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Why Does My Lawnmower Backfire? How can I fix it?

Are you startled by the abrupt backfire of your lawnmower? We’ve got the answers. Our detailed instructions will help you diagnose the problem and guide you on how to fix it.

Why Does My Lawnmower Backfire, and how can I fix it:

Lawnmower backfiring is often due to improper air-fuel mixture and delayed combustion. Fix by replacing fuel filters, repairing fuel pumps, tuning the engine, and cleaning/replacing spark plugs and wires. Installing anti-after-fire solenoids prevents backfiring when shutting off.

Don’t stop now! We’re about to dive into the nitty-gritty of lawnmower mechanics, exploring the causes of that startling backfire and providing you with a detailed guide to fix it.

What is the cause of a Lawn Mower Backfire?

Generally, a lawnmower backfires because the fuel mix is too lean. This means more air is in the gas mix, resulting in incomplete combustion. The gas that is pushed to the exhaust will still contain gas. When it reaches the exhaust, this gas will explode spontaneously and make a bang sound. This is the backfire.

Engine Backfiring in a lawnmower and other Small Engines:

Lawnmower engines are regarded as “small” internal combustion engines. The combustion of an air-fuel mixture in the engine’s combustion chamber produces power in internal combustion engines. This production of energy takes place in either two strokes or four strokes.

In both cases, combustion of the air-fuel mixture is the most vital stroke in which a spark ignites the mixture, causing an explosion and, therefore, a release of energy. Fuel combustion occurs in a controlled manner inside the combustion chamber.

Combustion or burning of fuel anywhere else will cause a bang called backfire. Backfiring in lawnmower engines can be caused by either fuel-related causes or engine-related causes. Both are explained below.

● Fuel-Related Issues:

Gas in the engine’s combustion chamber requires a certain amount of air to burn properly. Just like any burning, gas combustion requires oxygen. Each engine is designed to provide the correct air-fuel mixture to the combustion chamber during operation.

All strokes are timed perfectly for the complete combustion of fuel, maximum energy release, and smooth machine operation. If the air-fuel mixture is in an incorrect proportion, it will cause all sorts of problems for the engine. For example, if the fuel is too rich, it could result in your engine blowing smoke.

– Consequences of Process Delays in Combustion

The timing of all processes in the combustion chamber is vital. A process happening early or getting delayed even by some seconds can cause problems. An incorrectly proportioned air-fuel mixture can cause a timing disruption.

If too much air is in the combustion chamber, the engine will not create a spark with enough power to ignite the fuel for some seconds. While the ignition is delayed, other combustion chamber processes keep happening in the way they are supposed to be.

This causes the fuel to pool in the low spots along with the exhaust. This fuel can ignite outside the combustion chamber, producing an explosion you know will backfire.

In my two decades of experience, I’ve found that the most common reasons a lawnmower might backfire are a dirty carburetor, stale fuel, or a spark plug problem. These issues can often be resolved with a bit of maintenance and care.

– Backfire Fix: Fuel related

An air-fuel mixture with more than the correct amount of air is known as low-pressure fuel. If your lawnmower is backfiring, it could be because low-pressure fuel is pumped into its combustion chamber.

Look for reasons for low-pressure fuel. Possible causes of low-pressure fuel include old fuel filters and failing fuel pumps. Replace the old fuel filters and get the fuel pump repaired. If low-pressure fuel were the cause behind your mower backfiring, these fixes would solve the problem.

● Engine Related Issues:

As mentioned above, the timing of all engine processes is crucial for their smooth function. Engine-related issues that mess with the timing can cause backfire.

If your lawnmower is backfiring, check for its motor’s timing. If the motor timing is incorrect, the engine may ignite the fuel while the intake valves remain open. This can cause a backfire in the engine.

After prolonged use, the timing between the engine and the valve can become disrupted or out of the correct setting. This situation inside the engine results in backfires.

– Backfire Fix: Engine related

To solve the engine-related causes of backfires, take care of the following things.

  • Engine Tune-up: An engine tune-up will restore the engine and the valves to the correct timing settings. Both valves will open and close at the right time, preventing the fuel from getting burned in the engine.
  • Spark Plug: Clean or replace the spark plug. Clean the plug’s wires and the plug chamber. Doing this will ensure the ignition occurs correctly and prevent backfires from mistimed ignitions.
  • Fuel Filter: Keep your mower’s fuel filter clean and replace it with a new one every year. Consult your mower’s user manual for more info in this regard.

Moving forward from the issues that could cause engine backfires, let’s get into the details of when the engine backfires and what significance the backfiring time holds.

