface. Again, it could be better if the water is slightly warm for deep cleaning.

Step 5 —Wipe Off The Spark Plug

Once the washing process is done, use a soft, fresh towel to remove the remaining water from the spark plug surface.

Step 6 —Dry The Spark Plug

The last step is drying the spark plug properly using a fan heater or hairdryer. Keep the spark plug in a clean for a few hours and then reinstall it inside the lawnmower.

So, now you know the question of how to do you clean black spark plugs. Make sure you follow the steps carefully to keep it damage-free. In addition, cleaning your spark plug will improve the engine performance by up to 10-15%.

How Can You Tell When A Spark Plug Is Bad?

A bad spark plug doesn’t allow you to run your mower smoothly and efficiently. But the question is, how can you tell when a spark plug is bad? Here are the most common indications you will see on a bad spark plug:

Excessive Fuel Consumption

Your lawn mower engine will consume more fuel than earlier when a spark plug becomes foul or bad.

Struggle To Run The Engine

Are you struggling to run the mower engine even after trying several times? You may think it is because the battery is dead or no oil in the tank. But it can be associated with a bad spark plug. Unless the engine generates enough sparks, you can’t start the lawnmower.

Misfiring Engine

Misfiring Engine

A faulty spark plug may prevent you from running the lawnmower consistently. It may stop in the middle of the lawn cutting and disrupt the cutting process.

Unsmooth Engine Idle

Have you noticed any abnormal sound from the engine? It is supposed to operate steadily and consistently with typical sound. But, from a foul spark plug, you may notice rattling noises or even vibrations is coming.

Inspecting Visually

If you have a new spark plug at home, compare the new one with the old one and notice the difference. If the electrode or tip appears to be excessively worn, you want to replace it.

Can I Clean And Reuse Spark Plugs?

Are you facing any issues with your lawnmower spark plug? Are you wondering whether you can use it after cleaning or you must replace it?

Can I clean and reuse spark plugs?

Whether you can use your old spark plug or not after cleaning depends on its conditions and the type of spark plug you have. If your lawnmower has a platinum or iridium plug, you may reuse it after cleaning. But it should be free from physical damage or other issues.

If the spark plug is bad or faulty, cleaning it may not allow you to reuse it. Check the signs of bad or faulty sparks, which we have mentioned. So the question of can clean and reuse spark plugs relies on your spark plug physical condition.

Can You Clean A Spark Plug With WD40?

Do you think WD40 is only intended to serve as a squeak-removing agent for various objects? The multi-purpose nature of WD40 often stuns us with its surprising uses.

Can you clean a spark plug with WD40? YES! You surely can—using it correctly is all that is needed.

How To Clean A Lawnmower Spark Plug With WD40?

Cleaning the spark plug with an effective cleaning agent will remove unwanted dirt and grease from the surface and allows you to perform better. Here are the procedures:

  1. First of all, let the lawnmower engine cool down if you have used it recently.
  2. Remove the plug from the lawnmower and keep it aside.
  3. Then, take a new fabric and apply the WD40 solution to the fabric by spraying.
  4. Now, gently rub the fabric on the spark plug surface. Avoid overdoing it.
  5. Let it dry. Then, reinstall it. See the difference!

That’s the step you want to follow when you want to know the question of can you clean a spark plug with WD40.

What Is The Best Way To Stop A Black Spark Plug?

You have already known what causes a lawn mower spark plug to turn black. To keep the plug from turning black again, you want to know how to maintain it.

You are not someone who will waste money buying a new spark plug unnecessarily. Here are the following factors that can help you prevent blackening the spark plug:

Change Oil After A Certain Time

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Using the same oil for a long time may make the spark plug prone to be black. This is because old oil may enter into to system and make the plug black. Additionally, new oil prevents damage to the engine and carburetor.

Regular Cleaning

Regular cleaning is essential for any engine components, and so does for the spark plug. However, you don’t need to take the hassle of cleaning the spark plug after every cutting season. It will be best to clean the lawn mower spark plug once or twice each year.

Take A Mechanic Help

If you still see the black issue on the spark plug even after cleaning and regular oil change, take a professional mechanic to help to deal with the issue.

What Is The Recommended Interval For Replacing A Spark Plug?

You have figured out what causes a lawn mower spark plug to turn black, but you are still uncertain whether you should replace the spark plug or not.

The best-recommended interval for replacing a spark plug is around 100 hours of usage. Or, you can replace the spark plug after every season. When the engine sits idle for a long time at the end of the season, it may not work as perfectly as it did.

Replacing the lawnmower after every season or 100 hours of usage will reduce excessive fuel consumption and unwanted operating issues.

Therefore, you want to maintain and replace this vital component regularly to ensure efficiency.

Keep Your Lawnmower Spark Plug Always Inspected!

It is common to encounter problem machine parts. Whenever you notice any abnormality on the spark plug, you should always check them out.

As you are already the question of why does my lawn mower spark plug turn black, you can tell what problem your spark plug has. Most importantly, check out the physical appearance, whether it has any worn-out issue or not.

Final Thought

Knowing what causes a lawn mower spark plug to turn black can help you to make an informed decision. Also, you can decide whether you should replace it or just clean it will solve the problem. So, don’t forget to take care of your lawnmower appropriately to ensure engine efficiency.

Last update on 2021-10-22 / Affiliate links / Images from Amazon Product Advertising API