Why Does Lawn Mower Backfire?

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A lawn mower backfiring is an indication of another issue with the appliance.

This may be a simple problem with a spark plug or fuel system, or it could be severe, like an issue with the mower’s fuel.

No matter why your lawn mower has backfired, you need to fix the problem without delay to prevent harming the engine.

Considering when a lawn mower backfires can be useful, as this narrows down what the issue may be. 

We’ll cover some of the main reasons why a lawn mower may backfire in this post, as well as what to do in each situation.

Causes Of Lawn Mower Backfire

A backfire occurs when an engine’s blend of fuel and air catches light outside of its cylinder. Backfiring can impair the lawn mower’s exhaust system, muffler, and various engine components.

Backfire occurs in two main types:

Popping Or Spitting

This backfire occurs when the fuel-air blend within a carburetor catches alight. This generally happens when a warm engine is shut off or a cold engine starts.

After Run

This backfire occurs when unburned fuel in a muffler catches fire once you switch the engine off. This may indicate an issue with the fuel, ignition system, or carburetor. 

Lawn Mower Won’t Start After Backfiring

If a lawn mower won’t start after backfiring, the most common issue is with its fuel system.

There may be water in the gas or the fuel has gone bad after a while. In both cases, you’ll need to change its fuel after emptying the gas tank.

If emptying the gas tank doesn’t resolve the issue, the problem may be with the mower’s carburetor. 

The carburetor jets may be blocked, which means the correct amount of fuel and air cannot reach the generator. 

Another issue may be the carburetor float may be jammed, which stops fuel from entering the engine. 

Both instances will require rebuilding or cleaning the carburetor. Unless you are skilled with small engine repairs, this should be done by a skilled professional. 

Lawn Mower Starts And Backfires

A lawn mower that starts and backfires may be the result of a cold engine. 

If a cold engine starts, the fuel-air blend in its carburetor will be too thick. This means that there is an excess of fuel and insufficient air. 

As the engine starts to warm up, the air-fuel blend will start to even out, so the backfiring should cease. 

If a lawn mower keeps backfiring once its engine warms up, the issue may be with the ignition system.

A faulty ignition coil or dirty spark plugs may affect the engine, leading to a backfire.

To solve this issue, either replace or clean the appliance’s spark plugs and examine the ignition coil to check for damage.

As before, unless you are skilled with small engine repairs, save this work for a skilled professional. 

Lawn Mower Simultaneously Runs And Backfires 

Why Does Lawn Mower Backfire?

If a lawn mower runs and backfires at the same time, this is typically the result of a hot engine.

If an engine becomes too hot, its fuel will vaporize before it travels to the engine’s cylinder. This issue makes the fuel-air blend to be too thin, leading to a backfire.

Allow the engine to cool for a few minutes to solve the issue. Look at the engine’s oil level and add some if required.

Examine the fuel system to check for leaks, as a leaky carburetor gasket or fuel line can lead to a hot engine.

If your lawn mower backfires while it runs, this may be the result of a defective ignition system. The most common issue here is a dirty spark plug.

Try using a wire brush to clean the spark plug. If this doesn’t solve the problem, the spark plug may need changing.

Lawn Mower Backfires When Switched Off

A lawn mower that backfires once switched off is usually the result of a hot engine. If the engine is still hot, unused fuel can catch fire in the muffler, leading to a backfire.

This issue isn’t typically serious, so it should resolve by itself.

If the issue continues, there may be a problem with the carburetor. Try using a carburetor cleaner to clean the carburetor.

If this doesn’t solve the problem, you might need to change to a new carburetor.

Other Reasons For A Lawn Mower Backfiring

Backfiring is usually the result of a fuel system issue, but there are some other possible causes.

Faulty Exhaust Leak

If your engine has an exhaust leak, unused fuel can flow from the engine and ignite inside the muffler. This will lead to the engine backfiring. 

This can be fixed by repairing the exhaust leak, but it’s best to leave this to a skilled professional.

Off Valve Timing

If an engine’s valve timing isn’t correct, the engine may backfire. 

If the engine valves don’t open at the right time, unburned fuel can leave the cylinder and catch alight in the exhaust system. 

Poor valve timing can also make the engine run lean, leading to a backfire. Adjusting the engine’s valve timing can help solve this problem. 

Incorrect Gas Type

The wrong kind of gas in your engine can make it backfire.

For instance, if you use regular unleaded gasoline in a lawn mower that runs on unleaded, premium gasoline, the engine may backfire from running lean. 

Using the right gas type can help solve this issue. Look at your lawn mower’s manual to see what kind of gas is best for the appliance. 

Clogged Oil Or Air Filter

A dirty oil or air filter will prevent proper airflow to the appliance’s engine. This will make the engine run lean, leading to a backfire. 

Other than a clogged air filter, a dirty oil filter will also prevent oil from flowing into the engine. This may make the engine hot, leading to a backfire. 

To fix this issue, either replace or clean the oil and air filters.

Final Thoughts

If your lawnmower has started backfiring, the advice above should help you solve the issue. 

Note down when the appliance began to backfire, as this will help you figure out what’s wrong with the lawn mower. 

If nothing works, take your lawn mower to a professional, as it may need more extensive repairs.  

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