It’s a typical week and you’re out mowing the lawn again. It’s all going well until the mower stalls and dies. This can happen when the mower doesn’t get the spark, fuel, and air it needs to run or the engine is under load.
Keep reading and I’ll go over the items that can prevent your mower from continuing to run and perform as it should.
A lawn mower will keep shutting off and stall when it isn’t getting fuel due to old fuel, an insufficient fuel level, a clogged fuel line, a plugged fuel filter, or a bad gas cap. It can also stall when the engine isn’t getting air due to a plugged air filter or the wrong choke setting.
Additional items like a bad spark plug, bad safety switch, or plugged mower deck can contribute to a stalling problem.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
12 Reasons a Lawn Mower Stalls and Shuts Off
Many times the result of a lawn mower running sluggish and stalling is due to a lack of a sufficient fuel supply. This can be due to running old fuel through your mower. Old gas is known to gum up and corrode the fuel system.
Gasoline can begin to break down as quickly as 30 days after purchase. Most gasoline sold today has an added fuel called ethanol. It’s added to traditional gasoline to make fuel more environmentally friendly.
Ethanol is often created in the United States using corn, but other high-sugar and starch plants can be used.
Ethanol is okay to use in most vehicles, but it is not good for the small engine in a lawn mower. Because of this, it’s best to use unleaded gasoline with no more than 10% ethanol content in gas-powered lawn mowers.
Ethanol, being a plant-based product, will attract moisture out of the air. Water is corrosive to the fuel system.
This water and ethanol mix can leave behind gummy deposits restricting fuel flow causing your engine to stall because it’s not getting enough fuel to keep going.
Solution: If you find your fuel is old, drain the fuel tank using a fuel siphon pump to move the fuel from the lawn mower’s fuel tank to a container for proper disposal. Refill with fresh gas for your lawn mower.
Add a fuel additive like Sea Foam Motor Treatment to stabilize the fuel, reduce moisture and clean the fuel system.
Read more about the right gas to use in your lawn mower here. If you have a mower with a 2-cycle (2-stage) engine, refer to this article as the fuel requirements are a little different. 2-cycle engines require oil to be mixed with gas.
Insufficient Fuel Level
Just because your lawn mower isn’t out of fuel doesn’t mean it won’t stall because of a lack of fuel. Running your mower along a hillside or uneven ground can affect the fuel supply.
Solution: If you notice your mower stalling when you are running it over uneven ground or along slopes, add more fuel to the tank. As always, be careful running your mower along a hillside and avoid steep inclines as it poses a safety hazard.
Incorrect Choke Setting
The choke is engaged to start a lawn mower when the engine is cold. The choke restricts airflow so the engine receives a higher concentration of gas than the air normally required. Once the engine heats up, the choke must be adjusted so the engine is getting enough air to keep running.
Solution: Check the choke lever to make sure it has been moved to the correct position so it is no longer restricting airflow causing the mower to stall after the engine warms up.
Plugged Air Filter
Another item that can restrict airflow to the engine is the air filter. When the air filter is not replaced or cleaned regularly, it can become plugged with grass clippings and dirt.
Air isn’t able to pass through a dirty plugged filter as it can through a clean filter. Again, when an engine can’t get enough air, it can cause your mower to stall and shut off.
It’s important to keep the air filter clean. Not doing so can cause significant engine damage resulting in a large repair bill. Regularly check and clean your air filter.
Replace it if it is not in good condition or not able to be cleaned. NEVER run a lawn mower without an air filter, not even for a short while to finish mowing.
Solution: Replace your air filter annually and clean it several times throughout the mowing season.
Clean a lawn mower paper air filter:
- Remove the air filter from the air filter housing. The air filter housing cover is typically attached by clips, knobs, or wing nuts.
- Wipe out any dirt you find in the housing. Be careful to not allow dirt to fall into the air intake
- Tap your filter against a solid surface. What you are doing is loosening as much dirt as possible so it falls out of the filter.
- Hold the filter up to a light source. If you can see light shine through the filter, go ahead and reuse it. If you can’t or if you find it’s covered in oil or extremely dark in appearance, replace it with a new air filter.
- Install the filter and reattach the filter housing cover.
Clean a lawn mower foam air filter:
- Remove the foam filter from the air filter housing.
- Wipe out any dirt remaining in the housing. Don’t allow any dirt to fall into the air intake.
- Inspect the condition of your air filter. If it appears brittle, torn, or damaged, replace the air filter with a new one. If it is in good condition proceed with cleaning the filter.
- Wash the foam air filter using mild dish detergent and water to remove dirt and any oil you find on the filter.
- Rinse until the soap is removed and the water runs clear.
- Lay flat and allow the filter to air dry.
- Once dry, coat the air filter in foam filter oil. This oil helps trap dirt. The filter needs to be lightly saturated in oil, but not dripping oil.
- Install the filter and reattach the air filter housing cover.
Bad Spark Plug
A dirty spark plug can cause intermittent running issues and may cause your mower to stall. Remove the spark plug boot and remove your spark plug. Inspect it for a dirty tip, broken porcelain, or burnt electrode.
Solution: Clean a spark plug that has a dirty tip using a wire brush. I recommend replacing the spark plug if it appears very dark in color or if it is damaged.
Make sure the spark plug is gapped according to the engine manufacturer’s specifications and install it. Securely attach the spark plug boot. Running problems can be the cause of a loose spark plug wire.
