15 Reasons a Lawn Mower Starts Then Dies: Lawn Mower Stops Running

Over the years I have been asked by many homeowners and professional lawn care staff about why their mowers always seem to start up and then they die.

Most of the time, this problem is an easy fix so you don’t have to continue to fight with this problem

A lawn mower may start and then die due to old gasoline, a dirty carburetor, a wrong amount of engine oil, a plugged air filter, a bad fuel cap, a faulty ignition coil, a clogged fuel line, a plugged cooling system, or a dirty spark plug.

Whether we are working with single-cylinder or twin-cylinder engines, it seems like they all happen to present the same start-up and dying problem at some point in the life of the lawn mower.

When this problem happens to you, it may seem natural and easy to just run out to your nearest lawn mower parts store and buy a new spark plug. That’s not the first place I would start.

If your mower starts, you know you’re getting spark so replacing the spark plugs may not fix anything. Read on to find the most common reasons your lawn mower dies and how to fix it.

This will address what to do when your push mower, riding mower, or zero-turn won’t stay running.

Take safety precautions including removing the spark plug wires before performing any repairs.

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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.

This is Why Your Lawn Mower Starts Then Dies

1. Empty Gas Tank

I know you know gas is required to start a gas-powered mower. I only mention it because sometimes, out of frustration, simple things get missed. Check the fuel tank for fuel and fill it with fresh fuel if you are out or low on fuel.

You may have not noticed you were getting low on gas if the fuel gauge stopped working, you developed a fuel leak or you simply forgot the last time you put fuel in the mower.

2. Bad or Old Fuel

Old fuel is often the underlying problem when your lawn mower starts then dies. Bad Fuel in your mower can cause the mower to die after running.

Gasoline usually only lasts 30 days before it starts to break down and fail. It loses its ability to run well and hot because chemicals added to fuel today start to decay or fall apart.

Most gasoline available today has ethanol in its makeup. Fuels treated with ethanol tend to collect moisture from the air.

The water can evaporate leaving a residue in the fuel tank that can clog the fuel system.

If your fuel is old, drain the fuel tank, fill the tank with fresh fuel, and add a fuel additive to stabilize the fuel and clean the fuel system.

• Fuel Stabilizer Additive
  You may want to consider adding a fuel stabilizer to your fuel. Mix the stabilizer with the fuel in a fuel can before adding it to your lawn mower’s gas tank. This will prevent the new fuel from breaking down quickly.
  
  I like using a product called Sea Foam because it has cleaning agents in it for the engine. Not only is it better for the carburetor, but also for the valve train as well.
  
  It helps keep the carbon down on top of the piston when it is in use. Read more about Sea Foam in our article Why Use Sea Foam Fuel Additive in a Lawn Mower.

• Choose the Right Fuel
  • 2-cycle engine: Requires unleaded gasoline and 2-cycle oil mix. Read more about gas for push mowers.
  • 4-cycle engine: Requires unleaded gasoline. Do not mix with oil. Find out more about the right type of gas for lawn mowers here.

3. Plugged Air Filter

I’ve had customers bring in their lawn mowers for repair because they can’t keep them running. They tell me they have checked everything and their last resort is taking it to a mechanic to be fixed.

Many times, their issue was just a plugged air filter that the owner could have easily changed.

Routine lawn mower air filter change

You should be performing regular maintenance checks on your lawn mower to keep it running at its best. Checking the air filter is one item that needs to be inspected regularly.

You can find air filter cleaning procedures here.

4. Plugged Fuel Filter

When you have dirty, old, or contaminated fuel in the fuel tank, the fuel filter can become clogged. This will keep your engine from getting a sufficient amount of fuel to keep running.

Keep the fuel filter in good condition by using fresh clean fuel. Replace the fuel filter annually to reduce fuel system-related problems due to the filter.

5. Clogged Fuel Line

Old fuel can leave behind varnish and gummy buildups that can clog the fuel lines. The fuel line can also get pinched or kinked. This lack of fuel will result in your lawn mower dying.

I have seen fuel lines clogged with leaves, pine needles, and even plastic shavings from a new fuel tank clog the fuel line causing a mower to die.

Inspect the fuel line and check for kinks. Replace any lines that are damaged, dry, or cracked. If you find a clog, remove the fuel line from the mower and spray carburetor cleaner into the line.

