The sun is out, your grass is long and dry, and all the conditions are perfect for a day of mowing the lawn – only to find that your lawn mower starts then dies shortly after, or doesn’t start at all. This can be a frustrating discovery, and there are a number of different potential causes for a lawn mower that won’t start, or that stops after several minutes of use.
Some of the most common reasons why a lawn mower won’t start include issues with the carburetor, spark plugs, gas tank, oil reservoir, choke, air filter, and muffler. Read through this guide to troubleshoot the root issue of why your lawn mower is failing to start, along with how-to guides on what you can do to try and fix each issue.
Before following these troubleshooting tips, make sure you’re using the correct technique to start your lawn mower properly by following our guide: How to Start a Lawn Mower.
Carburetor is Dirty or Blocked
If your lawnmower’s gas engine is starting then stalling, it’s likely there is an issue with its carburetor. The carburetor is a part of your mower that’s responsible for mixing oxygen and gas in the right proportion to create combustion, regulating fuel flow to the crankshaft that powers the mower’s engine.
A dirty or clogged carburetor bowl disrupts the important job that the carburetor performs. Your engine may be able to start, but soon after you pull the cord, it stalls. This is due to contaminated fuel or fragments of dirt occasionally getting stuck in the carburetor, causing the contaminated fuel to pass through the compressor and stall the engine.
Signs of a Blocked Carburetor
- Engine stalls when mowing lawn
- Engine feels rough during operation
- Muffler emitting black smoke
- Mower consuming more fuel than usual
How to Fix a Blocked Carburetor
You can purchase aerosol carburetor cleaners to fix a dirty or blocked carburetor. These cleaners remove residue from the carburetor bowl using a high-pressure nozzle that sprays a substance that breaks down the carbon particles on the bowl. You can also buy a nylon cleaning brush to use alongside the cleaner to reach every corner of the mower’s engine. We recommend using the STA-BIL Carb and Choke Parts Cleaner along with this Needle and Brush Cleaning Kit to enable you to get as deep a clean as possible when unblocking your carburetor.
1. Remove the carburetor bowl and screws from the mower
Using a plug wrench, loosen the screws around the carburetor bowl and remove them along with the bowl from the mower.
2. Clean carburetor bowl, hole and screws
Use the carburetor cleaner to clean off as much residue as possible from the carburetor bowl and screws. Carburetor cleaners with directional nozzles are ideal to reach trickier corners. Clean the hole that sits underneath the carburetor using a thin wire.
3. Reattach the carburetor bowl and screws
Use your plug wrench again to reattach the carburetor bowl and screws. Make sure to not over-tighten the screws when reattaching the bowl as this can distort the seal.
Spark Plugs are Dirty, Defective or Worn Out
If your lawn mower won’t start or starts then dies soon after, you may have an issue with broken or worn out spark plugs. Spark plugs are responsible for igniting your mower’s engine by supplying electrical currents to the oxygen-gas mixture supplied by the carburetor, creating a small explosion that makes the engine produce power.
Carbon build-ups on the inside of the socket of the spark plugs and the spark plug caps can weaken them and prevent them from creating enough sparks inside the ignition chamber to power the engine. Also, over time, spark plugs end up becoming worn out and may stop functioning properly simply because they’ve reached the end of their lifespan.
Signs Your Lawnmower Spark Plugs are Bad
- You’re able to start the mower’s engine, but it quickly dies out when you try to throttle it. This is a sign that the spark plug is unable to create enough sparks for the engine to run at a high speed.
- Remove the spark plug from the mower and attach it to the inlet, then try to crank the mower’s engine. If you don’t see a tiny spark flash from the tip of the plug near the bent electrode and bottom pin, this is a sign that you need to replace the plug.
- Your lawnmower needs more fuel than usual. This is due to the wasted fuel that isn’t being ignited by the faulty spark plug, and the unburned fuel may produce white smoke.
How to Clean Lawnmower Spark Plugs
Spark plugs that are relatively new but aren’t functioning properly are most likely just dirty and can be fixed with a clean. Carbon build-ups on the plugs will prevent them from sparking, so you may only need to give them a deep clean to get them working again.
1. Remove spark plugs
Spark plugs are found at the front of the mower wrapped inside a black cable. Find them and use a socket wrench to remove them. Check the manual for your lawnmower to find the correct size wrench for your specific plugs.
2. Clean spark plugs
Clean off any carbon deposits or dirt using a wire brush and a cleaning solvent, such as this Brakleen Brake Parts Cleaner. Make sure to pay attention to the tip of the spark plug wire, removing any deposits or oil residues completely.
