Why Does My Lawn Mower Backfire? (causes & how to fix)

A lawn mower backfiring is something you definitely don’t want to hear. It is a clear indicator that something is wrong and needs to be fixed as soon as possible. If it goes unfixed for too long, the entire mower could become dangerous to use. Fortunately, the most common things that cause a lawn mower to backfire can be easily fixed. Today, I’m going to reveal the most common answers to your question: why does my lawn mower backfire?

Let’s start by defining what a mower backfire is, and what it sounds like. Typically, when your mower backfires there’s a loud bang. This may be accompanied by a poof of smoke, or you may even notice a spark or small explosion in the mower’s exhaust pipe.

In my experience, it will typically occur either while the engine is running, or while shutting down. That said, I’ve also had a John Deere riding mower backfire immediately after start-up.

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Why Mowers Backfire (most common causes)

Checked or fixed all these things but the mower is still backfiring?

There are three other causes that are also easy to rule out. These are:

• Issues with the mower’s muffler or exhaust,
• Problems with the engine running too hot, or
• Bringing the mower engine from high speed to a stop too abruptly.

I’ll go through exactly how to rule out these potential causes later in this article.

Here’s an in-depth explanation of each of the issues mentioned above, as well as how to fix them.

Using Wrong Type of Gas

Lawn mowers require specific types of gas in order to function properly.

The gas that you use in a lawn mower should have a very low amount of ethanol, a chemical that oxygenates the fuel to reduce air pollution.

If there is too much ethanol, the mix is “lean” or “skewed”, and the engine can’t use it.

There has to be a certain air-to-fuel ratio in smaller engines, and when that ratio is thrown off, the gas can’t oxygenate and the engine won’t run as it should.

This can result in the backfiring noise, which can damage your engine and make the mower potentially dangerous.

Fortunately, there is an easy way to fix this. All you have to do is change the gas blend or type that you use.

You can look for gasses that don’t contain ethanol or have a lower amount of ethanol than average gasoline.

Did you try changing the fuel and the backfiring is still happening? There is probably something else wrong.

How I Fuel Up My Mower

I use a product called TruFuel (you want the gray can) which is carried at Ace Hardware, The Home Depot, and Amazon in my mower and snowblower. It’s ethanol-free gas, which keeps my small engines in great shape, and offers the added bonus of not having to stabilize my fuel when my mower sits in the winter, and my snowblower sits in the summer.

If you’re wondering how to stabilize your fuel, I was recently quoted with recommendations on that subject in this article in Homes & Gardens.

Faulty Spark Plug

The spark plug is responsible for setting off the spark (hence the name) that ignites the air and fuel mixture in gas-powered mowers. This is what makes the engine run.

When the spark plug is damaged, it won’t be able to properly create the spark needed to power up the engine, and sometimes this can cause to backfire.

It’s not uncommon for spark plugs to develop problems after being used for a long time. Their wiring might become faulty, they could get damaged, and gunk and carbon can build up around them.

These things can cause the spark they produce to be weak, or nonexistent.

To fix this, check the condition of your spark plug or plugs. Are they coated in black gunk?

How to Troubleshoot This Issue

If money’s tight, you can take them out of your mower, clean them and pop them right back in the mower to see if that resolves your issue.

That said, it’s generally around $10 or less to buy a new one, and you can replace it yourself in a few minutes. I like to replace my plug annually (at a minimum I do it every other year) as a form of preventative maintenance.

Sheared Flywheel Key

The flywheel key is responsible for making the delivery of power to the motor as smooth as possible. It could get sheared and damaged if the flywheel key is old or has been used frequently.

This can cause the mower to backfire because there isn’t a smooth transfer of power from the engine to the rest of the mower.

General wear and tear to your mower (for example, caused by your mower running over an object) are the most common factors that cause sheared flywheel keys.

Flywheel keys usually last about five years, but being used constantly or not being well maintained can cause them to wear down faster.

Furthermore, if your mower runs over something hard or sharp, that can shear your flywheel key.

How to Resolve This Problem

Finding out whether your flywheel key is sheared (and fixing it) is easier than you think.

First, disconnect the spark plug to avoid the mower turning on by itself. After that, remove all of the screws holding the mower parts in place and set them aside.

Use a socket wrench to remove the flywheel nut and the recoil cut, and you’ll be able to see the flywheel key. You should align the flywheel key with the keyway hole in the crankshaft.

