Gas Lawn Mower Starts Then Dies (15 Causes + Fixes)

A lawn that might only usually take half an hour to cut can easily turn into an hour of sweat and tears, or at least sweat and several curse words. This is especially true when your mower starts then dies repeatedly. So, what is causing your lawn mower to keep stalling? Well, there are lots of potential issues that could make your lawnmower act up. Let’s check out the list of issues and go over how to fix your stalled lawnmower.

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Gas Lawn Mower Starts Then Dies (Overview)

There are several potential reasons that your lawn mower starts then dies. This is because a gas-powered engine relies on many different things working together simultaneously to achieve effective combustion. Any single problem or a combination of many issues can cause a lawn mower to start then die. Here are 15 examples:

Bad Gas
Faulty Choke
Bad Spark Plug
Bad Ignition Coil
Clogged Fuel Tank
Clogged Fuel Filter
Clogged Fuel Lines
Clogged Air Filter
Dirty Carburetor
Carburetor Adjustment
Blocked Gas Cap
Failed Cooling System
Engine Oil Level
Engine Oil Condition
Overworked Lawn Mower

Lawn Mower Starts Then Dies (What to Look Out For)

Many combinations of problems can cause your lawn mower to start then die. So, there are various symptoms you might notice when running your lawn mower. Here are a few examples of what to look out for:

Engine Has Trouble Starting
Engine Lacks Power
Engine Runs Rough
Engine Only Runs For a Short While
Mower Overheats, Then Stalls
Mower Only Runs With The Choke On
Black Smoke or White Smoke From The Exhaust

15 Reasons Your Lawn Mower Starts Then Dies (A Closer Look)

The conditions needed to start an engine differ from those needed to keep an engine running. As the load and engine speed increase, the engine needs more resources. This is why you can typically start your lawn mower for a few seconds before it dies. So, let’s look at each factor that can cause your lawn mower to start but not stay running.

Bad Gasoline

Does gas go bad? Yes, it does. Unprotected gasoline has a shelf life of about thirty days until it loses its ability to combust. After this period, your lawn mower will struggle to ignite the fuel vapor in the cylinder head. You’ll notice that your lawn mower will struggle to start, perform, and quickly stalls.

Faulty Choke

A lawn mower’s choke increases fuel flow into the carburetor and provides a rich fuel-to-air mixture. The choke is only designed to start a cold engine and should be switched off once the engine warms up. Therefore, if the choke isn’t ON, you’ll have trouble cold starting your mower. But if your choke fails to switch OFF your lawn mower will have trouble running. In other words, your lawn mower will start then die. This problem can be caused by forgetting to manually switch the choke OFF or if your lawnmower’s auto-choke fails to switch off.

Bad Spark Plug

Spark plugs can fail in a number of different ways. For example, they can fail to spark at the right time, or the sparkplug gap might be set incorrectly, causing a bad spark. Both of these issues result in the spark plug not delivering the ignition spark at the right time and strength. If the spark plug struggles to spark, then your mower will die. So, test your spark plug and re-gap it if needed. Failing this, you should install a replacement spark plug.

Bad Ignition Coil

For spark plugs to work, they need a charge from the ignition coil. For the ignition coil to work correctly, it needs to be in working order and gapped to the flywheel correctly. Typically, a bad coil will provide an unsteady charge to the spark plug, which can cause your lawn mower to stall. So, you’ll need to test the coil and check the gap distance.

Clogged Fuel Tank

When starting, the engine needs less fuel than when it’s up and running. A clogged fuel tank will prevent fuel from flowing at the needed flow rate, and your lawn mower will stall once it runs out of fuel. To fix this problem, you’ll need to drain your fuel tank and then clear the blockage.

Clogged Fuel Filter

You need to replace fuel filters once they become blocked with fuel impurities and junk that has made its way from the fuel tank. The fuel flow will slow if you don’t swap out the fuel filter once it’s clogged. So, just like a blocked fuel tank, your lawn mower will likely start then die once it stops getting enough fuel.

Clogged Fuel Lines

Fuel lines connect the many parts of the fuel delivery system. If junk or fuel gum/varnish builds up in the fuel lines, the fuel flow will slow. So, to correct the fuel flow, you’ll either need to clean the fuel lines or replace the fuel lines.

Clogged Air Filter

A clogged air filter reduces the airflow and alters the air-to-fuel mixture. When this happens, the carburetor will run rich and the engine will struggle and stall. So, either clean the air filter or install a replacement.