How to Replace Lawnmower Spark Plugs
Older spark plugs with a heavy carbon build-up will need to be replaced entirely. This is an easy and inexpensive process that you can do yourself. Check for a dark heavy carbon residue, or other signs of severe degradation such as cracks, as these indicate your spark plugs are beyond repair and need to be replaced. Ideally, you should replace your spark plugs every season.
1. Remove old spark plug
Locate the plugs at the front of the mower where they’re wrapped in a black cable. Remove the spark plug’s wire hook and use a socket wrench to remove the plug from its socket.
2. Measure space between electrodes
Measure the space between the tips of the two spark plug electrodes using a feeler gauge. Take note of the specifications for your mower’s model to know the recommended size of the gap and then purchase the right replacement plug.
3. If necessary, adjust electrode gap
Adjust the gap between the electrodes if necessary. Use a spark plug gauge to gently bend the curved electrode. When the gap becomes the correct width, the gauge should drag slightly as you pull it through the gap.
4. Install new spark plug
Install the new spark plug into the socket and reattach the spark plug lead. Take care not to over-tighten the new plug upon installation, only tightening it enough so that it feels snug in the socket.
Gas Has Gone Bad and Formed Residues
Gas that is left sitting in your mower’s tank for long periods will gradually evaporate over time. This leaves behind particles and gummy residues inside the engine that can end up clogging the carburetor, blocking the flow of gas into the engine and preventing it from powering the mower properly.
Signs of Gas Residue Blockage
- Mower starts but dies soon after
- Mower doesn’t start at all
How to Fix Gas Residue Blockage
If the tank of your lawn mower is under halfway full of old gas, you can try to dilute its impurities by adding new gas to the tank. If there’s more than half a tank of old gas, it would be better to get rid of what’s in there and refill the tank with fresh gas.
1. Drain out the old gas from the gas tank
Drain out all of the old gas from the gas tank using a fuel siphon – we recommend using this Fluid Extraction Pump from GasTapper. You can try to clear out as much gas from the carburetor as possible by running the engine.
2. Fill gas tank with fresh gas and a fuel stabilizer
After the tank is completely cleared of the old gas, add new gas along with a fuel stabilizer to the gas tank. Fuel stabilizers help to put off residue formation for a longer time, with some of the higher-quality products such as the STA-BIL Storage Fuel Stabilizer working for up to two years to keep gas fresh. Make sure that you use the correct ratio of fuel stabilizer to gas for your mower by referring to your owner’s manual.
Too Much Oil In Reservoir
If your lawn mower starts, runs for a few minutes, then dies, and you have already cleaned your carburetor, checked your spark plugs and cleaned your gas tank, there is a high chance that your mower’s oil reservoir is overfilled. This causes the engine to stall as it is fed more oil than it needs.
Signs of Overfilled Oil Reservoir
- White smoke coming from mower engine
- Mower engine starts then dies after several minutes of running
How to Check For Excess Oil
You should first check the oil level in your reservoir to find out whether it’s overfilled and if it could be the root of the problem.
1. Tilt the lawnmower
Tilt the lawnmower and prop it up with a heavy object so the oil can circulate to other parts of the engine. Let it sit like this for about an hour.
2. Try to run the engine
Try to run the engine, and if it runs for a good amount of time before it stalls, this is a sure sign that there are excess amounts of oil causing problems in your mower.
How to Drain Excess Oil
Draining off excess oil in your mower is an easy fix that you can do yourself.
1. Loosen oil drain plug
Find the oil drain plug and loosen it using the correct sized wrench.
2. Let out excess oil
Use a dipstick to check the current level of oil in the reservoir. Release excess oil into a suitable container, such as a pan, until the oil level is at ‘full’, making sure it doesn’t go above the maximum level.
3. Check there is adequate oil in reservoir
Too little oil can cause as bad, if not worse problems than too much, so do a final check with the dipstick to make sure the oil level is just right.
4. Run the mower
If your engine fires up and runs without creating billowing clouds of white smoke, the problem has been solved.
Choke is Loose or Faulty
If you’re having trouble getting the engine started on your mower, it could be down to a loose or faulty choke. The choke is a mechanism used to warm-up and start a cold engine. When you hold down the choke, it operates a metal flap that limits the amount of oxygen entering the carburetor, creating a richer fuel mix that warms up the engine. Once the engine has warmed up, you can release the choke and the engine will continue to run on its regular fuel amount.
A loose or faulty choke means that the metal flap fails to open, causing the combustion chamber to flood with too much gas and too little oxygen. The spark plug is then unable to produce a spark, and the engine will stall or fail to start at all.
