A lawn mower that won’t start can be baffling, especially if you don’t know much about engines. Most people learned the workings of the combustion engine in high school during science class. Vague memories of Nicolaus Otto’s 1876 early engines certainly won’t help get your lawn mower up and running again. So, instead of digging out an old high school textbook to find a solution, let me share with you the most common reasons why your lawn mower’s engine won’t start.
Why Your Lawn Mower is Not Starting (The Short Answer)
The most common causes for a lawn mower not starting usually stem from problems with the starter system (the battery), the fuel delivery system (the fuel pump), the air intake, and the ignition spark (the air filter and spark plug). A dirty carburetor or safety system can also prevent a mower from starting.
11 Possible Reasons Why Your Lawn Mower Won’t Start
I’m pretty sure that by the time you get to the bottom of my list, you will have found the reason why your lawn mower isn’t starting. So, let’s take a look at the possible causes.
Dead Battery
The first place to start is at the battery. Riding mowers, zero turns, and a few walk-behind lawn mowers use a starter motor and a battery to fire up the engine.
If your battery isn’t fully charged or is dead (I wrote about symptoms of a bad/dead lawn mower battery here), then the starter motor won’t be able to crank over the engine.
If you have a battery tester, you can check the volts of the battery and the cold cranking amps. Alternatively, you can have the battery tested at an auto parts store.
Bad Spark Plug
Old or damaged spark plugs cannot provide a spark inside the cylinder required to ignite the fuel vapor from the carburetor. No matter how many times you try to start the engine, if the plug is bad, the engine just isn’t going to start.
How do you tell if a spark plug is bad? You test it. When I need to test a spark plug, I like to use an inline spark plug tester. This is a little device that can be fitted in seconds and sits between the spark plug and the ignition cable. With the tester installed, you just need to crank the engine with either the key or pull cord and check to see if the tester lights up.
Alternatively, you can remove the spark plug and crank the engine. As long as the spark plug is connected to the ignition cable and touches a metal part of the mower, you’ll see a spark when you crank the engine if the plug is good.
Blocked Air Filter
The air filter is the one defense the engine has to stop any debris from entering the engine when it draws air into the carburetor. This specifically designed filter allows a certain amount of air into the engine during operation.
Once the air filter becomes blocked or dirty, the air getting into the engine is decreased, resulting in the engine not starting, poor performance, and stalling.
To check the air filter, you’ll need to pop off the air filter housing cover and remove the filter. Once the filter is removed, you can check it to see how blocked or dirty it is.
Blocked Fuel Filter
Like the air filter, the fuel filter is a protective measure the engine uses to stop dirt from entering the engine. Rather than filtering air, this filter removes any debris and impurities from the fuel.
Once the filter becomes clogged, it no longer allows fuel to flow through the fuel system, and the engine becomes starved. An engine that is starved by a clogged fuel filter is not going to start.
You’ll find the fuel filter located between the fuel tank and the fuel pump. Usually, these are small see-through devices that you can inspect visually.
Once you find the filter, you’ll want to see if it’s dirty. If it looks like it has a bit of debris or has turned black, then you’ll know you need to replace your fuel filter as it’s probably stopping your engine from starting.
Broken Fuel Pump
The fuel pump is the device that transfers the fuel from the fuel tank to the carburetor. Mower fuel pumps rely on a diaphragm, spring, and a vacuum from the engine.
It’s common for debris to enter the fuel pump and clog it up. If a lawn mower fuel pump is not working properly, it will starve the engine of fuel and result in your mower not being able to start.
A quick way to test a lawn mower’s fuel pump is to remove the outflow pipe from the pump and crank the engine. Once you crank over the engine, fuel will spurt from the fuel pump. If it’s broken, then no fuel will come out, and you will have pinpointed your problem.
Blocked Fuel Cap
You would think that such a simple thing as a fuel cap can’t stop a mower from starting, but it really can.
