Why Is My Lawnmower Spark Plug Black?

Approx Reading Time: 7 minutes

Mowing the lawn is one of the many chores a homeowner is responsible for. It can be a daunting task for those new to maintaining a lawn. As well as veteran mowers who simply find the task rather tiring or difficult. Whichever mower you are, you have probably come across a few questionable changes happening during the lifespan of your lawnmower, like the spark plug turning black. 

There are a few possible reasons why your spark plug is turning black: 

  • Carburetor isn’t working properly
  • The oil needs to be replaced
  • Incorrect fuel/oil ratio (with 2-stroke engines)
  • The engine needs to be replaced

Any of these reasons can turn your spark plug black over time. The best course of action is to start with the easiest fixes first before resorting to pricier options. 

A black spark plug is not the end of the world for your lawnmower. You can easily troubleshoot what could be causing the coloring of your spark plug before tossing out the whole lawnmower. 

Why Is My lawnmower Spark Plug Black? 

You May Have Dirty Parts 

Check out your lawnmower parts, like the air filter or carburetor. If these are dirty, simply cleaning them out may have your lawnmower running like new, and your spark plug won’t turn black again.

Cleaning the spark plug will momentarily fix the problem. You can do this with a metal brush, or some sandpaper – just make sure to let it cool down before touching it if you have been using your mower recently.

Check On Your Oil

You may have stored your lawnmower for the winter and tried to restart it after months of sitting in your shed. It most likely has old oil in the tank if you didn’t empty it before storing it. Be sure to clean out this oil before adding more or running your lawnmower.

It may work for a bit, but it may also cause your spark plug to turn black. 

Carburetor Problems

If the fuel and air mixture is too rich, it can lead to incomplete combustion. This happens when the carburetor lets too much fuel into the combustion chamber.

The resulting smoke will build up on your spark plug over time. If your spark plug keeps turning black, especially quickly, then it may be a carburetor problem. 

Check your mower’s manual for instructions on how to configure your carburetor properly. If everything is in order, you may want to get it checked with a mechanic, as replacing parts of a carburetor can be rather complicated.

Incorrect fuel/oil ratio

Most lawnmowers nowadays have 4-stroke engines, but there are still some 2-stroke models out there. With these machines, you have to pre-mix fuel and oil in the correct ratio. 

This ratio usually depends on the model itself – check your mower’s manual, or the manufacturer’s website if you aren’t sure about it. 

Adding too much oil to the mixture will not only generate a lot of smoke, but it can also also turn your spark plug black. Always use the correct oil/fuel ratio. If you don’t want to bother with mixing your own fuel, you can buy it pre-mixed – although it will be more expensive. 

Engines Age Too 

Spark plugs can turn black from an old engine. An older engine gives off more exhaust and carbon, thus building up more on your spark plug. It may also not readily fire your spark plug, and the lawnmower may not turn on or work for long.

A black spark plug is not a sure sign of a bad or old engine but can be one of the signs that your engine is getting up there in age. 

How Can I Stop My Spark Plug From Turning Black? 

Perhaps you’ve figured out the reason why your spark plug turned black from troubleshooting some other parts of your lawnmower. Now, you want to know how to keep it from going black again.

You don’t want to be wasting money buying new spark plugs all the time! Here are some things you can do to keep your spark plug from turning black: 

Keep New Oil In the System

Make sure you are using newer oil when running your lawnmower. Changing out the oil or only filling it up as much as you need to cut your yard can help prevent old oil from seeping into your system and causing your spark plugs to turn black.

Keeping old oil out of your lawnmower will also help prevent any other issues it can cause, like messing with your carburetor or engine. 

Clean Out Your lawnmower

Giving your lawnmower and all its parts a thorough cleaning can also help prevent spark plugs from turning black. If the plug is covered with carbon, then cleaning out all the lawnmowers’ parts will help prevent it from turning black.

This isn’t something you need to do every time you use your lawnmower, but at least once a year is would be nice. 

Get Your lawnmower Checked Out

If you are still having spark plugs turn black even after adding new oil and cleaning out all the parts, then you should get it checked by a lawnmower mechanic. You may have a bad engine or faulty spark plug connector.

A professional would be able to assess whether you can fix your lawnmower or if you need to start looking for a new one. 

How Can I Replace A Spark Plug? 

Replacing the spark plug can be relatively easy compared to replacing other parts of the lawnmower. There are multiple ways to get the spark plug replaced, such as: 

Do It Yourself

If you are lawnmower savvy, then you may already know how to replace it yourself. If you are more of a novice, it can be confusing at first.

You don’t want to mess it up or break something while trying to replace it. You can look through the lawnmower manual for step-by-step directions or look it up on YouTube:

As you can see, the process is very straightforward. 

Take It To A Mechanic

Another method you can do is take your lawnmower in to get the spark plug replaced. This can be a good alternative if you don’t feel comfortable doing it yourself or are worried you’ll do more harm than good. You can take it into a shop or have a mechanic come to your home and replace the faulty part. 

That said, I would only recommend this if you have other things that need fixing on your mower, as replacing a spark plug is rather easy. 

Phone A Friend

If you aren’t comfortable replacing the spark plug yourself and don’t want to pay to get it fixed by a mechanic, then call a friend or family member.

You are bound to know someone who’s dealt with spark plugs on lawnmowers before, so they may be able to help you out. They may want some compensation, like pizza, in return. 

How Often Should I Replace A Spark Plug?

Typically you want to replace your lawnmower’s spark plug every season or after every 100 hours of use.

This will help keep your lawnmower working well and decrease the number of issues you will find with your spark plug or the mower in general. Replacing it this way will also ensure that you lower your fuel consumption.

Maintaining your lawnmower is one of the best ways to guarantee that it lasts for a good number of years. That includes maintaining and replacing the spark plug. Don’t be afraid to reach out for help if you are still uncertain how to replace the spark plug or how to assess what is causing it to turn black.

It’s better to protect your equipment and do it the right way rather than spending even more money to buy a brand new lawnmower. 

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!