Plugged Fuel Filter
Dirty fuel can damage the engine. A fuel filter is used on your lawn mower to prevent dirt and debris from entering the fuel system. The filter strains dirt from the fuel as it comes out of the fuel tank.
The filter should be replaced annually and more often if you are running dirty fuel. A plugged fuel filter will restrict fuel flow and may cause your lawn mower to keep stalling.
Solution: Replace a fuel filter by shutting off the fuel supply using the fuel shut-off valve on the mower. If there isn’t a valve on your mower, use pinch pliers to crimp the fuel line to stop the flow. Remove the filter from the fuel lines.
Install a new filter paying attention to the arrow located on the filter. The filter must be installed with the arrow pointed in the direction of your fuel flow. This means the arrow should be pointed toward the carburetor and away from the fuel tank.
Clogged Fuel Lines
Gummy deposits left behind from running old fuel can prevent fuel from flowing through the fuel lines. To find a clog in the fuel line, you will need to check each section of the fuel line by stopping and starting your fuel flow to check for good flow from each section of the line.
You can control the fuel flow using the fuel shut-off valve located at the bottom of the gas tank. If your mower doesn’t have a shut-off valve, pinch pliers also work to crimp the fuel line to stop the flow.
Solution: Once you find a section of the line that is clogged, remove the line from your mower. Spray carburetor cleaner into the line to help loosen the clog. Blow out the line with compressed air. Repeat until you dislodge the clog and open the line.
If you are unable to unclog the line or the fuel line is dry and cracked, it’s time to replace your fuel line.
The carburetor is an important component of your mower to keep it running sufficiently by allowing the right ratio of gas to be mixed with air to create combustion in the cylinder.
Old gasoline is known to clog the fuel jet and make internal components stick so it is no longer able to function properly.
Solution: Before you take your carburetor off the lawn mower to clean it, make sure you are getting fuel to the carburetor. Next, remove the air filter and spray carburetor cleaner into the air intake.
Start the mower to check that the engine is running well with the injection or carburetor cleaner. If it does and then shuts off or starts to run sluggishly, you may have a problem with the carburetor.
Follow the instructions in this article to clean the carburetor. If cleaning does not help or you find internal components are damaged, you may need to rebuild the carburetor or replace it with a new one.
Bad Fuel Cap
The fuel cap used on your mower is designed with a vent to allow air to pass through the cap. When the vent becomes clogged, the fuel tank will form a vacuum. This will keep fuel from flowing out of the tank and to the engine causing your mower to stall.
To determine whether your fuel cap is the problem, remove the cap, start and let your mower run. If it runs well and your mower doesn’t sputter, stall and die, reinstall the cap while continuing to run the mower.
If you can replicate the stalling problem with the cap installed for a while, chances are the cap is no longer allowing air through the vent.
Solution: Replace a bad fuel cap with a new one. You can attempt to try to clean the cap first to remove the clog, but it may not fix the problem.
Plugged Mower Deck
Not only doesn’t a lack of air, fuel, or spark to the engine cause it to stall, but also putting the engine under load can also cause it to stall. The engine will have to work harder than usual when the mower deck is packed with grass cuttings, dirt, and debris.
When the blades need to turn through a deck packed with debris, the engine is strained and will bog down. Dull mower blades with a plugged mower deck further magnify the problem.
Solution: It’s best to keep your mower deck clean by regularly scraping the deck to remove buildup. A deck scraper, metal putty knife, or wire brush works well for this.
Avoid cutting wet or damp grass as it is more prone to clumping and sticking to the deck.
Incorrect Operation for Mowing Conditions
One speed does not work for all mowing conditions. A yard that has tall, thick, or wet grass must be mowed at a slower ground speed than a dry lawn without thick and tall grass.
Trying to quickly mow a lawn in these conditions will put extra strain on the engine and result in poor cut quality.
Solution: Assess your mowing conditions and adjust your ground speed. Always operate your mower at full throttle and avoid cutting wet grass. Slow down when you hear your engine bogging down and not running strong.
When mowing tall grass, it’s best to make multiple cuts. It will take a lot longer, but the cutting results will be better and you won’t overwork the engine.
Do this by setting your mower’s cutting height at its highest setting for the first cut and then lowering the cutting height for subsequent cuts.
Bad Safety Switch
A lawn mower uses safety switches designed to keep the operator safe. The number and type of safety switches used vary by the type of mower you own.
One of these safety switches will cause the mower to stall and shut down when it doesn’t sense the operator.
This could be the safety bar on a push mower or a seat switch on a riding mower. The switch can go bad and not make a good connection which can cause the mower to keep stalling.
This problem can also be caused when the operator using a riding mower is mowing in very bumpy conditions. Bouncing around in the seat can cause the seat to not make a good connection with the seat switch and the engine will cut out.
Solution: Test a safety switch using a multimeter. You can temporarily bypass the safety switch to identify a bad switch but only do this for troubleshooting purposes.
Never operate a mower without the safety switch. Never run a mower when a safety switch is bypassed. A safety switch can save you from serious injury. You never know when you’re going to need it.
Your Lawn Mower is Overheating and Shuts Down
A lawn mower can shut down because the engine overheats. It’s important to find and fix the cause of an overheating problem early to prevent extensive engine damage. Find out more in “7 Things That Can Cause a Lawn Mower to Overheat“.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.