Why Does My Lawn Mower Backfire? + How to Fix it

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Encountering lawn mower problems is something I’ve occasionally dealt with firsthand. Just recently, I was caught off-guard by that all-too-familiar loud bang – my lawn mower backfired.

It’s easy to understand how startling that noise can be, and you might feel concerned for your safety. So, to help you troubleshoot this issue methodically and safely, I’ve put together this guide on why lawn mowers backfire. I’ll tackle each possible cause and share insights to address this issue.

What Causes Backfiring in My Lawn Mower?

There are a couple of reasons why your mower backfires, and knowing what they are is essential to getting the right fix. While it may not directly impact your engine, it can damage the exhaust system if left untreated. 

Common reasons include bad gasoline, damaged spark plugs, and mishandled carburetor. There are also cases where backfires occur when you shut it down or it’s shut off. 

lawn mower backfiring

Here, I’ll detail the probable reason for these occurrences and the right fix to get your lawn mower back running. 

When Does it Backfire?

One telltale sign that your lawn mower is backfiring is when you hear a loud bang from the gas ignited outside the normal combustion chamber. A backfiring engine happens at different moments, either when you’re trying to start it while it’s running or as soon as you turn it off. 

Keep tabs on the moment when a backfire occurs, as this is essential in assessing what the issue is and how you can fix it. 

Lawnmowers are called small internal combustion engines, and they operate at high temperatures, which is another reason it tips over easily. Elevated temperatures can also prompt problems with the air intake, so it’s best always to clean the air filter and ensure increased airflow.  

Issue #1: Lawn Mower Backfires and Won't Start

If your mower starts then dies or you see splutter and backfires, the problem is with the engine’s timing. It can backfire through the card, so it’s best to check the flywheel. One common reason for lawnmower engine backfires are damaged flywheels.  

lawn mower won't start

Through time, it’s inevitable for the parts to tear and wear. However, flywheel breaks happen if you accidentally hit a hard object or rock when you’re using the mower, causing shears on your flywheel. 

To check so, partially disassemble the mower, and assess for sheared keys. The key should match the keyway hole on the crankshaft, but the engine timing will be faulty and altered with sheared keys. This results in synced spark plugs and engine valves. 

Solution

The most effective solution for a broken flywheel is to replace it. While doing so, be sure to thoroughly inspect the rest of the flywheel for any small cracks or damages, as well as assess the condition of the crankshaft. 

These are vital components of the machine, and addressing and replacing them will resolve the backfiring issues.

Issue #2: Lawn Mower Backfires While in Use or Running

What caused your lawnmower to backfire suddenly in the middle of operations? Check your carburetor. When a carburetor gets too lean or too rich, this can lead to incorrect carburetor settings. 

lawn mower that backfired

The carburetor is made to equally distribute air and pump an equal fuel mix for the proper combustion, but if there’s more air or more fuel, it will result in the lawnmower’s backfire. 

Solution

To fix this, identify the location of your lawn mower’s carburetor first. Once you’ve found it, you’ll need to adjust it to ensure it’s allowing the correct amount of gas into the engine. While this might seem straightforward, getting the balance just right can be a bit tricky. 

Based on my experience, I strongly advise enlisting the help of a qualified technician. These professionals have a deep understanding of the optimal air-to-fuel mixture and can expertly evaluate any other adjustments or repairs the carburetor might require.

Issue #3: Lawn Mower Backfiring While Turning it Off

Another common occurrence of backfiring happens just when you’re about to turn it off. When this happens, it can be frightening since the sudden noise is overwhelming. 

In my experience, the this kind of backfire happens when users operate the mower at full throttle and then abruptly brings it to a halt.

lawn mower backfiring

Like when you’re running, as soon as you drop your speed, you’ll hear loud heartbeats. This is basically what happens when you do that on your machine. The fuel continues to pump even when you’ve gone full halt already.  

Backfiring can also occur when the muffler gets very hot, then suddenly sparks with unburned fuel vapor, causing the bang sound. 

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

When you shut off the engine right away, it causes fuel to pump continuously and ignite the engine—overheating the engine results in your mower backfiring, which stems from the lack of airflow into the engine. 

Solution

Avoid engine backfire by gradually decreasing the engine speed before turning off your machine. Slowing the speed gradually ensures the excess fuel doesn’t snip through the exhaust system, increasing airflow. 