You’ll notice that on the top of a mower’s fuel cap, there is a tiny hole. This hole allows air to enter and exit the fuel tank. If your lawn mower’s gas cap is not venting (i.e. this hole becomes blocked), a vacuum can build in the fuel tank, preventing gasoline from flowing through the fuel system.
So pop off the fuel cap and take a look at the hole. If you find that it is blocked, more than likely from yard dirt, then it’s probably the reason why your mower is not starting.
Dirty Carburetor
If you have checked off all the other items on this list, then your mower’s problem likely comes down to an issue with the carburetor.
The carburetor has a small jet inside which vaporizes fuel so that it can be ignited in the cylinder head. Contrary to popular belief, liquid gasoline doesn’t burn; it’s the fumes from gas that do.
So, if this tiny jet becomes blocked, which is common if bad fuel or junk gets into the carburetor, it can no longer create the vapor needed to start the engine. The only way to check for a dirty jet is to remove the carburetor and inspect its interior.
Safety System
Finally, one part of the mower that gets overlooked is the safety features designed to stop the mower from starting. This isn’t a result of a malfunction but a feature that keeps you out of danger.
The seat, the brake, the blade engagement button (these apply to riding mowers), and even the bail lever are all systems that stop the engine from starting.
So, if you’re unsure of your mower’s correct starting procedure, grab the manual or look online at the manufacturer’s website for the proper method.
Electrical Lawn Mowers
It’s not only gas-powered lawn mowers that have problems starting; electric mowers, both battery and corded, can suffer from their own specific issues. Here are a few common problems that can stop your electric mower from starting.
Damaged Cord
Corded electric mowers have a long cable connecting the mower to the power outlet. These cables can take a real beating when dragged around the yard.
The easiest way to check the cable is to inspect it visually. Alternatively, you can use a multimeter to test the voltage coming out of the cable once it’s powered up.
Poor Electrical Connections
The connection on an electric lawn mower often suffers from some corrosion. This is usually due to the damp conditions the mower spends a lot of its time in.
The connection may be located where the battery/cord connects to the mower or, more commonly, where the cable connects to the electric motor.
By removing the mower’s cover, you’ll expose the electric motor. Here you’ll be able to see if the terminals are corroded and causing a problem with the lawn mower starting. Alternatively, you can use a multimeter to test the connections.
Worn Out Brushes
Electric motors require power to be transferred from the terminals/stator outside to the spinning rotor inside. To do this transfer, electric lawn mower motors use spring-loaded brushes that rub against the spinning internal rotor.
With the friction of the spinning, the brushes wear down and need to be replaced. This is really the only servicing an electric lawn mower requires.
Once you have access to the motor, you should be able to pop out the brushes by removing the terminal mounting screw. With some motors, you don’t even need to do this since the brushes are just clipped in for easy removal.
Once you have the brushes removed, you can inspect them to see if there is enough length left to make contact with the rotor. They are usually marked, showing when they need to be replaced.
How to Fix a Lawn Mower That Won’t Start
So, was I right–did you find the reason why your lawn mower won’t start? Now let me explain how you can get it fixed.
Charging the Mower’s Battery
Charging your lawn mower’s battery can be done in a few different ways. If you have an old charger, then you’ll need to disconnect the cable from the lawn mower. You can leave the battery connected if you have a modern smart controlled charger.
To disconnect the battery, a small wrench set will do just fine. Just remember that you disconnect negative, then positive, and then connect, positive then negative.
Next, you’ll need to set your charger setting. This tells the charger the battery’s voltage and specification, such as lead acid.
Both types of chargers will tell you when the battery is charged, but not all chargers will switch off. So, make sure you know how your charger works before you use it.
How long does it take to charge a lawn mower battery? That will depend on the type of battery you have, the battery’s voltage, and the charger you’re using.
Tools Required to Charge Your Lawn Mower’s Battery
- Battery Charger
- Small Wrench Set
Replacing the Spark Plug
To replace the spark plug on your lawn mower, you’ll first need to remove the ignition cable attached to the existing spark plug.