Dirty Carburetor

At the end of the fuel system is the carburetor. The carburetor is responsible for mixing fuel and air into a precise mixture so that fuel vapor is jetted into the cylinder head through a tiny jet. If this jet becomes blocked, fuel can’t pass through, and the engine will be starved of fuel. So, while there is enough fuel flow to start your mower, there’s not enough to keep it running. So, you’ll need to clean the carburetor and jet to restore the fuel flow.

Carburetor Adjustment

Depending on your mower’s design, the carburetor may be adjustable. This means you can set the carburetor’s high, low, and idle adjustments. Your lawn mower can run rich or lean if these adjustments are not set correctly. As a result, your lawn mower may start, but it will probably stall if the adjustment is wrong. So, to adjust the carburetor to the correct setting, you’ll need to set the adjustment screw to the correct settings.

Blocked Gas Cap

Gas caps on lawnmowers are usually, if not always, vented. This is because air needs to replace gasoline as fuel is consumed. So, if air can not enter the tank as needed, back pressure is created in the fuel tank, preventing fuel flow to the engine. The venting process won’t be achieved if the vent becomes blocked or damaged. So, inspect the vent and clear the tiny hole or replace the gas cap.

Failed Cooling System

Lawnmowers depend on either water or air cooling systems to disperse heat from the engine. So, if the cooling system is low on fluids or has blocked cooling fins, your lawn mower will sputter. This is one of the common symptoms of an overheating lawn mower engine. As your lawn mower gets hotter, it will eventually overheat and stall. Therefore, you should check the cooling fluids or the fin on top of the flywheel. A failed lawn mower cooling system is critical since overheating can cause extensive damage to the engine.

Engine Oil Condition

Old oil has less lubricating capacity than fresh oil. Your engine’s internal parts expand and create more friction as the engine runs. If your engine runs on old oil, the lack of lubrication will cause your lawn mower to overheat and stall. To fix this issue, replace the oil filter and oil as soon as possible.

Engine Oil Level

Not having enough oil in your lawn mower will again cause overheating because there isn’t enough oil to lubricate the engine. On the other hand, too much oil can create too much oil pressure, leading to problems such as piston damage. Therefore, you should check your engine oil level and make adjustments as necessary, including draining oil from your lawn mower if needed.

Overworked Lawn Mower

The last item on the list of reasons why your lawn mower starts then dies is overworking your lawn mower. For example, cutting very long, thick, or wet grass can cause your mower to work too hard. When this happens, your lawn mower may bog down and physically stop dead in its tracks or overheat and stall. Either way, your mower will start just fine but stall once the work becomes too much. So, try reducing the workload by mowing more regularly, overlapping your cuts, or raising the deck height. These methods should reduce the load on your lawn mower and prevent it from stalling.

How to Fix a Lawn Mower That Starts Then Dies (15 Fixes)

Now, let’s look at each of the individual fixes to repair your lawnmower if your engine keeps stalling. These fixes cover all fifteen potential reasons why your lawn mower starts then dies.

Replace the Gasoline

You’ll need to drain the gasoline from your lawnmower’s gas tank to replace the gas. This can be done by tipping the old fuel out of your lawn mower or using a fuel pump/siphon. Usually, you can tip a small lawnmower over to pour the fuel into a fuel-safe container. For larger lawnmowers like riding mowers or zero-turns, you can use a fuel pump/siphon to remove the bad fuel via the filler can. Alternatively, you can remove the fuel line from the bottom of the gas tank and allow the fuel to drain. But this does demand additional work. Personally, I would use a fuel transfer pump.

Tools to Remove Fuel From Your Lawn Mower

Repair the Choke

Lawnmowers typically use an automatic thermostatic choke that disengages once the engine warms up to a certain temperature. A small metal bar connected to the exhaust pipe moves once the engine warms up and closes the butterfly plate inside the carburetor. Look around your exhaust pipe and find this small metal bar. If you start your lawnmower from cold and allow the engine to warm up, you should see the bar begin to move and close the choke. Usually, the problem is that the bar has been knocked and can no longer complete its operation. So, use a pair of pliers to bend the bar by hand so the choke can close correctly.

