Why my lawn mower backfire is a recurrent question several turf owners tend to ask. It is a pop-like noise that comes from the mower because of ill-times combustion in the engine.

Backfire Reasons of a Lawn Mower

If this issue has happened to your mower more than once, it indicates an underlying problem with your machinery. Read this guide to learn why a lawn mower backfires and what you would do to get rid of it.

Why Does My Lawn Mower Backfire

💥 Quick Answer

Lawn mower backfires because of lowering the speed of the engine too fast, using high fuel and alcohol blend, a compromised spark plug, the engine temperature may be high. Additionally, due to sheared flywheel, problematic carburetor, water contamination, or problematic valve. Moreover, cutting damp grass, or a bumpy lawn.

– Lowering the Speed of the Engine Too Fast

If your lawn mower starts backfiring, then the reasons could be reducing the engine speed too rapidly, the latter is the main reason engines mostly backfire.

The engine tries to align with the decreased throttle, because excessive air is pulled into the gasoline mix, which makes it lean. As this lean blend is forced into the lawn mower’s exhaust system and combusts, a characteristic loud pop is heard, which is the backfire.

That is why you should always decelerate the engine slowly and gradually. We understand it might be tempting to turn the engine off after a day’s hard work. Doing so, however, would lead to extra air being pulled inside and creating an unnecessarily loud noise.

If your mower has a throttle that reduces speed, you must let it up slowly instead of releasing it all at once. Reduce the speed slowly and let the engine run idle for at least 20 seconds before completely turning it off.

– Using a High Blend Of Fuel With Alcohol

When you use sub-quality fuel, it would be is a key reason for backfiring in small engines. Gasoline that has a high quantity of alcohol or ethanol making the fuel blend is responsible for the bakcfire and also generally it is not looked upon for the health of the lawn mower in the long term.

Using a Fuel with Alcohol in Mower

Lawnmowers have been designed to work efficiently using a certain quality of fuel mix, which means that if you would add the wrong blend it can mess with the mower engine, and it will struggle to burn properly. Hence, you will experience occasional power losses and backfiring regularly, because of the source of energizing the machine.

The best way to fix this is to use pure fuel with zero levels of ethanol in it. We understand that gasoline this pure is not easy to procure and also not cheap either. Try to go for a fuel that has the lowest possible levels of alcohol in it, so that the engine would last longer and do its job perfectly.

– A Compromised Spark Plugs

If the spark plug of the lawn mower engine is compromised, then only a weak spark will be created, leading to constant backfiring. Gasoline is not ignited in the fuel chamber; instead, it ignites when the gases reach the muffler chamber. 

Spark plugs can get defective often, but luckily this aspect can be checked to see if it really is the reason for the backfire. As you go through your mower’s instruction manual to find out where its spark plug is located, it is often located behind the air filter, and you will need to unscrew it to gain access.

Once you take the spark plug out, see why it might be giving you problems. Over time the plug becomes coated with grease and gas residue, which prevents it from igniting a spark. 

Sometimes the spark plug wire needs to be connected properly, and only a little adjustment would help prevent the lawn mower from backfiring.

On the other hand, you must also note that if the plug or the wire has completely short-circuited, then it must be replaced by a new one. Don’t worry; spark plugs are available at all hardware stores at very low prices.

– Engine Temperature is High

If your backfiring lawn mower gets hot when you use it, that is where the concern lies. When the lawn mower engine is forced to run at a higher temperature than normal, it starts to stall and backfire at regular intervals.

That is why most lawnmowers, especially those with a small engine, have a separate space to allow adequate airflow. This exchange of air is imperative to cool down the engine while running. As you feel that the engine started to raise its temperature, what you are required to do immediately is to turn the mower engine off to allow it to cool down.

Checking Engine Temperature of a Mower

However, you still need to get the airflow into the engine fixed so that this backfiring doesn’t become a recurrent problem.

You cannot do engine modification at home unless you have good experience with mechanics. The people over at your local workshop, petrol pump, or hardware store might be able to help you with this. 

– A Sheared Flywheel

If your riding lawn mower backfires immediately upon starting, the problem lies in its internal components, a flywheel is the most commonly damaged part of the engine and might be the culprit here.

To elaborate further, the flywheel is a heavy, round wheel that is attached to the rotating shaft and responsible for the smooth transition of power from the motor to the blades. If anything comes in the way of its seamless rotation, your mower will backfire even if it has a smooth start. 

Sometimes the crankshaft gets damaged and causes lawn mowers backfire. It is the backbone of your engine’s combustion system and is sensitive. 

However, unlike spark plugs which are easily available and replaceable, flywheels, crankshafts, and other internal components will need professional help.

