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Why Does My Lawn Mower Backfire? 11 Causes Explained

When we use a lawn mower, we expect it to do the job without any problems. So, when it starts backfiring, this is not what we want to hear. A backfire is the loud explosion that happens when a high-pressure gas is suddenly released. 

It can be a result of an ignition source, such as the spark plug. Unless your engine is damaged by it, there is not much to worry about. A backfire can be irritating but it will not damage your engine. 

When your lawn mower has a low engine speed, it may generate excessive heat and give off backfires that are annoying but harmless. But it’s an indication that another component has some sort of damage. 

If you notice that your lawn mower is running slowly or producing more noise than usual, this could indicate hidden mechanical issues.

Lawn mower engine backfires
Lawn mower engine backfires

Reasons why a lawn mower engine backfire

There are many reasons why a lawn mower engine backfires. One of them is when the fuel mixture is incorrect which can be due to a poor fuel pump or clogged fuel filter

The main reason for the backfire typically occurs when the ignition timing is wrong and too much air in the combustion chamber

A faulty spark plug can cause a backfire as well, which is caused when sparks from the spark plug cannot reach the combustion chamber and ignite properly.

1. Incorrect fuel proportion

One of the main reasons that My Lawn Mower backfires is because there is not enough oxygen entering the combustion chamber to burn the fuel properly. 

Having an incorrect amount of fuel causes the engine to not have enough gas to burn efficiently. 

It causes the engine to produce excess heat and backfires. If the fuel is too rich, the lawn mower will most likely blow out smoke and backfire because it cannot burn the fuel efficiently. It causes timing disruption.

In order to avoid a backfire, it is important to have the right amount of fuel in the tank and the proper air-fuel mixture.

2. Too much air in the combustion chamber

One of the most common causes of a backfire is when the engine is not able to create a spark with enough power. As the ignition did occur in delayed time, the other combustion processes will also delay because one process is lied upon. 

The fuel inside would then pool in the low spots, which can then get ignited outside. Thus causes the backfire to occur. The low-pressure fuel causes backfire, which leads to engine damage because of the excessive heat.

When cutting your lawn, it is important to have a fuel filter change. In fact, the fuel filter should be changed whenever you find it’s faulty. This is because the old fuel filters can cause backfires and low-pressure fuel causes them.

3. Having an improperly adjusted carburetor

A carburetor is a device that mixes air and fuel in the right proportion for combustion.

If the carburetor is set too rich (too much fuel) or too lean (too little fuel), it can affect the quality of combustion and cause backfiring.

You should adjust your carburetor according to the manufacturer’s instructions or have a professional do it for you.

4. Having a damaged or misaligned flywheel key

A flywheel key is a small metal piece that connects the flywheel to the crankshaft and keeps them in sync. If the flywheel key is sheared or broken, it can cause the flywheel to rotate out of alignment with the crankshaft and disrupt the timing of ignition.

This can result in backfiring or other engine problems. You should inspect your flywheel key and replace it if needed.

5. The motor timing is not right

When your engine backfires, there are many reasons for this. A bad motor timing can cause a backfire in the engine due to bad ignition module. A backfire is a condition that happens when the intake valves are open when the engine ignites the fuel.

Faulty ignition module in lawn mower
Faulty ignition module

A backfire is caused by an engine’s uneven pressure distribution, or misalignment, which will affect how well it burns fuel and how much power it produces. 

It also may result in pre-ignition or knocking, which will damage your engine or create dangerous conditions for others nearby. 

6. Decelerating the engine too quickly

When you shut off your mower abruptly, the engine may still have some fuel left in the cylinder that has not been burned completely.

This fuel can escape into the exhaust system and ignite when it comes into contact with hot metal or oxygen. To avoid this, you should lower the engine speed gradually before turning it off.

7. Using gasoline with a high alcohol content

Alcohol-based fuels tend to burn faster and hotter than regular gasoline, which can make your mower more prone to backfiring. Alcohol can also damage some parts of your engine over time, such as the carburetor and the fuel lines.

You should use gasoline that has no more than 10% ethanol (E10) or preferably no ethanol at all for your mower.

8. Clogged fuel filter

The backfire could happen because of a wide range of problems, such as a clogged fuel filter. For the lack of enough gasoline, an engine can cause a backfire. 

Clogged fuel filter in a lawn mower
Clogged fuel filter

A low fuel pressure can be caused by a failing fuel pump, clogged fuel filter, or a lean mixture. A clogged fuel filter prevents air from getting to the combustion chamber, causing too much air to go in and not enough oxygen to combust- which leads to misfires. 

A faulty or blocked fuel pump will cause a low pressure due to a lean mixture and high vaporization of gasoline. A clogged fuel filter causes more damage than just backfiring. 

It prevents enough air from being mixed into the cylinder and combustion does not happen properly so that you can hear backfiring. This often means that your lawn mower will run out of gas before you can finish cutting your lawn.

9. Damaged component

A lawn mower backfires when the components are broken or damaged. When you start your lawn mower and it doesn’t engage, there can be a number of possible reasons. 

Some easy things to check are the crankshaft and the flywheel. If they seem to be working just fine, then the problem is with the blades of your mower. The blades might hit a large rock. 

The blades won’t engage when it starts, so the other components might be damaged. The lawn mower may have been damaged somehow. It won’t engage when you start it. 

The blades might hit a large rock. The blades won’t engage when it starts, so the other components might be damaged. The lawn mower may have been damaged somehow. It won’t engage when you start it.

10. Faulty spark plug

Spark plug is one of the most important parts of a lawn mower. The spark plug is a crucial part of your lawn mower that helps provide an ignition to the fuel in the cylinder and ensure a smooth-running machine. You might notice that the mower is running rich due to a faulty spark plug.

When it comes to spark plug, you should always make sure that the spark plug is clean and properly gap between the electrodes. This can be achieved by changing the spark plug whenever it needs. 

A man is changing faulty spark plug in a lawn mwoer
Changing faulty spark plug

When they are not working properly, they cause your mower to backfire or simply stop running altogether. If this happens, it’s best to check your spark plug for damage and gaps between the electrodes before replacing it with a new one.

11. Engine Speed is low

One of the most common causes of a backfire is when the engine is slowing down. When a lawnmower is shutting down, it might backfire because of the low engine speed. 

This can cause some damage to the other components and even harm people. When the engine speed lowers at a faster speed, the mower’s backfires may cause gas to make its way into the muffler. 

The gas will ignite into the muffler, and it’ll cause backfire. To avoid backfires, when you are shutting down your lawn mower, it is important to slow down the engine speed at a low speed, not too quick.

Is backfire is dangerous?

Backfiring is not dangerous to your engine or your lawn mower, but it can be annoying and indicate other issues that need attention.


A lawn mower engine backfires can be scary and irritating. And not every engine-related issue can be fixed even if you have a good understanding of how a lawn mower engine works. 

This is why it is important to have the right knowledge while fixing an engine problem or trying to prevent one. In this article, we’ve covered many of the possible causes that lead to an engine backfiring, as well as what to do for common problems that arise. 

You can’t fix all of the issues by yourself in your home. You should get help from professionals if you can’t figure out the issue or fix it.

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