There are many different types of walk-behind lawn mowers used today from small format mowers with 18″ mowing decks to larger format mowers with up to 60″ mowing decks.
They may use a manual recoil starter or a more complicated ignition system to start the mower. No matter what type or size of walk-behind mower you own, a mower that won’t start is equally frustrating.
A walk-behind mower won’t start when it has a plugged air filter, old fuel, dirty carburetor, clogged fuel lines, a bad gas cap, a plugged air filter, a bad safety switch, a bad battery, or a bad starter solenoid.
Keep reading for additional items that can prevent your mower from starting. Stay safe while working on your mower by following the safety precautions found in your operator’s manual.
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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.
15 Reasons Your Walk-Behind Mower Won’t Start
Empty Gas Tank
Gas is a necessity to run a walk-behind mower. As the owner of the mower, you’re already aware of this.
I only mention this simple fact because, sometimes when you’re frustrated with the mower not running, you simply skip the obvious stuff like gas in the tank.
Solution: Add fresh fuel to an empty gas tank.
Bad or Old Gas
Leaving old gas in your walk-behind mower can cause clogging of the fuel system and potential engine damage.
Because gas can begin to break down as soon as 30 days after purchase, it’s important to run fresh fuel and consume it within this short period of time.
When gas breaks down, it can start to lose its combustible properties. Furthermore, ethanol used in most types of gasoline today attracts moisture from the air that is corrosive to the fuel system.
It can cause gummy deposits to form restricting fuel flow. When your walk-behind mower can’t get a good flow, it will fail to start.
It’s important to select gasoline that has no higher than a 10% ethanol content and it must have a minimum 87-octane rating.
For small engines, like the one used in your walk-behind mower, it’s always best to use gasoline with the least amount of ethanol or no ethanol.
4-cycle engines used on most walk-behind mowers sold today use unleaded gasoline. There are many walk-behind mowers with 2-cycle engines still used.
Gasoline and oil are mixed and then placed in a 2-cycle fuel tank. Read more about fuel for 2-cycle engines here.
When you can’t consume the fuel you purchased within 30 days, it’s good practice to add a fuel additive to stabilize it. This will help reduce the problems you encounter from running old fuel.
A product like Sea Foam Motor Treatment will stabilize the fuel, reduce moisture and clean the fuel system. I discuss this further in “Why Use Sea Foam Fuel Stabilizer in a Lawn Mower”.
Solution: Drain the fuel tank using a fuel siphon. Add fresh fuel that includes a fuel additive to stabilize and clean your fuel system.
Gas Cap Won’t Vent
The gas cap on your walk-behind mower has a vent to allow air to pass through the cap to equalize the air pressure in the fuel tank.
When the vent is plugged, the fuel tank will form a vacuum and prevent fuel from leaving the tank. This will keep your engine from getting fuel and cause the mower to not start.
To check whether your gas cap is venting, remove the cap to see if your lawn mower will start and run. If it does, place the cap back on the fuel tank and allow it to continue to run.
You are checking to see if your mower eventually begins to sputter and shut down because your fuel supply is being restricted. If this happens, there’s a good chance your gas cap is bad.
Solution: Replace your gas cap with a new cap if you are unable to get it to vent correctly. You can attempt to clean it to remove the clog, but this doesn’t always work or it works temporarily.
Bad Spark Plug or Loose Connection
Without a spark, your walk-behind will not start and continue to run. The spark plug can become dirty with carbon buildup and oil.
Not having the spark plug properly gapped or the spark plug wire securely attached can also cause intermittent starting problems.
Your spark plug(s) can fail when the tip is excessively dirty from carbon buildup, it has a broken porcelain, or burnt electrode. A spark plug that is not properly gapped and doesn’t make a good connection can also contribute to the engine misfiring and starting problems in your walk-behind mower.
Solution: Remove your spark plug and inspect it for signs of carbon buildup, cracked porcelain, or burnt electrode.
Replace with a new spark plug when you find any of these conditions or a very dark tip. Make sure they are correctly gapped and the spark plug wires are secure.
If you choose to clean and reuse the spark plug because it is still in good condition and only a little dirty, use a wire brush to remove the buildup of carbon.
Air Filter Plugged
Air is one of the essential items to start a walk-behind mower. Because of the dirty dusty conditions you operate your mower in, the air filter gets plugged with grass clippings and dirt.
When the filter is regularly checked, cleaned, or replaced it can become so plugged, the air isn’t able to pass through the filter.