Tools to Adjust the Thermostatic Autochoke

Pliers

Replace the Spark Plug

Start by locating the spark plug in the cylinder head.
Remove the attached cable from the ignition coil.
Use a spark plug wrench to remove the old spar plug.
Thread your new spark plug into the cylinder head by hand until you cannot turn the spark plug.
Using your sparkplug wrench, lightly tighten the spark plug. Be careful not to over-tighten the plug as they can easily shatter.
Finally, reconnect the cable.

Tools to Replace a Spark Plug

Spark Plug Wrench
New Spark Plug (Engine Specific)

Replace the Ignition Coil

When installing a new lawn mower coil, you need to make sure that the replacement coil is specific to your lawnmower’s engine. Ignition coils are not universal. Then, research the specific distance needed between the coil and the flywheel. Typically, this information is on the new coil’s packaging or available online. Following this, you can begin the coil repair.

Lawn Mower Coil Removal

Remove the engine cover
Locate the coil that is positioned next to the flywheel
Disconnect the cable to the sparkplug
Remove the ground cable from the coil
Remove the coil mounting bolts
Lift the old coil from your lawn mower

Lawn Mower Coil Installation

Rotate the flywheel so that the flywheel magnets are positioned away from the coil
Position the new ignition coil onto your lawn mower
Loosely install the coil mounting bolts
Place the feeler gauge between the coil and the flywheel
Rotate the flywheel so that the magnets line up with the new coil
Tighten the coil mounting bolts once the coil gap is set
Connect the spark plug cable and ground cable
Install the engine cover

Tools to Install a Lawn Mower Coil

Clean Out the Fuel Tank

Empty your fuel tank by either tipping your push mower or uing a syphon.
Remove the fuel tank from your lawnmower.
Block off the outflow pipe using a clamp. I usually connect a small fuel line to the tank and clamp it closed.
Add a small amount of fuel to the tank and give the tank a good shake. This is usually enough to dislodge any junk inside the tank.
Pour the fuel out into a fuel-safe container. Depending on how much junk is inside your tank, you may need to repeat this a few times.
Finally, install your tank and connect it to the fuel system by reconnecting the fuel line.

Tools to Clean Out a Fuel Tank

Replace the Fuel Filter

To replace your lawn mower’s fuel filter, you’ll first need a replacement filter that is specific to your lawn mower. Fuel filters vary in filter material rating, allowing fuel to flow at a specific rate. For example, if you install a fuel filter with a very tight filter material, you will likely restrict the fuel flow. So, getting the right filter is key.

Use a pair of pliers to remove clips securing the fuel lines to the fuel filter.
Remove both fuel lines from the old filter.
Attach the fuel lines to your replacement fuel filter.
Attach the clips.

Make sure to install the filter in the right direction, as fuel is only designed to flow in one direction. If you look closely at the side of the filter, you should find an arrow showing which way it should be installed.

Tools to Install a Replacement Lawn Mower Fuel Filter

Pliers
New Fuel Filter (Mower Specific)

Replace the Fuel Lines

The first thing you need to replace your lawn mower fuel lines is a new line. You should be able to pop into a Home Depot or your local lawn mower repair store and purchase the exact length of the replacement line. Also, I suggest taking your old line with you to compare it with the new one. Many different lines are available, and it’s easy to grab the wrong one.

Remove the old damaged lines and install the replacement.
Remove the clips using a pair of pliers.
Remove the fuel line.
Remove the spring clips and set them to one side.
Measure the new line against the old one and use a pair of pipe cutters or a sharp knife to cut the new line to length.
Position the spring clips onto the new fuel line.
Attach the new fuel line to your lawn mower.
Place the spring clips to secure the new fuel line.

Because you have just installed a new fuel line, the fuel delivery system will be full of air, so starting your lawn mower will take a while. But after a few pulls of the pull cord or turns of the key, your engine should fire up once the air has been released.

Tools to Replace Lawn Mower Fuel Lines

Pliers
Hose Cutter
New Fuel Line

Cleaning & Replacing the Air Filter

Replacing your lawnmower’s air filter should only take a minute or two. All you need to do is remove the filter cover, remove the old filter, then install your replacement and replace the cover. It’s that easy.

But if your lawn mower air filter only looks dusty, you can try cleaning it. I always clean my lawn mower’s air filter a couple of times throughout the year instead of just replacing it. So, if you have an air compressor with an air gun attachment, then you can blast most of the dust out of your air filter.

