As frustrating as a stalling lawn mower engine can be, it’s an issue you can likely resolve quickly. If you know the root of the problem, have a few tools, and a spare hour, you can say goodbye to your lawn mower stalling issues and get on with mowing. So, let’s take the first steps and look at eight common reasons why your lawn mower keeps stalling.
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Why Does My Lawn Mower Keep Stalling?
Lawnmowers stall for several different reasons. Sometimes it may only take a few seconds for your engine to cut out, and sometimes it may take a while. Here are eight likely reasons why your lawn mower keeps stalling.
- Bad Spark Plug
- Bad Ignition Coil
- Bad Air Filter
- Bad Gas
- Dirty Carburetor
- Restricted Fuel System
- Vacuum in the Gas Tank
- Overloading Your Lawn Mower
Why Does My Lawn Mower Only Run For a Few Minutes?
That’s quite a list of potential causes why your lawn mowing might be stalling. Therefore, let’s dig deeper into each and see how these issues might affect your mower.
Bad Spark Plug
A spark plug’s main function is to produce a good strong spark with every rotation of the engine. This isn’t usually an issue with a new spark plug, but this can be difficult with an old spark plug. As a spark plug gets old, the center and ground electrodes burn down, increasing the spark plug gap and making it hard for the spark plug to ground and create the spark needed for combustion. So if your spark plug is at the midpoint where it sometimes misses a spark, then your lawn mower will likely stall.
So, you will want to test the lawn mower’s spark plug to check whether or not it’s faulty. If the spark plug is bad, then you’ll want to install a replacement.
Bad Ignition Coil
Following the cable connected to the spark plug, you come to the ignition coil. This small device produces a positive charge, which makes the spark plug spark. In order to produce the charge, the coil needs to function properly.
So what does this mean? The coil has an insulated wire wrapped around an iron core that forms an electromagnet. If the insulated wire is either damaged or the insulation is degraded, the coil will likely have a short and won’t make the necessary charge. So why does this make a mower stall? Well, the coil can work intermittently. One minute isn’t fine, and the next is not.
An easy way to check whether a coil is bad is to run a simple test. For this, you’ll need a multimeter and the coil’s resistance information. Then, all you need to do is test the resistance through the coil to see whether or not it falls within the range. If the coil is outside of its specific range, then the coil is bad and will be to be replaced.
Bad Air Filter
The air filter is the next likely culprit for your lawn mower’s stalling. An air filter can cause two different issues. If your lawn mower air filter is clogged, it will restrict the airflow into the engine, making it run rich (too much fuel and not enough air). On the other hand, if the air filter has a hole in it or is missing, then too much air will enter the engine, and it will run lean (too much air, not enough fuel). So either way, the fuel-to-air mixture will be off, and your lawn mower will struggle to run.
An easy way to check the air filter is by removing it from your lawn mower and inspecting it to see if it’s clogged or damaged. If it’s clogged, you want to clean the air filter or replace it. For a filter that is damaged, you’ll need to install a replacement filter.
Bad Gas
Does gasoline go bad and cause lawn mower problems? Yes, it does. If left unprotected, gas can go bad in just a month. As gas goes bad, it loses its combustibility and grows tiny bacteria. So, first off, gas that doesn’t want to ignite is going to be an issue. But also, fuel will oxidize and form a fuel gum or varnish that coats the inside of the fuel system, making fuel flow and fuel system operation difficult.
So how do you tell if the gas in your lawn mower has gone bad and is causing your mower to stall? Well, you can remove some of the gas from your mower and compare it to fresh gasoline. If the gas in your engine is good, it should be the same color as fresh gas. If the gas in your lawn mower is a darker yellow/brown, it is oxidized. Basically, this is the fuel gum and varnish.
So remove the bad gas from your lawn mower and fill the tank with fresh gas.
Dirty Carburetor
Even after you swap out the bad gas in your lawn mower, you still need to deal with the fuel gum and varnish. If you don’t remove it, your mower’s engine will likely struggle to get the fuel it needs to run smoothly. So if your mower is sputtering and stalling, then you need to take a closer look at the carburetor.
Fuel gum and varnish aren’t easy to spot as they’re just a sticky yellowish substance. But, if you diagnosed the gas in your mower was bad, or you haven’t cleaned your carburetor in a while, then cleaning the carburetor is going to be a good idea. Typically the fuel float can get jammed up, and the fuel jet can become blocked by bad gas, which can cause your mower to stall.
So, remove the fuel cup from the bottom of the carburetor to check the fuel floats’ operation. Then remove the carburetor jet to see if it’s blocked or not. Now it’s not just bad fuel that can cause fuel flow issues. Dirt that has managed to get past the fuel filter can also cause issues that result in your lawn mower stalling.
