why does my lawnmower backfire

Why Does my Lawnmower Backfire? – SOLVED

lawnmower backfire fix If your lawnmower has ever backfired, you will be familiar with how loud and quite frankly, scary this can be. But this is, unfortunately, something that happens with a lot of garden tools over time. When you first purchase your lawnmower, it will likely give you a smooth operation and very few problems.

The issue with this is that a backfire can cause damage to the engine, which could be costly for you in the long run. But it’s still a head-scratcher; why does my lawnmower backfire? In this article, we are going to look at this in a little more detail and put an end to your backfiring problems!

When you notice that your lawnmower engine is making a popping sound or something even louder that might resemble an explosion, it can be worrying. It is true that a lawnmower backfire is a small kind of explosion, but it is unlikely to ever cause any harm to you. But your engine may not be as lucky. The same goes for when your lawn mower starts and then suddenly dies.

The motor found in your mower is considered to be a small internal combustion engine. Inside the engine, there is a combustion chamber and this is where the magic happens and power is created. Here, air and fuel combust and this process can happen in either two or four cycles; this is where we get the terms ‘2-cycle’ and ‘4-cycle’ engines.

During this process, a spark is initiated as the fuel and air mix and this will always set off a mini-explosion of sorts. However, this happens inside the combustion chamber so is both controlled and won’t affect any parts of the engine negatively. However, if this process takes place anywhere else apart from inside the chamber, then we hear the ‘explosion’ and this is what is known as a small engine backfire.

Riding and Push Mowers

Both riding and push mowers that run on a gas engine may backfire at any time. But since a riding mower is typically a lot more powerful you may find that the backfire is louder but this depends on the size of the engine and the chemical reaction.

Quite simply, if fuel comes into contact with a spark outside of the combustion chamber, you are going to get a little explosion. So whether you are asking why you riding lawn mower is backfiring while running or why you keep experiencing a push mower backfiring, the common causes are likely to be similar.

Common Issues

There are people all around the world scratching their heads and wondering why does my lawnmower backfire? If you are one of them, the good news is that while there are several common causes of what causes a lawnmower to backfire, for the most part, they are easily remedied. Let’s take a look at what the issue may be and how you can solve it.

Fuel-Related Issues

One of the most frequently reported issues when it comes to a push mower backfiring is a problem with the fuel. When the fuel is inside the combustion chamber, it must have oxygen in order to burn and your lawnmower’s engine will have been specially designed so that it has just the right fuel to oxygen ratio for an efficient burn.

However, if for some reason, this ratio is interrupted, then problems are bound to follow. A good example of this is using the wrong type of could cause the engine to smoke. Another example of this is if the timing inside the chamber is off, this could result in the fuel to oxygen ratio being interrupted and, therefore the lawnmower may backfire.

Typically, this would happen if too much oxygen is present and in this case, no spark would be produced. But this doesn’t mean that the rest of the process will stop meaning that there may be a resulting build-up of fuel which has a chance of igniting once it is outside the combustion chamber.

How to Fix Fuel-Related Issues

Firstly, make sure you’re using the right type of fuel for your mower’s engine. You would most commonly notice this problem if the components of the engine are outdated and not functioning as they should. Components such as the fuel pumps and filters may simply need updating and this can often solve the problem.

Engine Related Issues

If there are no problems with the fuel, then it could be an issue with the engine. This can be concerning as many lawn mower owners may think that a faulty engine means the end of their mower’s life. In some cases, this may be true but where backfiring is concerned, it is likely a fixable problem.

Similarly to when there are problems with fuel, engine problems are often a result of timing with the processes. If you want your engine to run smoothly and without issue then everything needs to run like clockwork. If it doesn’t, then you may find that your lawnmower starts backfiring unexpectedly.

In much the same way that fuel could pool in the motor and ignite outside of it, the timing of the engine could cause fuel to spark in the wrong place. But in this case, it is less of a problem of where but when in the process. If the fuel sparks while the intake valves remain open, then a backfire will be the result. This is common in older engines but as we have mentioned, doesn’t have to mean replacing the entire lawnmower.