This is to loosen the blockage. Then blow compressed air into the line to remove the blockage. Repeat as necessary. If you are unable to remove the blockage, install a new fuel line of the same length and diameter.

6. Bad Lawn Mower Fuel Pump

If your gas tank sits lower than the carburetor, your lawn mower will have a fuel pump to assist fuel flow by pumping it to the carburetor. Most riding mowers use a fuel pump and most push mowers do not.

Over time, fuel can cause the seams of the pump or the pump itself to fail. While you can tell a pump is bad when it begins leaking fuel, internal damage will be hard to spot, so you will need to test your pump to check its operation.

To check your fuel pump, you are going to have to be able to start and stop the fuel flow. You can use the fuel shut-off valve if your mower uses one. If it doesn’t, use fuel pinch pliers to clamp the fuel line to stop the flow.

• First, check to make sure you are receiving fuel from the fuel pump.
  • With the fuel shut off, remove the line from the inlet port of the fuel pump and place it in a container. (Make sure the container is lower than the fuel supply so fuel can run downhill).
  • Start the fuel flow.
  • You should have fuel running into the container. If there isn’t fuel running out of the hose into the container, you need to look for a blockage in the fuel line and fuel filter.
  • Once you have verified the fuel flow to the pump, reattach the fuel hose to the inlet port.
• Second, check to see if the fuel pump is actually moving fuel to the carburetor.
  • Do this by removing the fuel line from the carburetor and placing it into a container to collect fuel.
  • Start your mower.
  • You should be receiving a steady or pulsating flow of fuel out of the fuel line if your fuel pump is still working.
  • If you aren’t getting fuel flow, shut off the mower and shut off the fuel valve. Make sure you don’t have any blockages in the fuel line to the carburetor.
  • If nothing is plugged it’s time to replace the fuel pump.

7. Dirty Carburetor

A dirty mower carburetor can be the cause of your mower not running. The carburetor is designed to regulate the amount of air with the right amount of fuel to create combustion.

You will need to check for the buildup of deposits left behind by bad fuel and clean your carburetor if necessary. This may sound complicated, but it really is not.

If you are not very mechanically inclined, you should bring your mower to a repair center, but if you are mechanically inclined then read on. Note: if your carburetor is in a condition that is beyond cleaning, you will need to replace it.

How to Clean Your Dirty Lawn Mower Carburetor:

• Spray carb cleaner in the air intake: Remove the air filter and spray some carb cleaner in the air intake. Start the engine to see if it will run. If your mower fires up and still won’t stay running then we need to get inside the carburetor.
  
• Gather tools needed: You will need pliers, a screwdriver, sockets, and ratchets so you to take your carburetor apart without destroying parts.
  
• Take a photo for reassembly. With all of the small parts in a carburetor, it is a very good idea to take several pictures while you are tearing down the carburetor so you can refer to them when rebuilding.
  
• Remove the throttle cable and choke cable if your mower has one.
  
• Undo the filter housing: Remove the filter housing and remove the screws and nuts attaching the carburetor.
  
• Slowly remove the springs: When removing the spring, be careful to not stretch out the springs. You may have to twist the carb a bit to get the springs off. There is a gasket located between the engine block and the carburetor you need to make sure you don’t tear it.
  
• Remove the bottom screw from the float bowl. The float bowl is located on the bottom of the carburetor. This is where gasoline is stored inside the carburetor. Have a rag ready to catch any gas remaining in the bowl.
  
  When you remove the bowl, you need to be careful not to damage the o-ring around it. This is the o-ring that looks like a rubber band. DO NOT get any carb cleaner or other substance on the o-ring as it will stretch out and you won’t be able to reuse it.
  
• Check the stem for clogged holes. This stem hangs down from the center of the carburetor and has holes in it. If these holes get plugged from old fuel it will not draw fuel up to the jet.
  
  Using a flashlight for assistance to locate the holes, take a thick wire to clean them out. Use carb cleaner to rinse out the unclogged holes.
  
• Check the carburetor for hard crusty white buildup. This white buildup is fuel additives including ethanol. You need to try to get as much of the white power material out as you can. It’s nearly impossible to get it all out.
  
• Reassemble the carburetor now that the carb is clean. Put it back together in the reverse order you took it apart. Remember to refer to the photo you took of the carburetor when reassembling so all parts are reinstalled in the right places.
  
