Lawnmowers are essential household machines with which we cut our grass and keep the lawn tidy. It is typical for the machinery to make some noise while in use, seeing as they all generally come with a small engine. The noise tends to increase as the lawnmower gets old and wears out.
However, apart from this normal noise, sometimes, there can be a loud unexpected bang that can startle you and make your heart jump. This noise is called a backfire, and it can occur while you are using the machine or when turning it off. Though the backfire itself can not harm your lawnmower’s engine, it might be a sign of a mechanical fault in the machine.
Read this if you ask yourself the question why does my lawnmower backfire a lot.
The lawnmower could backfire when you slow down the engine too fast. Sometimes, the type of fuel you use can also cause a backfire. There are other causes for this issue. In this article, we explain the common reasons why your lawnmower backfires and some possible solutions.
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Why does my lawnmower backfire?
Your lawnmower backfires when hot fuel goes into its exhaust or engine, and the following can cause this to occur:
- Lowering the engine speed too fast, or suddenly turning off the machine
- Using fuel mixed with ethanol
- When the engine temperature is higher than normal
- When the carburetor is not working well
- Damaged spark plug
- When the engine valves are sticky
As we have said, the backfiring might be a sign that something is mechanically wrong.
We will examine the causes and how to avoid this problem of backfiring below:
Lowering the engine speed too fast, or suddenly turning off the machine
When you turn off your lawnmower suddenly, this can cause it to backfire. This is because the engine operates at high speed while using it.
Suddenly turning it off causes fuel to spill out of its tank into the muffler. The fuel then ignites in the muffler, leading to the backfire.
Solution: Slowly reduce the speed of the engine before turning the lawnmower off. Once you finish mowing, leave the machine on without using it for at least 10 minutes. This will slow down the engine, so you can turn it off and prevent backfire.
Using fuel mixed with ethanol
Ethanol is a type of alcohol that is used in making wine and beer. Some fuel has a mix of this type of alcohol too.
Lawnmowers, especially the riding ones, may not be designed to properly burn this fuel. Hence, the lawnmower backfires when it’s in use and can go off.
We should also add here that sometimes water can mistakenly get into the fuel tank, mixing with the fuel. Backfire can occur in this case too.
Solution: Use fuel that is pure, without any mix of alcohol. If you suspect water in the tank, empty the whole and allow it to dry. Then refill the tank with fresh and pure fuel.
The engine temperature increases above normal range
Usually, the air cools off the lawnmower’s engine. When its temperature is too high, the exhaust can spit out tiny fire sparks. The temperature can go above normal if there are debris and grass stuck in the front net.
Solution: Turn off the engine, disconnect the plugs and clean the air filter of your lawnmower.
A faulty carburetor
The carburetor’s function is to mix the right amount of air and fuel to ignite the lawnmower’s engine. A faulty carburetor might combine too much air with little fuel, and this imbalance can result in backfiring in the engine.
Solution: Adjusting the carburetor might set this right. Professional service might be required to fix the issue. However, you can do it yourself with caution and a proper understanding of the screws.
There are two screws on the carburetor that can be adjusted. One of them regulates the mixing of air and fuel, while the other is for making minor adjustments.
First, you can clean the carburetor. Then keep adjusting the screws and test the lawnmower again and again.
Note that the solenoid controls the air and fuel mix in some lawnmower brands. If the solenoid is faulty, the mower might backfire when in use.
Damaged spark plug
If the spark plug is bad, or the gaps between the electrodes are irregular, it will result in a weak spark. The fuel ignites in the muffler, instead of the engine cylinder, leading to a backfire.
If the spark plug is bad or damaged, the lawnmower can stop working at any time while using it.
Solution: Replace the spark plug
When the engine valves are sticky
Lawnmowers engines usually come with two valves. One of them transports the gas and air mixture into the combustion chamber. The other one makes way for exhaust gas to leave the combustion chamber.
A sticky valve can take a longer time to close, and gas can through it to the muffler. The gas might ignite in the muffler, causing the engine to backfire and shut down.
Solution: Fixing problems with the valves would require getting into the engine since the valves are internal components. You would have to call a professional to get this fixed.
Lawnmower backfires and does not start
If the lawnmower backfires and the engine does not start, this indicates a problem with the engine’s timing. The engine makes a coughing sound and backfires, but does not power on. Instead, it dies off.
This problem might be from the flywheel of the equipment. Check to see if the backfiring sound is coming from the carburetor. If so, the flywheel might be damaged. The flywheel can go bad as you use the mower overtime. Hitting a hard surface can also damage it.
We recommend you seek professional assistance to properly diagnose the fault in this case. But if you have some DIY skills, you can decouple some parts of the mower.
First, you need to take out the spark plug. Then unscrew the blowers’ casing. Use a socket wrench to remove the flywheel nut, so as to reveal its key. If the key is not in line with the keyway hole that’s on the crankshaft, there’s a good chance that this is the problem.
Solution: Replace the flywheel key. Use the opportunity to check that everything’s in place on the flywheel and the crankshaft. Replace or fix what’s necessary.
How to prevent or reduce backfiring on your lawnmower
- Adjust your carburetor for improved performance – Read the manufacturer’s guide to see the amount of fuel required to run your lawnmower engine. In addition, adjust your carburetor from time to time till you identify the best screw point.
- Use pure fuel — Stick to gasoline brands that contain very little or no alcohol. As we have explained, most lawnmower engines can not properly burn fuel that contains ethanol or alcohol.
- Find ways to boost the cooling of your engine – You may ask professionals or manufacturers for advice on how to raise the air quantity to reduce the engine temperature.
- Reduce the engine speed slowly – Allow the engine to slow down before turning off the lawnmower. We recommend leaving the machine on but idle between 7 to 10 minutes before shutting it down.
- Consider using an anti-backfire device – Anti backfire devices prevent the fuel from leaving the carburetor once you turn the engine off. The gasoline can not go into the muffler and cause the engine to backfire once the device is installed. It is commonly called an anti-backfire solenoid and requires a professional to fix it in your lawnmower.
Though the backfire itself can not destroy your engine, it is usually a sign that something needs to be fixed. It can result in the lawnmower shutting off and the loss of fuel.
It can be annoying and scary when your lawnmower backfires often. We have explained the most common causes and possible solutions to the problem.
Not all of them can be fixed by the user, and most of them don’t need professional attention. But ensure you reach out to a professional if the issue is related to your engine. Attempting to open the engine yourself can lead to damage and might cause serious physical harm to you.
