Man confused as to the reason why his lawn mower won't start.

20 Reasons Why Your Lawn Mower Won’t Start

18 minutes

There are a variety of reasons why your lawn mower won’t start, and it usually happens when you least expect it. This could be due to wear and tear, fuel-quality problems, or lack of maintenance. These are some more general reasons though, so what are the 20 specific reasons why your lawn mower won’t start?

The 20 reasons why your lawn mower won’t start are a bad spark plug or loose connection, lack of fuel, bad or old fuel, a blocked fuel filter, a failed fuel pump, a blocked air filter, a fouled carburetor, a failed battery or loose terminals, a faulty safety switch, a faulty ignition switch, a faulty starter recoil, a broken recoil starter assembly, a faulty recoil starter pulley, a rewind pulley spring is loose, a faulty ignition coil, a faulty charging system, a broken flywheel key, a faulty gas cap, an incorrect operating procedure, or using the incorrect fuel.

3 General Reasons Why Your Lawn Mower Won’t Start

Old man trying to fix his lawn mower while out on the lawn.
Hopefully, he knows what he’s doing.

You can usually trace the cause of your lawn mower not starting by checking for the following problems. Before going into them, please remember if you don’t feel comfortable doing any of these yourself, you can always contact a professional for help. With that said, here are 3 general reasons why your lawn mower won’t start.

1. Too Much Or Too Little Fuel

The mixture of fuel and air is essential to your lawn mower’s performance. Anything that interferes with this mixture will prevent the engine from starting.

If you have too little fuel, you’ve either run out or there’s a blockage in the fuel line. Alternatively, you need to clean or reset the carburetor. You may also need to check your fuel pump and replace it. 

Too much fuel is usually caused by an incorrect float level in your carburetor or the mixture screw is not correctly adjusted. Your carburetor has two screws, one for setting the idle speed and the other for the fuel mixture. If you wish to reduce the amount of fuel, turn the mixture screw in a counter-clockwise direction while the lawn mower is running. You will hear a difference in how the engine runs as the revs are reduced slightly. 

Avoid making big adjustments when adjusting the fuel mixture. Just turn the screw a little bit each time and wait to see what the engine does.

Turning the mixture screw clockwise will increase the amount of fuel. You’ll know that you’ve gone too far when the motor starts to backfire or run roughly. Just turn the screw back a quarter turn and that should solve the problem. 

2. Too Little Air

A clogged air filter or an incorrectly set idle speed may prevent the engine from running or starting. 

Without air, combustion cannot take place. If the amount of fuel in the mixture is too high, your lawn mower won’t start. You’ll need to reduce the amount of fuel and increase the amount of air for the engine to start and run properly.

To check your air filter, first, remove the cover. A spring clip, nut, or screw may be holding it on. Once the cover is off, remove the filter and inspect it for dirt and debris. You may be able to clean it by knocking some of the dust and dirt out of it with your hand. 

However, you will want to replace badly clogged air filters to avoid excessive fuel consumption and wear on the motor.

The idle speed can be adjusted by turning the idle screw clockwise to increase the idle speed and counter-clockwise to reduce the idle speed. The idle screw is attached to a butterfly valve inside the throttle body. As the butterfly valve opens, it allows more air into the motor, which increases its speed. 

3. A Lack Of Spark

Your battery may be dead if it’s an electronic start or the switch may be faulty. The battery provides the initial electrical current to turn the motor, which in turn causes the magneto (that is not a typo, it’s a real thing) to generate a spark in the cylinder.  If the motor turns over when you turn the key but doesn’t start, you may have a fouled spark plug, faulty ignition coil, or starter solenoid.

Lawn mowers with a pull-start don’t have batteries, as you provide the initial turning force by pulling the starter cord.

The one thing necessary to get a motor running is a spark. Without a spark, your lawn mower won’t start or run. As the engine turns over, the magneto creates a 20,000-volt spark across the spark plug gap. This ignites the fuel/air mixture inside the cylinder, pushing the piston down and turning the motor over. 

Now that we’ve gone over some general reasons your lawn mower won’t start, let’s go over the specific ones. 

20 Specific Reasons Your Lawn Mower Won’t Start

Unhappy woman sitting in the grass with her lawn mower turned over.
Not a good idea for lawn mower problems, it seems.

Here’s a list of 20 reasons why your lawn mower won’t start and how to fix them. Before attempting any work yourself, please read your owner’s manual. Also, remember that if you don’t feel comfortable doing something yourself, you can always get in touch with a professional.