If you find your air filter is the fault for the starting problem, remove it and clean it using the instructions below. Never run your mower without an air filter at risk of damage to the engine.
If you have to purchase a new filter, don’t mow thinking you can get by without a filter until you get a new one.
Trust me, a small amount of dirt into the engine can cause scoring of the cylinder, blown gaskets, and other engine damage that could result in needing an engine replacement.
Solution: Identify the type of air filter on your walk-behind and follow the instructions to clean it.
Clean a foam air filter:
- Remove the air filter cover. It’s usually held on with knobs, clips, or wing nuts. Wipe out dirt on the cover.
- Carefully remove the air filter from the housing so you don’t allow any dirt to fall into the air intake.
- Wipe out any remaining dirt left in the housing.
- Inspect the foam filter to make sure it is in good condition to be reused.
- Replace with a new filter if it appears very dark in color, torn, or brittle.
- Reuse if it appears in good condition
- Wash the filter with mild dish soap and water to remove dirt and any oil on the filter. Rinse to remove all soap until the water runs clear.
- Place the filter flat to dry. Leaving it in the sun will help it to dry faster.
- Once dry, lightly coat the filter with filter oil. The filter should be fully coated in oil, but not dripping oil. Oil is used to help trap dirt. Use a paper towel to soak up excess oil if needed. (If you are using a new filter, coat it with filter oil).
- Place the filter back into the housing and secure the cover.
Clean a paper air filter:
- Remove the air filter cover. It’s usually held on with knobs, clips or wing nuts. Wipe out dirt on the cover.
- Carefully remove the air filter from the housing so you don’t allow any dirt to fall into the air intake.
- Wipe out any remaining dirt left in the housing.
- Tap the filter against a hard surface. Don’t tap the paper element as it will damage it. Tap using the plastic or rubber part of the filter.
- Knock as much dirt loose as possible so it falls out of the filter. Don’t use compressed air as this will damage the filter.
- Hold the filter up to a light source. If you can see light shine through the paper element, go ahead and reuse it. If you can’t or if it is covered in oil, it’s time to use to the new air filter.
- Install the filter and secure the air filter cover.
Fuel Line Restriction
Bad or old fuel can cause clogging throughout your fuel system. This includes the fuel lines. You usually are not able to visibly see a clog in your fuel lines because they are made of black rubber material. You will have to check one section of the line at a time.
Do this by starting and stopping fuel flow using the fuel shut-off valve located under your fuel tank. If you don’t have a shut-off valve, just use pinch-off pliers to crimp the line to start and stop the flow.
You will want to check each section of the hose to check for clogging. Read more about locating a clog in the fuel line in “Your Lawn Mower Isn’t Getting Fuel“.
Solution: Once you find a clogged line, you need to try to remove the blockage. Do this by shutting off the fuel supply and removing the line from your walk-behind mower.
Spray carburetor cleaner into the tube to loosen the blockage. Then use compressed air to blow air into the line and remove the clog. Repeat as necessary.
If you are unable to dislodge the clog, you will need to replace your fuel line. You can purchase a mower fuel line at your local hardware store or online.
I do recommend replacing your fuel lines if the lines are dry or cracked even if you don’t find a clog in them.
Bad Fuel Pump
Many walk-behind mowers do not require a fuel pump because of the placement of the carburetor in relation to the fuel tank.
When the carburetor sits higher on the mower than the fuel tank, your walk-behind will use a fuel pump. It needs a fuel pump to get fuel up to the carburetor. Fuel cannot run uphill without it.
Old fuel sitting in the fuel pump can degrade the pump or it can just fail over time. It is no longer able to build pressure off the vacuum in the crankcase to move fuel from the fuel tank to the carburetor. If you notice your fuel pump is leaking, it must be replaced.
Otherwise, check whether your fuel pump is performing as it should by confirming you are getting a good supply of fuel to the inlet port on the pump.
Once this is confirmed, shut off the fuel flow, disconnect the hose off the carburetor and place it in a container.
Turn your fuel supply back on and start the mower. Watch the flow of fuel into the container. There should be a consistent or steady flow of fuel being pushed out of the pump and into the container.
If the flow is good, stop your mower and reattach the fuel line to the carburetor.
Solution: Replace your fuel pump if you are not receiving a constant or pulsating flow out of the pump.
Plugged Fuel Filter
The fuel filter should be replaced annually and more often if you find your fuel to be very dirty.