● During Starting:

Lawnmowers aren’t supposed to backfire when you are starting them. If your lawnmower backfires when you try to start it, it indicates that the mower is damaged somehow.

Many mowers have parts that serve to safeguard other, more expensive parts. These inexpensive parts, such as the flywheel, sacrifice themselves to protect the expensive parts like the crankshaft.

– Obstacles During Mowing

If, during mowing, your mower runs over a large rock or its blades hit an obstruction, the flywheel breaks and won’t engage when starting again.

This will cause your engine to stutter or backfire as you try to create it. A mower with a broken flywheel won’t start usually, so it can’t be used until it is repaired.

If your mower backfires when you try to start it, the indication is of damage. In this case, you must take your mower to a professional for repair.

The most challenging backfiring issue I’ve ever fixed was when the backfiring was caused by a combination of problems, including a dirty carburetor, old fuel, and a faulty spark plug. It was a true test of my expertise.

● During Shut-off:

Sometimes, when you mow, turn the engine off, or when your mower is idling, it will backfire. This is most probably happening because you are slowing your engine too fast.

The engine speed builds up during operation, and when you slow it down too quickly, it could pump gas to the mower’s muffler. This gas could ignite there and cause a backfire.

To reduce this kind of backfire, make sure that you gradually reduce speed and let the motor idle for 10 to 15 minutes before turning off the engine.

Understanding Engine Overheating

Another potential problem that could be the reason behind your mower backfiring when you try to shut it off is engine overheating.

Engine overheating can be reduced by increasing airflow to the engine. If your mower backfires during shut-off and you suspect it could be because of engine overheating, take it to a professional or contact the manufacturer to learn ways of increasing airflow to the engine.

● After Shut-off:

Sometimes, your mower backfires right after the engine is turned off. Backfiring occurs after problems with the carburetor or muffler cause shut-off.

Get your carburetor checked for incorrect settings and your muffler for wrong construction. Getting these two components adjusted will solve the backfiring after the shut-off issue. Using the wrong kind of gasoline can also cause after-shut-off backfires.

Gasoline that has alcohol ignites differently than gas that does not have alcohol. Gas with alcohol can get torched in the muffler, resulting in a backfire. Switching to gas with lower alcohol content or alcohol-less gas could solve the mower’s backfiring problem after shut-off.

Does a backfire damage a lawnmower engine?

Generally, a backfire does not damage a lawnmower or another small engine machine. The explosion you hear is in the exhaust and not in the engine. But when your lawnmower experiences a lot of backfires, it can damage the exhaust and engine over time. I always recommend fixing the backfire issue.

A lawnmower exhaust is not designed for backfire explosions; the additional heat and strain can damage it over time. The engine can experience extra wear and tear depending on the reason for the backfire. They are resulting in a shorter life and potentially engine failures.

Anti-backfire Equipment:

Some mowers can install particular parts to prevent them from backfiring after shut-off. Such features are called anti-after-fire solenoids. Installing them will let you turn off the mower’s engine at any speed without causing fuel to shoot to the muffler.

Installing anti-after-fire solenoids requires professional help. Take professional service if you want to equip your mower with such equipment.

One misconception I’d like to clear up is that backfiring is always a sign of a major problem. Sometimes, it can be fixed with a simple solution like changing the fuel.


Engine backfires, especially a backfiring lawnmower, can sound dangerous, mainly when your lawnmower catches you off guard almost every time there is a backfire. An explosion on the machine sounds crazy, but it is not something to worry about.

The causes of backfiring engines are mentioned in this article. Using this info, you can determine why your mower is backfiring and look for possible fixes. So, the next time your mower backfires, put the acquired knowledge to use and get the issue fixed before it causes permanent damage to your mower.

• The Common Problem: Clogged Carburetor

When your mower backfires: A common problem is a clogged carburetor. This can lead to the engine backfiring. The carburetor is an essential component of the engine. If it is not correctly adjusted, it can also be caused by a faulty or weak spark.

This will lead to a build-up of fuel vapor in the fuel tank and can affect the performance of your mower. Fixing it immediately is essential to ensure your mower is working correctly.

You can take it to a shop specializing in minor engine repair. This guide should help you understand what’s with your mower and how this will lead to a solution.

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  1. Nellie Perkins says:

    Is there a section on troubleshooting the ignition system? It seems like an important aspect of diagnosing and fixing lawnmower backfires.

    • Allard Vdhooft says:

      Yes, there is a troubleshooting section on the blog post dealing with ignition system issues that may cause lawnmower backfires. Check it out for detailed guidance.