1. Bad Spark Plug Or Loose Connection

Spark plugs don’t last forever. They suffer damage, wear out, or crack. Carbon can build up on the electrode and prevent a spark from jumping the gap. The spark plug fits inside the cylinder and consists of two electrodes with a small gap between them. When the magneto sends a high voltage charge into the spark plug, the electricity moves from one electrode to the other across the gap, creating a spark.

The quickest way to check for a spark is to remove the spark plug from the engine and leave it connected to the lead. The lead is constructed of thick rubber and is connected to the spark plug with a rubber cap.  Hold the spark plug near to the chassis then crank the engine. You should see a spark try to jump the gap. If it doesn’t, buy a replacement. 

You can wear rubber gloves or hold the spark plug with a pair of insulated pliers. But, it really isn’t necessary, as the spark will jump from the spark plug to the chassis very easily and the insulation on the spark plug lead is thick enough to prevent any electrical charge from reaching your hand.

2. Lack Of Fuel

The most common reason that your lawn mower won’t start is a lack of fuel. This may be because the tank is empty or you may have a blockage in the fuel line. 

If the tank is full and no fuel is reaching the carburetor, you can remove the hose connecting the tank to the carburetor to check it for blockages. The carburetor is located behind the air filter. To reach it, remove the air filter cover. You may have a clogged carburetor due to old fuel gumming up the jets or inlet.

Use a carburetor cleaner (also sometimes called a carb cleaner) to clean out the carburetor. You may have to remove the carburetor from the lawn mower and disassemble it to clean all the parts. Unless you have some experience doing this, leave it to a skilled mechanic.

3. Bad Or Old Fuel

Fuel breaks down over time. If you don’t use your lawn mower for a few months and then try to start it, the fuel left in the carburetor may have evaporated and left a thick sludge behind.

This sludge gums up the carburetor and prevents fuel from flowing through it. Use a carburetor cleaner to remove the sludge. Otherwise, you may have to remove and rebuild the carburetor if it has gummed up the inside too badly. This job is best left to a mechanic unless you have some experience in removing and disassembling a carburetor. 

4. Blocked Fuel Filter

The fuel filter catches any debris that may interfere with your engine. Over time, it clogs, and you’ll need to replace it. If you have old fuel in your fuel tank, it may cause the filter to clog more quickly.

5. Failed Fuel Pump

Your fuel pump can become clogged with oil if you’ve overfilled it. There are three ports on a fuel pump: the inlet and outlet ports, and a pulse port. The oil may enter the pump via the air tube from the crankcase. It travels along the tube to the pulse port, which then allows it to mix with the fuel, as the fuel enters through the inlet. The outlet port pumps the oil and fuel mixture into the carburetor.

The pressure from the crankcase provides the force, via the connecting tube, for the pump to work. But if the crankcase is full of oil, and if oil enters the tube connected to the pump, it may not work. To fix it, you’ll need to remove the pump and purge it of any oil. Don’t forget to clean the pipe as well and remove the excess oil from the crankcase if it is overfull.

6. Blocked Air Filter

If you’ve used your lawn mower in very dusty conditions or where a lot of debris is flying around, the air filter will eventually become clogged.

It is also a good idea to check your air filter before cutting season, as you may need to replace it with a new one. The air filter is located behind a cover on the side of the engine. To get to the air filter inside, undo the clip or screw holding it in place. Pull the filter off and replace it with a new one.

7. Fouled Carburetor

Your carburetor can become clogged with old fuel. This is common if you haven’t used your lawn mower for a long time with fuel still in it.

To get your lawn mower running again, you’ll have to clean out the carburetor with a carburetor cleaner or it may require a complete overhaul. If, after you have sprayed the carburetor cleaner into the throat of the carburetor, it still won’t run, it is best to have a mechanic remove and clean the carburetor. Unless you have some experience doing this, rather let a professional do the job. 

To clean the carburetor, you’ll have to remove the air filter and then remove the carburetor. You’ll then have to take the carburetor apart and clean all the parts, replacing O-rings and resetting the float in the float chamber.

To avoid the hassle, it’s a good idea to run your lawn mower until it stops from a lack of fuel. This is especially useful if you are packing it away for a long time. With all the fuel used up, there is nothing to gum up your carburetor. 

8. Failed Battery Or Loose Terminals

If your lawn mower has a starter motor and battery, then the battery will eventually wear out. This will require a replacement battery. You can check your battery by hooking up a multimeter to check the voltage both when the battery is just sitting there and when it is under load. 

A battery tester will place a load on the battery automatically while testing it. If you only have a multimeter, you can switch on the lawn mower lights or try cranking the starter while checking the voltage drop. Just make sure that you keep your hands and feet away from the blades.