Just be aware that you need to blow the dirt out of the filter and not into the filter. In other words, blow the dirt from the inside to the outside. This is the opposite direction that air would flow into the filter when in use.

Tools to Clean a Lawn Mower Air Filter

Clean the Carburetor

To clean your lawn mower carburetor, you can either remove the carburetor jet or use a carburetor cleaning spray. Let’s take a quick look at both options.

Cleaning A Lawn Mower Carburetor Without Removing It

You can use a cleaning spray directly into the carburetor via the air filter housing to clean a lawn mower carburetor without having to remove or disassemble anything:

Locate the carburetor and the attached air filter housing.
Remove the air filter housing cover.
Remove the air filter.
Start your lawn mower.
Spray the carburetor cleaner through the air filter housing into the carburetor.

Give the carburetor a one or two-second blast of cleaner. Just make sure that you don’t spray too much, as the cleaner will cause the engine to stall. When you spray the cleaner, the engine RPMs will drop until the cleaner burns off. Then, when the RPMs rise again, you can give the carburetor another spray. I typically give my lawn mower five or six sprays in total.

Tools to Spray Clean a Lawn Mower Carburetor

Carburetor Cleaning Spray

Cleaning A Lawn Mower Carburetor Jet

Getting your lawnmower jet out of the carburetor requires several different steps. So, to make things a little easier, I have listed all the steps.

Steps to Remove a Lawn Mower Carburetor Jet

Remove the carburetor fuel cup.
Remove the fuel float mounting pin.
Remove the fuel float and attach the float pin.
Remove the jet securing bolt.
Remove the carburetor jet.

Steps to Clean a Carburetor Jet

Once you remove the jet, you should be able to hold it up to the light and see through the jet hole. This is where fuel is forced through to create the fuel-to-air mixture vapor. If a clogged jet is why your mower starts then dies, you’ll probably see dirt or very little light. So, you need to clear the blockage and clean the jet well.

Use a pin or a thin metal wire and poke out any debris causing the blockage.
Use your carburetor cleaning spray to remove any fuel gum or residue.
Reinstall the jet using the five removal steps in reverse order.

Tools to Clean A Lawn Mower Carburetor Jet

Adjust Lawn Mower Carburetor Settings

Since there are so many different types of carburetors, there isn’t one process that will fit all situations. Some carburetors are factory-set and do not allow for any adjustment. Also, some lawnmowers don’t actually have throttles that allow you to control the RPM, so there is no way to adjust the idle speed.

However, you can make adjustments if your lawn mower has both an adjustable throttle and carburetor adjustors. To adjust the idle speed, you need to turn the idle screw to either increase or decrease the engine speed.

Set the throttle to idle.
Adjust the idle to a speed where the lawn mower isn’t going to stall or rev too high.
Then increase the throttle to maximum and adjust the high screw so the engine doesn’t under or over speed.

Some carburetors will tell you how many turns the adjustment screws need. But a lot of the time, it’s a case of trial and error. If you take your time and listen to the engine, you should be able to set the carburetor to the right settings.

Tools To Adjust a Lawn Mower Carburetor

Carburetor Screwdriver

Clear the Gas Cap Vent

Gas caps can get damaged when you’re out mowing. Usually, the tiny vent gets knocked, closing off the vent. You should be able to re-open the hole using a pin. I use a hook tool set, as the plastic can be pretty tough. So, a regular little pin might not be strong enough. When you re-open the hole, be careful not to make the hole any bigger, as gas may leak out of the cap.

To check if the gas tank is creating back pressure, remove the gas cap and listen for a rush of air. This is the tank releasing the pressure. To fix this, repair the vent to allow air into the tank as gas is used. Alternatively, you can replace the gas cap entirely.

Tools To Repair a Gas Cap Vent

Hook Tool Set

Repair the Cooling System

I will break the process down into water and air systems to simplify things. These repairs will allow your lawn mower to keep the engine temperature within working limits.

Water Cooled Systems

Water-cooled systems normally have a coolant reservoir, water hoses, and a radiator. Unless one of these components has failed, the problem will likely be low cooling fluid and/or clogged radiator fins.

Locate your coolant reservoir.
Check the level using the indicator on the side.
Top off the fluid if the coolant is below the “Low” indicator.
Locate your lawn mower radiator and inspect the fins to see if they are clogged. If clogged, air won’t be able to pass through the radiator and disperse the heat.
Blast out the debris with your air gun and a compressor.