If it’s not, then you know that you have a sheared flywheel key. If this is the case, you need to replace it with a new one and reassemble the mower. While this is a project many people can DIY, it’s also one that might be better left to a professional at a small engine repair shop.

Here’s a nice quick video of the process on a push mower so you have an idea of what to expect:

Carburetor Issues

Carburetor issues are a final reason that a lawn mower might be backfiring. The carburetor is responsible for adjusting the amount of air and fuel that enters the engine.

If too much air or gas gets into the engine, the ratio will be off and won’t ignite correctly, which can cause the engine to backfire.

Carburetors are prone to clogging, especially if the fuel you’re using has gone bad or isn’t the right type. Also, if you don’t clean and maintain your mower, your carburetor can end up clogged.

Furthermore, if it’s dirty or not well-adjusted, air and fuel won’t be able to get to the engine and the mower will backfire.

To fix this, adjust and clean out your carburetor, and repeat this process every few months to avoid backfiring in the future.

What I Do to Maintain a Clean Carburetor:

I own two inexpensive products (total cost is typically around $8 to get both of them … that gives me 2+ seasons of coverage) for my carburetor maintenance.

Here’s what they are and how I use them:

• I spray Gumout starting fluid in the air intake (behind my mower’s air filter) cleaner spray into the mower every time I mow my lawn. Not only does this help my gas mower cold start immediately, this spray inhibits corrosion and keeps your carb running cleaner, longer.
• Every year (usually in the winter when I’m less busy with projects) I clean my mower’s carburetor. Again – I like Gumout’s spray product for this.

Sometimes the Problem is Pretty Straight Forward

A few final causes for a backfiring mower are:

• Poor Muffler/Exhaust Design – A poorly designed muffler can cause an engine to backfire. If your mower backfires at random times, or under heavy load or on cold start, then this could be the cause.
• High Engine Temperatures – If your engine is running too hot, this can cause a backfire. Most mower engines are either air-cooled or water cooled, and I find this cause to be more likely in water-cooled engines when something is off with that system. If you find your mower backfires after it has been running under load a while, and especially on hot days – check this.
• Abrupt Engine Speed Decrease/Stoppage – If you are running your mower full bore and then immediately come to a complete stop and shut it off, this can cause a backfire. If your mower backfires when you shut it down, I recommend trying to be intentional about slowing your mower’s engine speed gradually before you cut the power to rule this out as the cause.

When To Take Your Mower In For Professional Repair

Have you tried all of the fixes I talked about above, but your mower is still backfiring? There might be something else going on that needs professional repair.

As I mentioned earlier, you can fix most lawn mower issues yourself, doing things like cleaning parts or purchasing new ones.

But if you’re constantly finding yourself having to repair your mower or replace the parts, that’s a strong indication that there’s another issue with your mower.

Taking it in to see a professional may save you time and aggravation, which may be money well spent.

If you find that you’re fixing your mower multiple times a year, it’s most likely time to replace it.

A new mower can be expensive, but it’s an exciting purchase that can rejuvenate the pleasure you once took in caring for your lawn, and turn mowing from a chore into something you look forward to.

In my view, that can be money well spent as well.

How To Avoid Lawn Mower Backfiring

The best way to prevent your mower from backfiring is to ensure you stay on top of maintenance.

You should give all the parts a thorough cleaning every few months, make sure that you’re using the correct fuel, and ensure that you’re not accidentally mowing over objects which can damage the blade (or worse, the engine).

This significantly decreases the chance that your lawn mower will end up with expensive problems.

Don’t Panic if Your Lawn Mower Backfires

A backfiring lawn mower isn’t all that unusual. As I’ve explained here, there are several different issues that can cause it.

As long as you figure out the cause and deal with it appropriately, you should be able to fix your backfiring mower in short order by yourself. Though in some cases, you will need to bring it to a professional for repair.

As I mentioned, staying on top of maintenance is the best way to keep your mower running the way it should.

To help you with that, I recommend you read what I recommend you do each spring, and here’s how I suggest winterizing your lawn mower at the end of the season.

Taking these small steps each year can really extend the life of your mower and help you avoid problems.

Additional Resources

• Why Does My Engine Backfire or Afterfire by Briggs & Stratton (https://www.briggsandstratton.com/eu/en_gb/support/faqs/browse/engine-backfire-afterfire.html)