Restricted Fuel System
The fuel filter can restrict fuel flow through the fuel system as it slowly gets clogged by fuel impurities and debris in the fuel. Now this is expected as this is the purpose of the fuel filter. But there does come a time when the fuel filter is too blocked and needs to be replaced. So if you have a transparent fuel filter, you can simply look through the filter and check for discoloration and junk. However, if your filter is a non-transparent type, you’ll need to remember when you last replaced the filter. If it’s been a while, say more than a year, then you’ll likely need to install a replacement fuel filter.
Vacuum in the Gas Tank
As your lawn mower uses fuel, the volume of gas in the tank reduces. As the gas level drops, air takes its place, preventing back pressure. Back pressure in the gas tank prevents fuel from flowing to the engine. So, your lawn mower’s gas cap has a vent that allows air to enter the tank. If this vent gets blocked, the air won’t enter, and back pressure will cause your engine to stall.
For example, fifteen minutes or quicker after starting your mower, fuel will stop going to the engine, and your mower will stall. Then, the pressure will slowly release, and you’ll be able to start up your mower again. But the whole cycle repeats once you get your mower fired up again. Start, stall, start, stall, etc.
So how do you know if your lawn mower gas cap is venting or not? Well, there are a couple of ways you can check. The first is to visually inspect the gas cap and see if the tiny hole is blocked.
Alternatively, fill the gas tank, then remove the fuel line and allow the tank to drain into a separate fuel-safe container. If the fuel flows continuously until the tank is empty, then the gas cap is venting fine. But if the fuel stops flowing when there is still plenty of gas in the tank, then the cap is an issue. One way to confirm this is to remove the gas cap once the fuel stops flowing. If the fuel immediately begins flowing again, then you know the gas cap vent is blocked or damaged.
If the vacuum makes it difficult to remove the cap, then check out my article on how to remove a stuck gas cap.
Overloading Your Lawn Mower
Overloading your lawn mower does two things: overheating and bogging down the engine. A lawn mower can overheat for a number of different reasons, such as low oil, clogged cooling fins, and a bad air filter. But using your lawn mower to cut grass that is way too thick and long will not only get your mower overheating, but it can also stop the cutting blade dead in its tracks. For example, if the blades don’t have the power to cut through the grass and push the clipping out of the chute, then the blade can easily get bogged down and stop.
If the blade is engaged or directly connected to the engine and stops suddenly, the engine has no choice other than to stall or break something. Therefore, check what your mower is cutting and see if it’s ok or too much.
How Do I Stop My Lawn Mower From Stalling?
Hopefully, you’ve figured out why your lawn mower keeps shutting off. Now, it is time to check out the solutions. Here are eight fixes that should keep your mower running and make stalling a thing of the past. Let’s take a closer look.
- Replace the Spark Plug
- Replace the Ignition Coil
- Replace the Air Filter
- Replace the Fuel Filter
- Replace the Gas Cap
- Remove Bad Gas
- Clean the Carburetor
- Adjust Your Mowing Technique
Replace the Spark Plug
Once you have found your lawn mower’s spark plug, you’ll want to remove the attached cable (the ignition coil cable) and tuck it out of the way. Then use a spark plug wrench and remove the old spark plug from your lawn mower’s engine. Next, thread your new spark plug into the engine by hand until you can no longer turn the spark plug. Then, use your spark plug wrench to tighten the spark plug. Don’t make it too tight, as you don’t want to snap the plug. Finally, attach the coil ignition cable to the new spark plug, and you’re done.
What You’ll Need to Replace a Lawn Mower Spark Plug
- Replacement Spark Plug (Mower Specific)
- Spark Plug Wrench
Replace the Ignition Coil
Replacing a lawn mower ignition coil is a bit more involved and takes several steps. First, you need to buy a replacement coil specific to your lawn mower and know the specific coil gap. Typically, the coil gap information is included in the documents that come with your new coil. Failing this, you can always check online. So let’s go over the steps to replace a lawn mower coil.
Steps to Replace Ignition Coil
- Remove the ignition cable attached to the spark plug.
- Remove the lawn mower engine cover/blower housing.
- Disconnect the ground cable from the ignition coil.
- Remove the coil mounting bolts.
- Lift the old coil from the lawn mower.
- Position the new coil onto the lawn mower.
- Loosely install the coil mounting bolts.
- Place your feeler gauge between the new coil and the flywheel.
- Rotate the flywheel until the flywheel magnets face the new coil.
- Allow the flywheel magnets to pull the new coil to the magnet.
- Tighten the coil mounting bolts.
- Check that the coil gap matches that of the coil specification using your feeler gauge.
- Remove the feeler gauge.
- Connect the coil ground cable.
- Connect the coil to the spark plug.
- Install the blower housing.