For the adjustment of the latter you may contact a manufacturer and look into the matter further and see if they provide any repair services. If not, then your local mechanic will surely know what to do.

– A Problematic Carburetor

The carburetor in the mower is responsible for letting adequate air and gas into the combustion chamber for carrying out combustion. If there is a problem with the carburetor itself, then the balance between air and gas is disturbed, and this leads to backfiring during both the working of the mower and its deceleration.

Replacing Problematic Carburetor

A carburetor that isn’t adjusted properly will often have this issue and produce a lean mixture of gases. However, you must note that the latest carburetors come with two types of adjustment screws that are mounted on the exterior: one of the screws control the mixture intake, and the other is for smaller adjustments.

You can tell that the concern lies in the carburetor because the mower will backfire under all conditions. In addition, you may adjust a loose carburetor by emptying it and tightening the screws with no hassle. However, if you do no wish to compromise your handiwork, then, it is safer to take help from a professional instead.


– Water Contamination

Water contamination will cause engine backfire and loss of power in most lawnmowers. There might be other signs of water contamination that might have missed your notice. While working normally, it might begin to stall and stutter altogether.

The combustion of the fuel mixture in the chamber does not occur properly, and you may even  start to notice reptiles like snakes coming out occasionally. If you have been using your mower in the rain, this is the most likely cause of such contamination, because they would get into your lawn mower for their safety.

Remove the spark plug to gain access to the carburetor to treat contamination. Attach a hose to a container to drain all the fuel out of the carburetor first, and then you should take the carburetor out fully and clean it thoroughly. 

Allow the carburetor and the other parts to dry fully before you put them back together. Swipe a tissue paper across the inner surface of the carburetor to confirm there is no moisture left before refilling it with fresh gasoline that’s lower in alcohol blend.

– Problematic Valves

Valves are an essential component of lawnmowers that need to open and close efficiently for mixing gas for combustion. They help to seal off the firing chamber so that combustion can take place in it. 

Checking Valves of a Mower

Note that one valve is primarily responsible for letting gas and air into the cylinder, and the other allows exhaust gases and dirty air to exit the cylinder. However, over time, it is natural for the valves to start having problems opening and closing, especially if you don’t oil your machine’s components frequently. 

However, you shouldn’t try fixing the valves yourself, especially if you aren’t qualified enough. Messing with the internal parts of the mower’s machinery by an inexperienced person might lead to the development of more issues.

– Cutting Damp Or Wet Grass

Lawnmowers should not be used to cut wet or moist grass blades. This can create serious problems for the machine and its parts over repeated uses, as moisture gets into the machine and starts to get problematic in different ways. 

Hence, we do not recommend watering the lawn the day of the mowing before you have begun cutting grass. Wait to cut grass early in the morning until the dew drops all dry up. 

Similarly, it is not a smart move to cut grass in the rain. Doing so will also lead to the contamination of various machine parts with water, and they might even start to develop rust, which would lead to new concerns. 

– Using Mower On A Bumpy Lawn

Mowing an uneven and bumpy lawn can seriously damage the internal and external parts of the lawn mower machinery because of the unsteady motion it is being run on.

Mower on a Bumpy Lawn

Even if your mower is large and sturdy, this does not mean that it should be used in a lawn strewn with rocks, pebbles, and random twigs and branches, it is getting away from its normal way of functioning.

Some less expensive and older models do not have enhanced safety features that enable them to resist riding over hard surfaces. Their internal parts are more prone to damage, and you will soon begin to experience recurrent backfiring.


– Why Does The Lawn Mower Backfire Immediately After It Is Turned Off?

A carburetor is a problem If the lawn mower backfires right after it is turned off. The carburetor might get clogged up due to gas and grease, or its screws might be undone. A clogged muffler can also cause this problem. That is why you must check both the muffler and the carburetor separately.

– Can Continuing Backfire Damage Mower Engine?

If engine backfire is not corrected, it can damage the engine over time and decrease fuel efficiency. 


Finally, you have made it to the end of this comprehensive lawn mower guide

  • The number one reason a mower won’t start and backfire is rapidly decelerating the engine.
  • Try to use gasoline with minimum levels of ethanol; otherwise, the engine will backfire.
  • If the engine gets hot quickly, it could lead to loud backfires until it cools down.
  • Using a mower on hard and rocky surfaces will derange the internal components, producing backfiring.

Now that all the possible reasons for lawn mower backfiring have been discussed, you are better equipped to see why your mower is undergoing this problem. As you read this guide, you now know why the backfire is taking place.

5/5 - (19 votes)