Why Does My Lawn Mower Backfire?

If your lawn mower backfires, you’ll know about it. I had a lawn mower that had this problem a few years ago, and I can tell you that the loud bang took me completely by surprise. Not “I wet myself” kind of scary, but it did make me jump. If you’re facing this problem, you’re probably wondering “Why does my lawn mower backfire?” and searching for information on what you can do about it. And that’s exactly why I’ve put this guide together for you.

Why Does My Lawn Mower Backfire?

When a lawn mower backfires, you’ll hear a rather loud bang, which results from gas being ignited outside of the normal location in the combustion chamber. This can happen when trying to start your mower, while you still have your mower running or when you’re shutting it down. Common reasons why a lawn mower may backfire include decelerating the engine too quickly, the type of gasoline that you’re using, a compromised sparkplug, the way the carburetor is set up (running too rich or too lean), and a sheared flywheel key.

I’m going to go into more detail about each of these potential causes of lawn mower backfiring below, and talk about a few fixes you can try in order to solve the issue.

When Does Your Lawn Mower Backfire?

A lawn mower may backfire at different moments while you’re using it. It could backfire while you’re trying to start it up, when you already have it running, or perhaps as you’re shutting it off.

Taking note of when your lawn mower is backfiring can be helpful, as it can help to narrow down what may be causing the issue, with certain factors more likely at certain times. Below we’ll run through each of the three different scenarios and outline the most likely cause in that situation, as well as possible solutions for you to try.

Why Does My Lawn Mower Backfire And Not Start?

If you’ve been trying to start your mower up, but have not been able to and all you see is your lawn mower spluttering and backfiring before dying, this often indicates that there’s something wrong with the engine timing. And if it’s backfiring through the carb, I’d recommend you check the flywheel out. A damaged flywheel is the most common cause when it comes to timing issues with mower engines.

How do lawn mower flywheels break/get damaged? Well, it can just be gradual wear and tear that happens over time, but typically the most common reason a flywheel breaks is when you run your mower over something like a rock and hit it. If it’s a nasty collision, it can result in the flywheel key shearing.

How can you check if this is what has happened in your case?

You will need to partially disassemble your mower, but it’s pretty quick and easy to do. Remember, we’re just checking here to see if the key is indeed sheared.

You’re going to want to disconnect the spark plug connector first to prevent any accidents. Then you’re going to need to remove any screws that are holding the blower housing in place (there will likely be 2 or 3). After that, you just need a socket wrench to remove the flywheel nut, so that you can take off the starter recoil cut. Once you’ve done all that, you’ll be able to see the flywheel key.

The key should be in line with the keyway hole on the crankshaft. If it’s isn’t, it has been sheared and is almost 99.9% certain to be causing issues with the engine timing. The spark plug will not be in sync with the engine valves, resulting in backfiring.

Solution: This is an easy one. You have no option by to replace the sheared flywheel key. The condition of the rest of the flywheel should also be checked for damage (and if it is damaged, replacing), as well as the condition of the crankshaft.

See Also: How to Tell If Your Lawn Mower Crankshaft is Bent

Lawn Mower Backfires While Running

If you can start your mower up just fine, but after it has been running for a while, you start to hear it backfire, there are a few different reasons that this could be happening. The most common one though is a carburetor that is running either too lean or too rich.

If you’re not sure what that means, it’s quite simple. The ratio of fuel to air is out of whack. The carburetor’s job is to make sure that the mix of air and fuel is just right for combustion to take place. If there’s too much air and not enough fuel, or vice-versa, backfiring can occur.

Solution: Your carb will need to be adjusted so that the right amount of gas is fed into the engine. If you’re not sure how to do this, it’s probably wise to leave it to a qualified technician.

Why Does My Lawn Mower Backfire When I Turn It Off?

This is personally when I have had issues with backfiring in the past (I later learned that it’s actually referred to as “afterfiring” which makes sense). I had mowed my whole yard trouble-free, but when I came to the end and started to shut the mower off, BANG, it backfired.

After doing some research, I found the likely cause. Essentially it’s running your mower at full pelt and then coming to an abrupt halt. You see, when you drop your engine speed too fast, fuel continues to be pumped through it, as it’s not expecting that it’s going to stop so suddenly.

However, as the ignition is now off, there is no spark from the spark plug, so the fuel mixture (gas + air) is not ignited in the combustion chamber. Instead, it makes its way into the exhaust system. The muffler inside the exhaust is very, very hot, and when the unburned fuel vapor comes into contact with it, it ignites and that’s what produces the BANG, or backfiring.

It’s actually quite logical when you think about it. Some modern, higher-end mowers that have electric fuel shut-off valves can be shut off with the engine running at full throttle, but most mowers shouldn’t be shut off in this manner.

Solution: In this case, the solution is very simple. To prevent a lawn mower from backfiring when shutting it off, you need to gradually bring down the engine speed and let it idle for 30 seconds minimum before you turn it off. This will ensure that excess fuel doesn’t make its way through to the exhaust system.

None of the Above? Here Are Some More Things to Check

There are lots of other reasons that can cause a lawn mower to backfire. The ones we’ve discussed above are good starting points, depending on when the backfiring is taking place. But if they don’t fix the issue, check out the following things, which all could potentially be the cause of the problem.

Worn or Damaged Spark Plug

In addition to gas and air, the third vital component that your lawn mower engine requires for combustion to take place effectively is a spark. This is provided by the spark plug. Over time, the condition of the spark plug can deteriorate – they often become caked in carbon and gunk – and they can either cease to work, or produce a much weaker spark.

When the spark becomes much weaker, it may not be sufficient to ignite the fuel vapor in the combustion chamber, meaning it makes its way to the exhaust, which, if hot enough, will ignite it and produce the backfire.

Solution: Remove your spark plug and check its condition. If it looks fairly clean, it’s probably not the issue, but you could replace it just in case. If it’s dirty, give it a clean with a wire brush and then reinstall it.

The Gas You’re Using

You might be thinking “gas, is gas, right?” but there are lots of different types of gas that you could use in your lawn mower. One thing you may have heard people talk about is the ethanol content in gas.

It’s widely recommended that you use a premium gas with low, or better yet, zero ethanol content for numerous reasons. One of those being that you’ll likely avoid having water in your gas lawn mower (no fun + can lead to engine damage). Another reason is to prevent backfiring.

You see, gas that has high ethanol content ignites easier, and that can lead to backfiring.

Solution: Check whether you’re already using a gas with either no ethanol or at least one with very little ethanol content. If you’re not already using one, try changing to one and see if that resolves the issue.

Final Thoughts: Your Plan of Action

A lawn mower that backfires can take you by surprise and your first tendency is to think there’s something seriously wrong. But many times, all that is required is a slight tweak to either the mower’s hardware, or the way in which you use your mower.

To quickly summarize, here are the things I suggest you check:

• Check if your flywheel is damaged – is the flywheel key sheared?
• Is your carburetor running either too lean or too rich?
• Are you decelerating the engine speed too quickly before shutting your mower down?
• If you’re using gas with high ethanol content, change to one that has low or zero ethanol
• Check the spark plug – does it need changing

I hope this guide helps you to get back to backfire-free mowing soon. Good luck!