(To prevent excess fuel, here’s how much oil should you put in your lawn mower!)

fuel tank of lawn mower

While many mowers have shut-off valves, it’s not recommended to use this when shutting off an engine running at full throttle, causing fuel to pump and ignite the engine. 

Issue #4: Lawn Mower Backfiring When it's Turned Off

Does backfiring still happen even when the lawn mower has turned off already? Unfortunately, in this case, yes. In some instances, lawnmower backfire can still occur even after shutting off, and the culprit is either the muffler or the carburetor. 

If any of these two are incorrectly adjusted, or the muffler is on faulty construction, engine backfiring will occur. 

Another possible reason why a lawnmower backfire occurs after shutting off is you’re using the wrong type of gasoline. Backfire occurs when you use fuel with high alcohol components, as alcohol ignites differently. 

Solution

To fix this dilemma, adjust the carburetor and muffler accordingly. Make sure these two components are prioritized. However, if you believe the problem is with gasoline, switch to gas with lower alcohol content. An alcohol-less gas would be better. 

fixing lawn mower

Other Possible Issues

Beyond the issues I’ve discussed, there are other potential culprits behind your mower’s backfire.

I’ve come across situations where a compromised exhaust, irregular engine speed, damaged combustion chambers, or malfunctioning anti-backfire equipment were the root causes.

Bad or Low-Quality Gas

There are different types of gas, and one problem that may be causing your machine to backfire is the type of fuel you’re using to power your mower. Some gas types contain ethanol, and while its use is a point of contention, I personally wouldn’t recommend it for your lawn mower.

lawn mower gas tank

Instead, I strongly recommend opting for premium gas with zero ethanol. Not only is it more efficient but also helps in preventing various issues.

Fuel contaminated with water can damage the engine and lead to backfires. Similarly, gas with a high ethanol content can ignite more easily, which often results in backfiring.

Solution

Check the content solution of the gas you’re already using. If you notice a high ethanol content, it’s time to drop that fuel and purchase a new one with lower or no ethanol content. Try to restart the machine with new gas and see how it goes. 

Interesting Read

Damaged or Defective Spark Plug

Your lawn mower engine requires a spark for combustion to take place. It would help if you didn’t have a damaged or worn-out spark plug to get sparks. When this happens, it can lead to combustion problems, lawn mower engine surging, and backfiring. 

mower spark plug

A spark plug is one of the most used components in your lawn mower engine, and it’s likely for this part to deteriorate over time. Building carbon and gunk can weaken your it, resulting in weaker sparks.

Solution

To check whether your spark plugs are in good condition, remove them and visually inspect the plug. Clean it if you notice visible dirt, and use a wire brush to ensure you’re able to clean it properly. 

If it’s broken and has visible wear, it’s better to replace it immediately. This will help you ensure that you prevent a mistimed ignition.

Engine Issues

One of the important components of running an engine smoothly is synchronizing the engine processes and ensuring the combustion chamber runs off smoothly. The motor timing may be causing the engine to ignite the fuel with the intake valves still open, causing backfires in the engine. 

lawn mower engine

When this happens continuously, it may disrupt the engine timing and cause disruptions on the valve. Using the mower in the summer can also prompt a hot combustion chamber that damages the engine, leading to incomplete combustion. 

Solution

Conduct an engine tune-up, which will restore the engine and valves to their correct timing settings. After a tune-up, this will re-align your valves to open and close while preventing fuel from getting burned. 

Read Next: Riding Mower Won’t Move Forward or Reverse

Using Special Equipment to Avoid Backfires

Some mowers have special parts as a preventative measure for backfire incidents. These are anti-after solenoids [1], allowing you to turn the engine off at any speed without excessive fuel shooting into the muffler. 

To troubleshoot a faulty solenoid, you’ll have to require help from a professional, so you must add a licensed repair facility to your directory. 

FAQ

Will a backfire damage the mower's engine?

A backfire caused by a fuel issue can damage a mower’s engine. Even with anti-after-fire solenoids, your engine will still malfunction if the engine is misaligned and there’s too much air or fuel. 

Damage to the engine can also happen when the air-fuel mixture combusts outside the engine cylinders. When the fuel is too lean, there will be excess air in the gas, causing incomplete combustion. 

Conclusion

When a lawn mower backfire takes place, you’ll automatically think there’s something wrong with the machine. And even with a small engine, you’ll think you need a professional. 

But whether you take it to a professional or try the solution yourself, you’ll have to first understand why does your lawn mower backfire by observing when it happened. This way, you can diagnose the issue correctly yourself or before you take your mower to a technician.  

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Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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