Why Does a Lawn Mower Backfire? (and how to prevent it)

Why Does a Lawn Mower Backfire?
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Is your lawn mower’s engine popping, hesitating or backfiring? Maybe the engine just doesn’t sound right. At this point you may be wondering why lawn mowers backfire. You’ll also be interested to know if this is this causing any damage to your engine and what you need to do about it.

One of the most nerve-racking sounds you can hear when starting a lawn mower is a backfire which tends to be a common problem after winter storage. Most commonly, lawn mowers backfire because of a failing spark plug or a dirty air filter resulting in incomplete combustion.

 Let’s look into what that means and what you should do about it.

What Happens When a Lawn Mower Backfires?

When you try to start your lawn mower and it backfires, there may be ab engine timing issue but most likely it’s a result of incomplete combustion of the fuel. In other words, the fuel is not being burned properly which can cause stuttering, unusual sounds and even black smoke being blown out of the mower.

You may find that your mower will not start at all, lose power or run rough.

Other symptoms of your engine misfiring include:

  • Inefficient fuel consumption
  • Underperformance
  • Smell of gasoline

This can certainly be a cause for concern, so let’s dive a little deeper into why you may be experiencing some of these problems.

Why Does a Lawn Mower Backfire?

Engines often backfire when there is an improper air to fuel mix. It can be difficult to know what exact component of your lawn mower is causing the problem but we’ve found some of the most common causes and put them together below.

Of course, with most mechanical issues, a certain level of troubleshooting is required to get to the root cause of the problem.

We’d recommend starting by checking the fuel supply, the air filter and the spark plug first. Removing the carburetor and checking for a malfunction there is probably the last thing you should check since it can be tricky to remove and put back on.

Air Filter

Sometimes after running your mower for some time, the air filter can become blocked with all kinds of debris like crud, grass clippings or paint chips. The air filter in a lawn mower acts to prevent these particles from entering the engine as a first line of defence. Once it becomes clogged, you may experience strange engine sounds, a misfiring engine, reduced fuel efficiency or even black smoke from the exhaust.

Spark Plug

Over time your spark plug will accumulate wear and tear. A failing spark plug is one that is unable to produce enough current to spark the fuel. A light gray color is usually a good indication that it’s operating properly. Sometimes it can become chipped or black and carbonated over time. A worn or damaged spark plug can the engine to stall or have poor fuel economy.

Fuel Filter

The fuel filter screens out dirt and debris from entering the fuel. Old fuel can clog up the fuel filter which may prevent it from effectively filtering out any impurities. If you’re unable to see any light through the filter, this may be an indication that it’s gone bad.


The fuel you’re using in the mower may be old or have a higher amount alcohol than necessary.

Incorrect fuel maybe preventing the proper function of your engine by clogging up essential components.

You might also want to check that there is no water contaminating your fuel. A good way to do this is to take off the float bowl to take a look inside. You may see tiny droplets of water in the bottom and if this is the case, you need to drain the fuel out and replace it.


Your carburetor is responsible for creating the right fuel to air mixture in order for complete combustion to take place.

Sometimes, the residue from old fuel can create problems in the carburetor. The holes may become clogged with debris resulting in malfunction and of course, could result in incomplete combustion so they will need to be addressed.

Does a Backfire Damage the Mower’s Engine?

Overtime it could cause extensive wear to the engine and other essential components of your lawn mower.

A backfiring lawn mower will result in suboptimal lawn mowing and wasted fuel so there’s plenty of reason to get it fixed straight away.

How to Stop a Lawn Mower From Backfiring?

Proper maintenance and regular service of your lawn mower can go a long way to help your mower’s engine to run smoothly.

Below are some steps that you can take to in order to prevent your mower’s engine from backfiring.

Before you replace anything on your mower, it’s best to consult the owner’s manual for the best practice although we do appreciate this may be long gone for some of you out there!

Unclog the Air Filter

You can start by making sure the air filter is unclogged. Dust, grass and other debris can block it up. A dirty air filter can stop good air flow from reaching the engine so you’ll want try and remove the air filter and use some compressed air to clean it up.

You may even want to replace the air filter if it’s really dirty. They are cheap and it’s not difficult to do.

Clean the Spark Plug

This probably needs to be replaced about once a year or so but you may just need to clean it rather than put a new one in. You can also use an ignition tester to see if it’s faulty. Double check the lead while you’re at it too. A damaged lead could cause the circuit to be incomplete.

If the plug is fouled with oil and sand, the best tool to clean it with is a pneumatic spark plug cleaner. You can connect this to an air compressor and it can be used over and over. On the other hand, spark plugs aren’t that expensive. If only have one mower it may be worth just replacing it.

Use the Correct Fuel for Your Mower

Always use fresh fuel when filling the fuel tank along with a stabilizer to make sure the fuel quality remains optimal. Always remember to following the manufacturers recommendations on what fuel you should be using. If you have too much water in the fuel, you will need to drain it out and start over. This can be caused by leaving your lawn outside which can cause you to accumulate water in your float bowl.

Once you’ve checked the air filter, spark plug and added a little fuel to make sure there’s plenty to get down into the carburetor, it’s likely a fuel flow issue. You can start by spraying some starting fluid into the throat of the carburetor to see if it will run in short bursts. If it will run, it’s probably something to do with the carburetor which you’ll need to take out.

Clean the Carburetor

When you remove the carburetor, be sure to take note of how the mechanisms attach together so you can put it back in the right order.

You can also try cleaning the carburetor with carburetor cleaner to eliminate any clogs. Use a paperclip or a guitar string to help clean out the holes, and combine this with an air compressor for the best results.

This should be enough to get the job done, but if it wasn’t effective, you may need to purchase a carburetor repair kit in order to either replace any faulty components or put in a brand new carburetor.

Replace the Fuel Filter

A degraded spark plug will need to be replaced to prevent the engine from stalling. You will need to check it for any damage and replace if necessary.

Winterize Your Lawn Mower

If you’re planning to put your lawn mower away for the winter, you’ll want to winterize it before doing so. This is a great yearly practice that you should do to keep your mower in the best condition possible. If you’re unsure on how to do this, we have an extensive guide here to walk you through what you need to do.

In Summary

The above steps should cover any issues that might be causing your lawn mower to backfire. If the problem persists, you may need to get someone to look at it. Nevertheless, troubleshooting on your lawn mower can save you a lot of money and can actually be quite rewarding too!

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