What You’ll Need to Install a New Lawn Mower Ignition Coil
- Screwdriver Set
- Socket Wrench Set
- Feeler Gauge
- Replacement Coil (Mower Specific)
Replace the Air Filter
Replacing a lawn mower air filter is probably one of the quickest mower service tasks. All you need to do is remove the air filter cover, pull out the old filter, and pop a new air filter back in. Additionally, you can use a shop rag to wipe down the inside of the air filter housing and cover. But other than these few steps, that’s all there is to it.
What You’ll Need to Replace a Lawn Mower Air Filter
- Replacement Air Filter (Mower Specific)
- Screwdriver Set
- Shop Rag
Replace the Fuel Filter
Again, replacing a lawn mower fuel filter is pretty straightforward and can be done in just a few minutes. Here are the steps that you will need to follow to replace your inline fuel filter.
- Using pliers, move both spring clips gripping the fuel filter down the fuel lines.
- Remove both fuel lines from the old fuel filter.
- Discard the old fuel filter.
- Orientate the filter so that the arrow points to the fuel line heading to the carburetor.
- Connect both fuel lines.
- Reposition the spring clips so that they grip the fuel lines and the fuel filter.
What You’ll Need to Replace A Lawn Mower Fuel Filter
- Replacement Fuel Filter (Mower Specific)
- Pliers
Replace the Gas Cap
If you are having an issue with your lawn mower gas cap not venting, then an easy solution is just to replace the cap entirely. These tend to be relatively inexpensive. Just remember that lawn mower gas caps are not universal, so it’s best to order an original part, otherwise known as an OME part (Original Manufacturer Equipment), to save you from getting a cap that still causes stalling.
Remove Bad Gas
Depending on what type of lawn mower you have, you may just be able to pour out the old gas. But this method won’t work for large riding mowers like tractors and zero-turns. So instead, you can use a transfer pump or a fuel siphon. Some of you probably think you could just remove the fuel line from the bottom of the tank and let the fuel drain out. Well, you can, but I think this is more work that is required. I’m all for the easy approach.
To start, place the inlet end of your transfer pump or siphon into your lawn mower fuel tank and the outlet end in a fuel can. Then just start the transfer process. Once done, you can add new fuel to your lawn mower.
To prevent your gas from going bad in the future, I recommend using a fuel stabilizer in your fuel, such as STA-BIL. These types of products prevent the process of fuel going bad and will also help keep your fuel system clean.
What You’ll Need to Remove Bad Gasoline From a Lawn Mower
Clean the Carburetor
There are a few different ways that you can clean a carburetor. Some are quick and easy, and a few are much more thorough and require a better understanding of engines and how they work. So here I want to explain the two methods you can use to clean both the carburetor fuel intake and the carburetor air intake. These are the quick and easy methods.
Carburetor Fuel Intake Cleaning
What is the fuel intake? Well, this is where the fuel passes from the fuel tank through the various components and then reaches the carburetor. Once the fuel reaches the carburetor, it then has to pass the fuel float, float needle, then through the fuel jet.
You can use a fuel additive to clean these parts without taking your mower and carburetor apart. An additive is a cleaning liquid added to the fuel tank, which passes through the entire fuel system, removing dirt, residue, and varnish.
So all you have to do is add a suitable product to your fuel tank and start mowing. As you use your mower, the additive will clean your fuel system and carburetor.
Carburetor Air Intake Cleaning
The air flowing through the air filter travels through the carburetor air intake to reach the cylinder head. For example, the air passes through the air intake, choke valve, throttle valve, mixing chamber, and then into the top of the cylinder. You can spray a cleaner directly into the carburetor to clean these parts without taking it apart. Here are the steps you can follow if you want to use this method of carburetor cleaning.
Steps to Clean the Carburetor Air Intake
- Remove the air filter cover.
- Remove the air filter.
- Start the lawn mower engine.
- Ensure the lawn mower is set to full RPM.
- Spray the carburetor cleaner directly into the air intake using a quick blast.
- Allow the engine to smoke and drop in RPM.
- Once the smoke stops and the RPM increases, repeat the spraying process.
- Repeat the process several times or until the engine performance improves.
- Once done, reinstall the air filter and air filter cover.
My Recommended Cleaning Products
- Air Intake – Sea Foam Spray Quick Acting Top Engine Cleaner
- Fuel Intake – Sea Foam Motor Treatment Multi-Use
Adjust Your Mowing Technique
To prevent your lawn mower from overheating and stalling, try to lessen your mower’s workload. For example, if you have tall, thick grass, you can set your lawn mower to a higher cut setting or double cut your lawn (mowing once with a long cut followed by a second shorter cut). Making it easier for your mower to cut through difficult grass will significantly reduce stalling. My best tips are to keep your lawn mower blades sharp, don’t let your lawn get too long, and try not to mow wet grass. All of these methods should stop your lawn mower from overheating and stalling in the future.