If you have a 12V battery, then it will show a charge of around 12.8 – 12.9 Volts (V) when it is not in use and the ignition is turned off. If the voltage is below 6-7V, then you can try charging it, but you’ll likely need a new battery.

When you crank the engine over, the voltage should drop to between 8V and 10V. If it is any lower than this, then your battery is either about to fail or in the process of failing. Cranking the engine entails turning the key on the ignition to engage the starter motor. 

Terminal Issues

Check that the positive and negative lead on each battery terminal is in good order. If the insulation is melted, cracked, or very brittle with exposed wiring, replace it. The metal battery clamps on the end of each lead should be securely tightened to the battery terminals.

If there is corrosion on the positive and negative battery terminals, clean it off with 3 or 4 teaspoons of baking soda mixed with a cup (250ml) of water. The acid will bubble and become harmless, so you can wash it off with a little water. Before pouring the mixture over the acid, wear protective glasses and a pair of work gloves to protect your hands. If you get some acid on your hands, rinse them in fresh water. You can also rinse your hands in a solution of baking soda and water to neutralize the acid. You can also have a professional do all of this for you instead.

Smearing a little petroleum jelly on the battery terminals will help to prevent corrosion from occurring. You can do this after you have removed the powdery white acid buildup or on any new battery before it has had a chance to corrode.

9. Faulty Safety Switch

The safety switch is another mechanical device that can fail after years of use. You will need to test it with a multimeter for continuity. The multimeter sends a small current through the switch to measure its resistance.

If the switch is faulty, it is best to replace it with a new one. Replacing the switch is a simple procedure and can be accomplished by unscrewing the existing wires and inserting the new switch in the old one’s place. If you don’t feel confident in doing this yourself, a mechanic can do it for you very easily.

10. Faulty Ignition Switch

To test your ignition switch for continuity, you’ll have to remove it from your lawn mower and connect a multimeter across the battery and solenoid terminals.

When turning the key, you should get a reading on your multimeter to show that the circuit is completed. If there is no reading, then the switch is faulty and should be replaced with a new one.

11. Faulty Starter Recoil

On a lawn mower that uses a rope pull, there is a recoil spring inside that returns the rope to the spool.

If the spring breaks, you won’t be able to start the engine when pulling the rope. You’ll notice that the rope just hangs loose outside the lawn mower.

Depending on your lawn mower’s brand and age, you can either replace the spring in the recoil assembly or you may have to replace the whole recoil assembly as a unit.

12. Broken Recoil Starter Assembly 

When you pull on the starter rope, the recoil assembly engages the lawn mower crankshaft. 

If the assembly is broken, you can pull on the rope as much as you want, but the tabs in the assembly will fail to extend and engage the crankshaft. Replacing the recoil starter assembly is just a matter of unscrewing the assembly from the motor and replacing it with a new one.

13. Faulty Recoil Starter Pulley 

If the recoil starter pulley on your lawn mower is faulty, remove it and check to see if the pulley is in good order. If it is broken, then you must replace it.

14. Rewind Pulley Spring Loose

Sometimes the spring inside the recoil pulley comes loose. It is then just a matter of turning it to get the tension back in the spring and then reattaching it to the clips. To do that, you need to remove the pulley assembly from the top of the lawn mower. Then unscrew the cover so that you can reach the spring. 

You can complete the tensioning process by winding the spring up inside the pulley housing until it is completely taut and then attach the cover to lock it in place. Holding the pulley with a screwdriver or clamp, feed the rope into the pulley, and tie a knot to keep it in place. Then carefully let the rope wind up on the cassette as the spring unwinds. If the plastic tabs are broken, you’ll have to replace the whole assembly.

15. Faulty Ignition Coil

Ignition coils fail after extended use. 

The coil sends a spark via the spark plug to the engine while it is running. When the coil fails, you’ll not get a spark at the spark plug. After checking your spark plug and finding it is working, the most likely culprit is the ignition coil.

The best way to test your ignition coil is to use an ignition coil tester. You plug this into your spark plug and then try to start the lawn mower. If you observe a spark, then it is likely the spark plug is the problem. If there is no spark, the ignition coil is likely to be faulty.

You can confirm this by removing the ground wire from the coil. The ground wire is attached to the underneath of the coil. You can find the coil by following the spark plug lead back to where it attaches to the coil. The thin ground wire connects to the coil and can be unplugged by pulling on the plastic connector. Then connect the ignition tester to the ground wire instead of the spark plug. The other end of the spark plug tester is connected to a lead that is attached to the lawnmower chassis. Remove the spark plug from the cylinder and set it aside. 