• Mix fresh fuel with a fuel stabilizer before you add fuel to the mower. Pour the fuel mixture into the gas tank and give it a chance to fill the bowl of the carburetor. Start your engine.
  
  If you are starting with a pull cord, give the rope a yank. It may not start on the first pull, but it should start after several pulls and continue to run.

8. Too Much Oil in the Crankcase

When there is too much engine oil in your mower, your engine may smoke. It may run terribly and eventually shut down. Too much oil may create smoke causing your air filter to plug up if the engine oil can’t pull clean air.

As a result, the engine may pull air and oil from the crankcase causing the air filter to become plugged. The lack of air can cause the mower to shut down.

For more details on the damage of overfilling your engine crankcase with oil read “This is What Happens If You Put Too Much Oil In Your Lawn Mower“

9. Low Engine Oil Level

Oil is used to lubricate the internal components of your engine. When there isn’t enough oil, friction will begin to build creating a heat that will overheat the mower so it shuts down.

This heat can become so intense the oil becomes thick and the engine parts begin to melt.

If your mower shuts off due to a low engine oil level, you may be one of the few lucky ones who can add fresh oil to the crankcase to start and run your mower.

Unfortunately, most of the time a mower shuts off due to a lack of lubrication, and the engine has been severely damaged.

Have a small engine mechanic diagnose the engine damage caused by running the mower while low on oil. Depending on the age and model of your mower in addition to the cost of the repair, you may be better off purchasing a new mower.

10. Bad or Dirty Spark Plug

A dirty spark plug can foul out causing your engine to stop running. It may have produced enough spark to start your mower, but it may be unable to keep it running. Remove your spark plug and check the condition.

Look for signs of carbon buildup or cracked porcelain. You can attempt to clean your spark plug to remove dirt and reuse it. A damaged or excessively dirty dark spark plug must be replaced.

When the spark plug wire(s) is loose or the gap is not correct, it may cause intermittent running or starting problems. Make sure your spark plug has the correct gap and the wires are the spark plug wires are securely attached.

11. Choke is in the Wrong Position

Many mowers utilize a choke. A choke is used to restrict airflow to allow more fuel into the combustion chamber while the engine heats up.

A mower uses the choke to start a cold engine. When the choke is left on after your lawnmower starts, your engine receives more fuel and less air than it requires causing it to shut down.

12. Broken or Clogged Fuel Cap

Fuel caps are designed to be vented. If the vent is clogged or the cap is broken your engine may die because it cannot get fuel. If your cap won’t vent, a vacuum is formed in the tank which then doesn’t provide fuel.

Loosen your cap and start your mower to check to see if your cap may be broken. Replace with a new cap if you are able to start your lawn mower with the cap loosened allowing it to vent.

13. Faulty Ignition Coil

The winding on the ignition coil can separate and short out when a mower is hot.

When this happens, the spark plugs are unable to get the voltage they need to create spark. This can cause your mower to die after it’s been running for a while.

Identify a bad ignition coil using an ohmmeter to check for a break in continuity. Replace the ignition coil if you find there is a break.

14. Clogged or Damaged Cooling System

Cooling fins can become damaged or packed full of grass and mud. When this happens, the fins are no longer able to circulate air around the engine block to keep it cool.

When this happens, your mower can overheat and die in the middle of mowing.

Remove the engine cover and clean the cooling fins. Replace any damaged fins. Remove any dirt around the engine and engine cover.

15. Mower Dies From a Plugged Mower Deck

A mower deck that is packed with grass and other debris can cause the engine to have to work harder. Rotating the blades through a lot of debris puts an extra load on the engine.

When you add dull blades to a packed deck, you compound the problem.

An engine that is under load can overheat and shut down. Take extra precaution when working under the mower deck as the blades are sharp. Remove the spark plug wire before beginning any work.

Scrape the deck to remove all packed dirt, grass, and debris. Sharpen the mower blades.

To help alleviate this problem in the future, scrape the deck regularly, avoid cutting wet grass, and make sure you are always running your mower at full throttle when cutting grass.

In Summary – Mower Starts, Runs and Dies

Your mower starts but won’t stay running because of a fuel restriction, too much oil in the crankcase, a plugged air filter, a dirty spark plug, a clogged carburetor, or your engine running too rich.

If you have checked all of these items and are still having problems with your mower starting and then shutting down, it’s time to bring your mower to a lawn mower repair shop.