Signs of a Loose or Faulty Choke
- Engine doesn’t start on first pull
- Engine becomes flooded with fuel
How to Fix A Loose or Faulty Choke
You can take some steps to tighten a loose choke or fix a faulty choke so that it opens and closes properly.
1. Clean the choke
Spray carburetor cleaner along the choke shaft to loosen any bits of grit or debris attached to it. The grit will then be dislodged during the mower’s next use, and the choke will be able to function properly again.
2. Tighten choke screws and spring
A loose choke can be fixed by tightening any loose screws holding the choke in place. You may need to replace the choke spring if it’s failing to pull back to its closed position.
Gasoline Cap is Clogged
A blocked gasoline cap can also prevent your mower from starting. In your mower’s fuel tank cap, there is a small hole that enables the fuel tank to take in air, which in turn allows gas to flow freely out of the tank along the fuel line, and finally ending up in the carburetor.
If the hole in the gasoline cap becomes clogged, the air pressure in the gas tank becomes destabilized and it creates a vacuum that prevents gas from leaving the fuel tank. Without any gas reaching the combustion chamber, the engine will fail to start.
Signs of a Blocked Gasoline Cap
- Engine completely fails to start
How to Fix a Blocked Gasoline Cap
1. Remove the fuel cap
Remove the cap on the fuel tank and check the breather hole. If it’s clear and air is able to pass through it then it doesn’t need any attention, but if it’s blocked you’ll need to clean it out.
2. Clean the breather hole
Clean out the residue causing a blockage in the breather hole using a nozzle cleaner or a small length of wire that’s thin enough to easily fit through the hole.
3. Make sure fuel tank isn’t overfilled
While you’re here, check that the fuel tank hasn’t been filled beyond the maximum fuel level and take care to never fill the tank past this mark. When the tank heats up it can cause the gas to expand, and the extra space in the fuel tank is to accommodate for when this happens. If the fuel tank is overfilled, gas can escape through the breather hole, creating the risk of fire when the mower is running.
Air Filter is Dirty or Clogged
The air filter in your lawn mower can become dirty or clogged over time, which can prevent your lawn mower from running properly. When the air filter becomes clogged with dirt and debris from your yard, it obstructs the flow of air to the fuel. This will cause your lawn mower to die after a few minutes of running.
It’s important to note that you should never run the mower’s engine without an air filter. The filter performs the essential job of stopping dirt and debris from getting into the machine. This debris is capable of damaging the cylinder walls, valves, and pistons, and can severely damage your engine, leaving you with a completely defunct lawn mower.
Signs of a Dirty or Clogged Air Filter
- Mower engine runs for a few minutes then dies
How to Fix a Dirty or Clogged Air Filter
1. Clean it with compressed air
The material that air filters are made from is sturdy and porous, so you can clean out light dirt with a can of compressed air such as the Dust-Off DPSXL Disposable Duster.
2. Wash it in lukewarm water and dish soap
If the filter is too dirty and needs a deeper clean beyond the compressed air, you can remove it from the mower and hand wash it in a bowl of lukewarm water and regular dish soap. Let it air dry and then reinstate it into the mower.
Muffler is Blocked
If your lawn mower won’t start, or stops within a few minutes of running, another common cause of this is a blocked muffler. The muffler is a part of the mower that suppresses loud exhaust noises that come from the engine.
When the muffler becomes dirty or blocked, this can increase the noise of the exhaust, as well as causing the engine to stall. You should remember to check the muffler every so often to make sure that there aren’t any blockages or that it has any leaks of excess oil.
Signs of a Blocked Muffler
- Mower runs for a few minutes then dies
- Engine doesn’t start at all
- Mower stalls
- Exhaust making louder noises than usual
How to Clean a Blocked Muffler
1. Remove the muffler
The muffler will be very hot after use, so turn off the engine and leave it to cool off for a few minutes before you touch it. Wearing safety gloves and goggles, remove the damper from the engine by turning it counterclockwise. You can refer to the manual for your mower if you have trouble removing it.
2. Clean the muffler
Once you’ve removed the muffler from the mower, remove any debris or obstructions and then spray it with carburetor cleaner. Let the cleaner sit for a few minutes, then remove any dirt and debris from the rear-facing muffler by firmly shaking it downwards.
3. Reinstate the muffler
After the muffler has dried out, put it back on the mower’s engine. When reinstalling it, make sure that it’s closely fitted and that there isn’t any exhaust coming from the joint.
It’s often the case that the lawn mower engine will smoke white, blue, or black smoke, in addition to stalling or failing to start at all. See our article Lawn Mower Smoking: Reasons Why & How To Fix Them for more information on how to fix a smoking lawn mower.