Next, use your plug wrench to remove the existing spark plug and set it to one side. Now insert the new spark plug into the lawn mower.
There should be no reason to test the gap on your new spark plug, as it should have already been gapped correctly.
Next, with the new spark plug loosely threaded back into the engine, use your plug wrench to tighten the new spark plug snuggly. Finally, reattach the ignition cable, and you are done.
Tools & Parts to Replace the Spark Plug
- New Spark Plug
- Plug Wrench
Replacing the Air Filter
Unclip and remove the air filter cover the same way you did when inspecting it, and set it to one side. Next, remove the old filter/s and discard them in the trash.
Now use a cloth to give the inside a good wipe down. If it has some old oil and gasoline residue, then you can use a little WD-40 to help remove the grime. Then check your new filter/s and see if they need to be oiled.
Specific oil helps keep the filter supple and stops them from drying out and cracking. However, oil shouldn’t be used on paper filters, only on sponge filters. If your filter/s does need to be oiled, then you can use a can of spray filter oil to complete this stage.
With the air filters prepped, you can go ahead and insert them into the lawn mower’s filter housing. Finally, reattach the filter housing cover, and the fix is complete.
Tools & Parts to Change the Lawn Mower’s Air Filters
- New Filters
- WD-40
- Cleaning Cloth
- Filter Oil Spray as Needed
Replacing the Fuel Filter
Installing a new lawn mower fuel filter isn’t difficult. First, you’ll want to switch off the fuel using the fuel cut-off valve. This is to stop the fuel from flowing out once the filter is removed.
Next, grab yourself a pair of pliers and remove the spring clips holding the fuel lines to the fuel filter. You just need to slide them down the fuel line, so they no longer rest on the filter.
Then twist the filter while holding the fuel lines. This helps loosen the lines from the filter. Once the filter no longer grips the fuel lines, you can go ahead and pull off both fuel lines. Just be careful as a small amount of fuel will come out of the filter and line.
Next, reattach both fuel lines to your new fuel filter. You’ll notice an arrow on the side of the filter stating the direction the fuel flows in, so make sure to install the filter in the correct fuel flow direction.
Then use your pliers to reposition the spring clips so that they sit over the lines and the filter and grip everything in place.
Finally, switch the fuel back on to test the mower. It will take an extra few pulls to get fuel back through the system and fill the new filter.
Tools & Parts to Change the Lawn Mower’s Fuel Filters
- Pliers
- New Fuel Filter
Installing a New Fuel Pump on a Riding Mower
Lawn mower fuel pumps vary as there are a number of different designs, but the principle is the same. So, here I want to talk you through a standard fuel pump that is connected to an OHV, which most riding mowers and zero turns have.
First, turn off the fuel cut-off valve to prevent fuel from leaking from the fuel lines once you remove them. So, locate the fuel pump and remove the spring clips from the three pipes. To do this, use a pair of pliers and slide the clips down the pipes.
Next, slide off the three pipes from the fuel pump. So that you don’t confuse which pipe is which, use a marker pen and number them 1, 2, 3, then do the same on the old pump.
Then you want to use your socket wrench to remove the two bolts that hold the pump to the mower. Once the bolts are removed, you can go ahead and remove the old pump.
Position the new pump on the lawn mower and insert the bolts you previously removed. Use your socket wrench and tighten up the bolt nice and snug, but not too tightly as these pumps are usually made of plastic and can crack.
Once the new pump is mounted, attach the three pipes using your marked numbers as a reference and slide the spring clips back into the correct position. If you look closely at the pipes, you should see some indent from their original position.
After completing this step, you can switch the fuel back on and start the mower. A few extra cranks will be needed to get the fuel back through the system.
Tools & Parts to Change the Lawn Mower’s Fuel Pump
- Pliers
- Marker Pen
- Socket Wrench Set
- New Fuel pump
Unclogging the Fuel Cap
To unclog a fuel cap, you just need to poke the hole with a pin or needle, whichever you have at hand. Use whichever tool you select, push it through the hole, and dislodge the dirt. There probably isn’t going to be much, just enough to cause a problem
Tools & Parts to Unclog a Fuel Cap
- Small Needle or Long Pin
Replacing the Carburetor
Cleaning a carburetor is a task that not everybody will want to undertake. But if you’re up for the task, I’ll cover how you can swap your old one for a new one. Depending on the particular engine, replacement carburetors start at about $30.