Then pull the starter cord to see if there is any spark present. If there is no spark, then the ignition coil is broken and needs replacement.

16. Faulty Charging System

The battery on your lawn mower is charged by the engine as it is running. If the battery runs down, then it may be the charging system that is at fault.

The engine charges the battery through a voltage regulator hooked up to a generator that is turned by the lawn mower engine. If you connect your battery connection from the voltage regulator to a multimeter and turn on the ignition, you should see a voltage reading. If not, then the wiring from your ignition is faulty or the switch is broken.

To locate the voltage regulator and battery connection, you will see a small matchbox-sized metal box attached to the side of the lawnmower, just below the flywheel. Two black wires feed alternating current into the regulator and a red wire feeds the direct current to the battery. The red wire is the one you are looking for. 

If there is an inline fuse fitted, it may have blown. Look for a plastic fitting on the red wire. Unplug it and you’ll find a plastic fuse inside. Check to see if it has blown (black or melted inside) before chasing a wiring problem.  

You can check that the engine is producing alternating current by hooking up your multimeter to the ac connector on the regulator. If there is no AC, then you know that the problem originates with the generator.

To check that the regulator is sending the correct amount of current to the battery, you can connect the multimeter in line with the red regulator lead to the battery. The charging voltage should be 14-15V. Anything less and the regulator is faulty and needs replacement.

17. Broken Flywheel Key

The engine is protected from damage when the blades hit something that stops their rotation, such as a large rock or log. This protection comes in the form of a key that fits into the crankshaft to engage the flywheel. The small metal key breaks off when the blades stop suddenly. You will then not be able to start the lawn mower.

The flywheel key must be replaced before you can use the lawn mower again.

18. Faulty Gas Cap

The gas cap allows air to pass into the fuel tank as the fuel level drops. If it does not, a vacuum is created which eventually stops the engine from starting.

To check if this is the problem. Try starting the lawn mower with the fuel cap off. If it starts and runs perfectly, replace the cap.

19. Incorrect Operating Procedure

There are safe ways to start and operate most machinery, and your lawn mower is no different.

If you fail to follow the correct procedures, the lawn mower will not start to ensure your safety and prevent damage to the machinery. Failure to set the parking brake on ride-on lawn mowers will stop them from starting. 

Also, cold engines require more fuel to start. If you fail to use the choke, then your engine will fail to start. Once the engine is running, reset the choke to its original position otherwise the engine will stop due to an excess of fuel in the air-to-fuel mixture.

Your lawn mower may still run, but there will be clouds of white smoke coming out of it. The smoke is not overly harmful to you, but it is an indication that the fuel mixture is too rich. The excess fuel will cause a buildup of carbon on the spark plug and valves which will eventually need replacement. The engine will also be running at excessively high revs which generates extra heat. This also causes excessive wear on all the moving parts and can lead to the engine seizing.

20. Using The Incorrect Fuel

If you use the incorrect fuel, you can damage the engine or cause it to not start.

2-stroke engines use a mixture of fuel and oil to lubricate them. This mixture is usually 40:1 (fuel to oil) for engines built after 2002 and 32:1 for engines built before that year. You can tell if your lawnmower has a 2-stroke engine as it will only have a single fuel fill port. 

4-stroke engines have both a fuel and oil fill port. Don’t use fuel with a rating lower than 87 octane and an ethanol content that exceeds 10%. And whatever you do, don’t put oil in the 4-stroke engine fuel tank. I learned this the hard way as a youngster and never forgot the lesson.

I was tasked with cutting our lawn as a kid. My dad, in his wisdom, stored gas in an old BP oil can. When I ran out of gas, I looked in the tool shed for the can and saw a shiny new Chevron can. As you can probably guess, the “gas” poured into the lawn mower was “real smooth”. It then failed to start. After receiving a thorough whipping for damaging our lawn mower, I sat with my old man late into the night, stripping the motor and cleaning the oil out of the fuel tank. Needless to say, I was very particular about what I filled the tank with after that. 

Conclusion: 20 Reasons Why Your Lawn Mower Won’t Start

So, there are the 20 reasons why your lawn mower won’t start. It might seem like a lot, but take it one step at a time, and your lawn mower will be good as new in no time.

So, what do you think? Which problem do you think your lawn mower is most likely experiencing right now? Let us know which one and why in the comments below!

Todd L Miles
Lawn Mowers Enthusiast
I started "Landscape and Lawns Care" to provide clients with lawn care with better service, better products and, most importantly, better ethics. My promise to every customer is to give the greenest grass possible while controlling weeds, insects, and diseases! The most important thing is that I strive to always do the right thing for you, your lawn and your wallet!