Tools to Repair a Water-Cooled System

Air Cooled Systems

Typically, the cooling fins are on top of the flywheel under the pull cord.

Remove the recoil starter mechanism and the engine cover to get to the cooling fins.
Use your air compress and air gun attachment to blast out any junk.
Once the fins are free of debris, reinstall the recoil starter and engine cover.

Tools to Repair an Air-Cooled System

Check the Oil Level

Check the oil level when your lawn mower is cold so you don’t get a false read.

Locate your lawn mower dipstick and remove it from the engine.
Use a clean shop rag and wipe off all the oil from the dipstick.
Return the dipstick to the engine, ensuring that the stick is fully seated back into its original position.
Finally, remove the dipstick again and inspect the oil level.

The end of the dipstick should have hatched markings. Your lawn mower is fine as long as the oil level sits within this hatched area. You need to top off the oil if it’s below the hatch marking. So, add a small amount of oil and recheck the oil level using an oil funnel. Repeat adding oil and checking the level until you are happy you have enough oil in the engine.

Tools to Check & Top Off Lawn Mower Engine Oil

Change Lawn Mower Engine Oil

Depending on your lawn mower’s design, you can use different methods to remove the engine oil. For example, you can usually tip over small lawnmowers to pour out the engine oil. In contrast, larger lawnmowers, such as tractor-mowers or zero-turn mowers, need the oil drained from the engine’s sump. So, let’s take a look at both methods.

Walk-Behind Lawn Mower Oil Change

To change the oil in a 4-stroke walk-behind lawn mower, pour the old oil out by tipping the lawn mower upside down. This can vary between lawnmowers, but typically, you tip your lawnmower over with the oil filer cap facing down and the fuel filler cap up. Also, if you can, empty the gas tank so fuel doesn’t spill. If this isn’t possible, remove the gas cap, place a ziplock bag over the gas tank opening, and then screw the gas cap back on. The ziplock will prevent fuel from spilling out of the gas cap vent. So, let’s take a look at the steps.

Steps To Change Oil on a Walk-Behind Lawn Mower

Ensure the engine is cold.
Remove the oil filler cap.
Place an oil catch pan next to your lawn mower.
Tilt over your lawn mower.
Catch the old oil in the oil catch pan.
Allow all old oil to drain.
Upright your lawn mower.
Add fresh oil to the engine (see owner’s manual for oil volume).
Check the oil level.
Top off as required.
Drain excess oil if the engine is over-filled.
Replace the oil filler cap.

Tools to Change Oil on Walk-Behind Lawn Mower

Riding Lawn Mower Oil Change (Inc. Oil Filter)

Changing the oil on a riding mower takes several steps. I have outlined each step in a bullet point, which will make following along much easier. Let’s take a look.

Steps to Change Oil on a Riding Mower

Ensure the engine is cold.
Apply the parking brake.
Choke the wheels.
Remove the spark plug cable.
Place an oil catch pan under the engine’s sump.
Remove the oil drain plug from the sump.
Allow the oil to begin to drain.
Remove the oil filler cap to increase the flow of oil.
Once the oil has stopped flowing, remove the oil filter.
Install the oil drain plug.
Coat the new oil filter gasket with fresh oil.
Install the new oil filter.
Add fresh oil to the engine (see owner’s manual for oil volume).
Check the oil level.
Top off as required.
Drain excess oil if the engine is over-filled.
Replace the oil filler cap.
Remove the chokes.
Attach the spark plug cable.
Wipe off any oil spills.

Tools to Change the Oil on a Riding Lawn Mower

Adjust Your Mower Cut Height

If your lawn mower struggles to mow and overheats, it could be as simple as adjusting your lawn mower’s cut height. For example, if you have too much grass in the cutting deck at one time, your lawn mower engine may not have enough power to clear the deck and cut.

So, you can raise the cutting deck to reduce the load on your lawn mower. This lessens the volume of grass in the deck and allows your lawn mower to mow efficiently. If your lawn mower is still struggling, say because the grass is very long, try cutting half a strip at a time. This will reduce the load on your lawn mower by half.

Another way to help your lawn mower and stop it from stalling is to remove the grass clippings bag and allow the grass to discharge on your lawn. How does this help? The clipping bag reduces the airflow in the cutting deck, keeping the grass in the deck for longer. So, if your mower is struggling, the quicker you can get the clippings out of the deck, the better.