Strategies For Beginners when your lawn mower is backfiring

Dad's guide for when your lawn mower is backfires

A backfiring lawn mower is a common issue most lawnmower owners encounter. The sound of a loud bang is enough to scare anyone, especially any young ones who have just recently started watching dad operating a lawn mower.  A lawn mower backfiring could be due to several reasons, such as a clogged carburettor, high temperatures, poor fuel quality, etc. 

Backfiring can cause damage to your machine’s exhaust system and reduce its life span. So, it is important to limit the frequency of these backfires. Fortunately, most cases of backfiring can be easily fixed. Continue reading as we discuss common reasons why a lawnmower is backfiring and how to fix it.

How to Identify When Your Engine Backfires?

When a lawnmower engine backfires, it will make a loud, popping sound. This is caused by gas being ignited outside of the combustion chamber’s usual position. Your lawn mower may backfire at various times. A lawnmower could backfire while you’re trying to start it after it’s already running or while you’re switching it down.

A lawnmower rarely backfires while starting. If this occurs, it may indicate that the lawnmower’s system has been damaged. For example, the flywheel might be damaged. If the flywheel becomes damaged, the lawnmower may begin to stutter and backfire.

In addition, a backfiring machine could be caused by speeding up the machine. This is the most common cause of a lawn mower backfiring while shutting down. A lawn mower can also backfire when you’re switching it off. However, this incident is rare except when the carburetor or air filter is damaged. This may also be caused by the fuel you’re using. Read on as we delve deeper into the causes of a lawnmower engine popping sound.

Causes of a Backfiring Lawnmower and Quick Fixes

Lawnmower engines are four-stroke engines that power the blades and wheels using controlled explosions in the combustion chamber. The causes of an engine backfiring could be due to various reasons. Here are eight possible causes and solutions for a backfiring lawnmower.

1. Poor fuel quality

Your lawnmower may backfire if you fill the machine with bad gas. Gas with a high ethanol content will usually lead to your engine backfiring. A fuel with more alcohol blends is also less suitable for small engines. To keep your lawnmower’s engine safe and operating properly, use fuel containing soft alcohol blends.

How to fix it: Drain the bad gas from your fuel tank and replace it with newer, more suitable fuel. We recommend using fuel with at least an 87-octane rating.

2. Clogged carburettor

The carburetor is an important component of any engine because it prepares and supplies the engine’s air/fuel mixture. Unfortunately, clogged carburetors are a common issue typically caused by old or bad fuel. Backfires may also occur if the carburetor is not correctly adjusted.

Therefore, make sure you adjust your carburetor occasionally if you want to keep your engine running properly. It is also possible that problems will arise if the carburetor adjustment is set too lean. Backfires caused by the carburetor usually occur when the engine is starting up. 

How to fix it: First, use a carburettor cleaner or disassemble the carburettor and clean it to unclog it. Ensure the carburettor is correctly adjusted after it is thoroughly cleaned.

3. Faulty spark plugs

Spark plugs are another important component of the lawnmower engine. A faulty spark plug produces a weak spark. As a result, this weak spark may not properly ignite the fuel vapor in the cylinder. This will lead to the fuel vapor igniting when it reaches the hot exhaust muffler, causing a loud backfire.

How to fix it: Remove the spark plugs and inspect them. If you notice a lot of dark-colored carbon buildup, you can use a metal brush or sandpaper to clean the spark plug. However, if you notice a damaged component on the spark plug, we recommend replacing it right away.

4. Abruptly reducing engine speed or shutting down

When the engine’s speed is suddenly lowered or abruptly shut down, it attempts to decrease its throttle to achieve the same timing. This decreased throttle will result in a lean air-fuel mixture forming. This means that there is more air than fuel. Excess air causes incomplete combustion, and when the lean air-fuel mixture reaches the exhaust, it combusts there. This late combustion will result in a lawn mower engine popping sound.

How to fix it: To avoid backfiring, reduce the engine speed gradually and let it idle for at least 30 seconds before turning it off. This will prevent excess fuel from entering the exhaust system.

5. High temperatures and pressure

Backfiring can also be caused by high engine temperatures. So, if your engine backfires, check the engine’s temperature. If the lawn mower is turned on but not in use, the temperature should be normal. Low fuel pressure could be the cause of the high engine temperature. This means that the fuel-to-air ratio is off, and there is excess air in the combustion chamber. As a result, the engine will be unable to generate additional power to ignite the fuel. 

How to fix it: Check the fuel pump on the machine. Clogged fuel lines and filters can also be the source of the issue. Examine these parts and clean, or replace them if necessary.

6. Clogged air filter

The air filter is in charge of keeping the engine clean. Therefore, the engine will not run properly if the filter becomes clogged with debris. Because air filters allow clean air to enter your engine, a dirty filter can prevent air from entering the intake, resulting in a poor fuel-air ratio. This could be the reason your lawn mower is backfiring.

How to fix it: The fix for this issue is quite straightforward. Simply replace the air filter with a new one.

7. Broken flywheel key

Flywheels are connected to the transmission and can be found after the engine’s crankshaft. A backfiring engine can be caused by a damaged flywheel key. Your flywheel key can be damaged when you mow and run over large, solid objects like stones. 

The flywheel is connected to the crankshaft by the key. Therefore, when the key is damaged, the connection is broken. This will lead to the lawnmower failing to move forward and eventually backfire.

How to fix it: Visit a repairer and replace the flywheel’s key. To ensure your flywheel doesn’t get damaged, remove any large, solid objects from your lawn before mowing. 

8. Irregular maintenance or tune-up

Backfires can also be caused by timing system issues, which can lead to engine failure. A negative pressure difference is created when the piston inside the cylinder moves down. When the valve is opened, the fuel-air mixture reaches the combustion chamber. The valves open and close at a particular time and crank angle.

This is known as valve timing. The timing and valve clearance can both be adjusted.  If either of these two components is out of balance, the lawnmower’s performance suffers and may cause backfiring. If the engine’s timing is off, it may flare up before the intake valves close. The timing between the engine and the valves can change over time. This will lead to the engine backfiring and require tuning.

lawn mower is backfiring

How to fix it: To avoid this problem, make sure the timing between the engine and the valve is set correctly. So, take your machine to a repair shop for proper servicing and tune-up.


A backfiring lawnmower is a common occurrence. Luckily, this issue is rarely fatal and typically easy to fix. A backfire can happen at any time, such as when you start the engine, when the engine is running or when you’re shutting down the machine.  Hopefully, this guide has allayed your fears. Hearing that popping sound for the first time can be worrying. Simply take note of the common causes and solutions for a backfiring lawnmower and examine your machine to find out the exact cause.  

Frequently Asked Questions about engine backfiring

1. Is a backfire harmful to an engine?

Backfiring doesn’t harm the engine. A backfire occurs when combustion occurs outside of the engine chamber. Therefore, it has no damaging effect on the machine.

However, backfiring may have an adverse effect on the lawnmower’s exhaust system. This is because the exhaust is not designed to withstand the heat and energy generated by the combustion that occurs when the machine backfires.

2. Is it possible for a lawnmower to explode?

Only gas-powered lawnmowers have a remote chance of exploding. Although it is extremely rare, it takes numerous volatile factors to occur simultaneously for an explosion to happen. Because of the hot temperatures of the engine, there is a risk of a fire starting while it is running. If the fires are not quickly put out, they may spread to the fuel tank and lead to an explosion.