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

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Encountering problems with your lawn mower is frustrating. Hearing that loud bang will catch you off-guard, and now you’re wondering why it suddenly backfires. 

Given how surprising the noise can be, you might be frightened or alarmed for your safety. Here we put up a guide to relieve you and help you understand why does your lawn mower backfire. 

What Causes Backfiring in My Lawn Mower?

There are a lovely 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, we’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 you’re having trouble starting your mower and instead 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 best solution for a broken flywheel is to replace it. Check the rest of the flywheel, assess for small cracks or damages, and inspect the crankshaft conditions. These are vital parts of the machine, and getting these two checked and replaced will solve the backfiring problems.  

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, adjust the carb until it holds the right amount of gas into the engine. This may also need professional help, so we highly recommend hiring a qualified technician. 

They know the right air mixture and can assess what other possible fixes can be done to the carb. 

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. 

Our experts did a few experiments on this and found that the common reason for lawnmower backfire incident is when you’re forcing your mower to run at full speed and then suddenly bringing 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. 

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

Apart from the common problems we’ve mentioned, there are other possible issues with your machine that can cause a backfire. 

These include a compromised exhaust, faulty engine speed, damaged combustion chambers, and anti-backfire equipment that’s not working. 

Bad or Low-Quality Gas

Contrary to the common misconception, all gas is the same. It doesn’t mean that if you’re using small engines, you can toss in any available 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. There are gas types with ethanol content, and while it’s a debated topic, it’s not recommended for your lawn mower. 

lawn mower gas tank

We recommend using a premium gas with zero ethanol as it’s more efficient and will prevent you from experiencing troubles. 

Using gas with water can lead to engine damage and prompt backfires. At the same time, a high-ethanol gas can ignite easier, causing backfires. 

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. 

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 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. 

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 understand why does your lawn mower backfire by observing when it happened. 

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson

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