Removing the Lawn Mower’s Old Carburetor
First, remove the air filter cover and filter and set them to one slide. Then, use your socket wrench to remove the couple of bolts holding the back plate of the air filter housing to the mower. These bolts also pass through the carburetor and hold it in place, so once the bolts are removed, the carburetor will be loose.
Next, switch off the fuel using the fuel cut-off valve. Then, use your pliers to remove the spring clip holding the fuel line onto the carburetor. With the clip removed, you can pull off the fuel line.
Next, you’ll need to remove the governor/throttle linkage from the carburetor. To do this, twist them out of the brackets with a bit of persuasion. This was the last thing holding the carburetor to the mower, so now that it’s fully disconnected, you can set it to one side.
Install the Lawn Mower’s New Carburetor
Now it’s on to installing the new carb. Hopefully, the new carburetor came with two new gaskets– one that sits between the carburetor and the engine and another between the carburetor and the air filter back plate. If it didn’t, you need to salvage the old ones as long as they are still in good condition. If they are worn or damaged, you’ll have to get replacements.
So, you’ll need to first attach the governor/ throttle linkage to the new carburetor. Then, attach the fuel line and position the spring clip. Now thread your bolts through the air filter back plate, the gasket, the new carburetor, the second gasket, and finally into the engine. Repeat this with both bolts and make sure to get everything in the correct order.
The last job is to tighten the bolts and reinstall the air filter and cover. That’s it – see, it’s not too difficult. With the new carburetor installed, you can switch the fuel back on and crank the engine.
Tools & Parts to Replace a Carburetor
- Socket Wrench Set
- Pliers
- New Carburetor
- Gaskets as Required
Electric Mower Fixes
That’s the gasoline powered mower out the way. Now we can move on to the electric mowers. If your electric lawn mower doesn’t start, let’s see if these fixes can get it working again.
Replacing the Cord
Replacing the cord on an electric mower is as simple as it sounds. There’s not much you can do with a damaged cord. You could try to repair it, but this will mean you have to cut a section out, and the cable will get shorter. So, I recommend replacing it with a new one.
Cleaning the Connections
First, remove the connectors from the terminal. Then take some sandpaper and run it over the corroded surface. Once you have removed most of the corrosion, spray the metal area with some contact cleaner. Give the cleaner a minute to soak in, then brush it with an old toothbrush.
Next, use a cloth to wipe down the area you have been working on. Once you are happy with the outcome, repeat the process to include the connector and the terminal. When complete, you can reattach the connector to the terminal. If you find the connector is a little loose, try to give it a little pinch with a pair of pliers.
Finally, make sure everything is connected correctly and that everything is dry.
Tools & Parts to Clean Electrical Contacts
- Sandpaper
- Contact Cleaner
- Toothbrush
- Cleaning Cloth
- Plier
Changing the Brushes
The last item on my list is changing out the bushes on your electric motor. While most electric mowers allow you to change brushes, some don’t. So, if your lawn mower gives you the option to change them, then this is what you’ll need to do.
First, remove the mower’s cover and expose the top of the motor. A screwdriver should be enough to remove the top. Then you’ll need to remove the old brushes. If you have ones that are just clipped in, you can unclip them, slide out the old ones, and slide in the new ones. For the rest of you with replaceable brushes, you’ll need to remove the terminals.
Use your screwdriver to remove the screw holding the terminal in place. Once this is removed, you should be able to slide out the old brush and slide in the new one. You need to repeat this for both the positive and negative terminals. With the new brushes installed, reattach the terminals and replace the cover.
Tools & Parts to Replace Motor Brushes
- New Brushes x2
- Screwdriver
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