How to Fix Engine Related Issues

Where a fuel-related issue can be easily fixed, a problem relating to the engine may require a little more hard work. However, you will normally find success by addressing one of the following issues.

Lawnmower Backfires During Starting

If your lawnmower backfires when trying to start then this is, unfortunately, never a good sign. It could signal that the mower is in need of repair since unlike other problems, when a lawn mower struggles to start without backfiring, this is not normal and may need a professional to take care of the problem.

A petrol-powered lawnmower is an expensive bit of kit and this is because there are a lot of intricate parts that cost a lot to manufacture and can cost an arm and a leg when it comes to replacing them. For this reason, many gas lawn mowers have other, less expensive parts that are designed to break first. You might see this as some sort of barrier to the pricey parts or even some kind of weird, garden tool self-sacrifice. But in any case, a mower that backfires upon starting isn’t a good sign.

One of the first parts to go would be the flywheel. This will normally be the result of some sort of direct damage to the mower, such as mowing over a large stone or hard piece of debris, but rather this than the crankshaft breaking as that would be a much more expensive repair. Once the flywheel is broken, it will not work and this can cause the lawn mower engine popping sound or backfiring that is causing you so much concern. In some cases, the mower will refuse to start at all so the only solution is to have it repaired.

Lawnmower Backfires During Shut Off

A lot of gas mower owners will quickly shut down their gas mowers when they have finished mowing. During this short shut down period, backfires can be pretty common. If you notice that your lawn mower pops when turning off then there is usually a very simple cause for this which can be quickly and easily solved.

Typically, this problem occurs because the mower engine is being slowed down too quickly. As you use your lawn mower the engine speed will increase over the course of the mow. However, just as it needs time to build speed, it also needs time to slow down. If it doesn’t have this time then the muffler could become filled with gas where it will spark and result in a backfire.

In order to avoid backfiring like this, it is important not to turn the mower off as soon as you finish mowing. Give it the time that it needs to slow down by leaving it running in idle for ten minutes.

However, this may not be the only reason that your lawnmower backfires when shutting down. There may be an issue with overheating which is typically related to a lack of decent airflow around the engine. This is a problem that needs to be addressed by a professional repairs-person.

Lawnmower Backfires After Shut Off

If you have managed to start your mower, complete the cut and then get it to shut down without it backfiring, it can then be pretty disheartening when it backfires the second it shuts down. If this happens, you’ll likely find that there is an issue with either the muffler or carburettor. In this instance, you will need to have the lawnmower checked and repaired.

However, you might also experience problems if you are not using the correct type of fuel. Normally, using a non-alcohol fuel will reduce the number of backfires, almost completely eliminating them.

Anti-Backfire Equipment

If you continue having problems with your lawn mower backfiring at shut off then there are special pieces of equipment known as anti after fire solenoids which can be used to prevent the problem in future. However, this is not something that the average homeowner can fit on their own and it is important that you take your lawnmower to a professional to have these fitted.

Once installed, this feature will mean that you no longer have to idle the mower after use and before shutting off. This can make the mowing experience a lot more convenient.


Gas-powered lawn mowers are very effective and powerful tools that can make light work of mowing your grass, especially if you have a larger garden. However, they are not without their problems and one of the most common is that a lawn mower will backfire at any point during operation. If you’re still running into issues with your mower, consider buying a secondhand mower or cheaper model that won’t break the bank.

There are several causes for this but for the most part, they are easily fixed either at home or by taking the mower to a professional repairs person.

Andrew Fisher

Andrew Fisher

Andrew is a dedicated father of three who really takes pride in his lawn and garden. You'll find Andrew behind the scenes of almost everything Edge Your Lawn produces. When he's not helping readers find all the information they need, he's in his backyard working on his lawn and garden landscaping. This year he hopes to build an outdoor deck and sort out his veg patches.

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