The fuel filter is used on your walk-behind mower to strain dirt and other contaminants to keep them from entering the fuel system and causing clogging and damage to the engine.
The filter can become plugged with deposits from running old or dirty fuel.
Solution: A clogged fuel filter must be replaced with a new one. You will find an arrow on the side of your new fuel filter.
Make sure the filter is installed with this arrow pointed in the direction of your fuel flow. This means the arrow should be pointed toward the carburetor and away from your fuel tank so it functions as designed.
The carburetor on your walk-behind mower regulates the mixture of gas and air needed to create combustion in the engine’s cylinder.
When your carburetor is gummed up from and old fuel, it can prevent your engine from getting the gas and air mixture it needs to start and run. Your carburetor will need to be cleaned and any failed components must be replaced.
Solution: If you are a little mechanical you should be able to handle cleaning your carburetor. Your carburetor includes a lot of small parts.
If you are unsure about cleaning your own carburetor, you can have your local small engine mechanic clean your carburetor or you can replace it with a new carburetor.
Clean the carburetor by taking it apart and using carburetor cleaner to clean the carb including the float bowl and needle. You can find steps for cleaning your carburetor here. If your carburetor is too dirty to clean adequately, you should replace it.
Bad Safety Switch
Your walk-behind mower may use several safety switches designed to keep the operator safe. The manufacturer installs safety switches to prevent the mower deck to run without the operator present.
There are other safety switches installed. The number and types of safety switches vary with each model. Refer to your operator’s manual to locate all of the switches on your mower.
Solution: Test the switch using a multimeter or you can temporarily bypass the safety switch to identify a bad switch. Do not operate a mower without the safety switch.
Never run a mower when a safety switch is bypassed. You never know when you will encounter a situation where the safety switch can save you from serious injury.
Bad Ignition Coil
Before checking for a bad ignition coil, make sure your spark plug is in good condition. The ignition coil provides voltage to the spark plug so it can start the engine. If the spark plug isn’t able to fire, the engine will not start.
Solution: Check the continuity of the ignition coil using an ohm meter. If you find a break in the continuity, replace the ignition coil.
If your walk-behind mower uses a recoil with a rope to start the mower. A recoil is known to wear out over time. You may have pulled the rope and it no longer retracts.
Sometimes, the rope can be restrung on your recoil and other times parts, including a pulley, spring or clips, may have failed.
Solution: You can attempt to replace the spring and restring your walk-behind recoil. If it does not work because other components in your recoil are damaged such as the clips or the pulley, you are better off just replacing the recoil assembly.
Bad Battery or Blown Fuse
If you use a walk-behind mower with an electric start, the battery may be bad or it might not have a sufficient charge to start your mower.
Try to start your mower with the manual recoil. If the mower starts with the recoil, you have a problem with the electric start.
Solution: Charge your battery using the instructions found in your owner’s manual. You also need to check for a blown fuse. Replace a blown fuse.
If the fuse is fine, replace a battery that doesn’t hold a charge. If you still have problems, have an experienced mechanic troubleshoot the electric start system.
Bad Starter Solenoid
If the mower uses a starter solenoid, its failure to work properly can cause the mower not to start. A walk-behind mower solenoid is an electromagnetic switch that is like an on-off switch that actuates the starter motor to turn over the engine.
A click or hum when turning your ignition key is an indication to check your solenoid. Another indication your mower solenoid may be bad is when a wire attached to your solenoid gets hot and begins to smoke or melt.
Find out more about starter solenoids in “How to Tell Your Lawn Mower Solenoid is Bad“.
Solution: Replace your solenoid if it is found to be bad.
Incorrect Operating Procedure
Safety features are installed on walk-behind lawn mowers requiring certain steps to be followed when starting and operating the mower. This varies by the type of walk-behind mower and its features.
Solution: Refer to your operator’s manual to ensure you are operating your lawn mower correctly so you don’t set off the safety features that shut off your lawn mower or don’t allow it to start.
Additional Causes of Starting Problems in Commercial Walk-Behind Mowers
Commercial walk-behind mowers have additional features not found on all walk-behind lawn mowers. These may include ignition switches, 12-volt batteries, and charging systems.
These items can cause a commercial mower not to start when they are faulty and not operating correctly.
For more information on determining whether your battery or charging system is bad, check out “5 Things that are Draining the Life Out of Your Lawn Mower Battery“.