A lawn mower that won’t start can be baffling, especially if you don’t know much about engines. High school science classes were the best place to learn about the workings of a combustion engine. 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.
The Short Answer: Why your lawn mower isn’t starting (The Short Answer).
Most lawn mowers that don’t start are caused by problems in the starter (the battery), the fuel delivery system (the pumps), the air intake, ignition spark and fuel system (the fuel pump). A dirty carburetor can also stop 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.
The battery is the place you start. Starter motors are used to start riding mowers, zero turns, and a few walk behind lawnmowers.
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.
A battery tester is a tool that can be used to test the voltage and cold cranking amplifiers. Alternativly, your battery can be tested at an auto part shop.
Bad Spark Plug
Spark plugs that are old or damaged will not ignite the fuel vapor coming from a 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 can you tell if a sparkplug is bad? It’s easy to test it. An online spark plug tester is my favorite tool for testing spark plugs. This small device is easy to fit between the spark plug’s ignition cable and spark plug. Once the tester has been installed, you can crank your engine with either the key or pull cord to see if it lights.
Alternately, you could remove the spark plug from the ignition cable and turn 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 engine’s air filter is the only defense against debris entering the engine. This filter is designed to allow air into an engine when it is not in use.
The engine will not start if the filter is blocked or dirty.
To check the air filter, you’ll need to pop off the air filter housing cover and remove the filter. After the filter has been removed, you can inspect it to see if it’s blocked or dirty.
Fuel Filter Blocked
The engine uses the fuel filter to protect it from dirt and other contaminants. This filter does nothing to filter air, but it removes fuel-borne pollutants.
Blockage of the fuel filter can cause engine to become starved. A blocked fuel filter can cause engine to stop working.
You’ll find the fuel filter located between the fuel tank and the fuel pump. These transparent, small devices are easy to see 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 that it will need to be replaced and that it’s probably stopping your engine from starting.
Broken Fuel Pump
The fuel pump transfers fuel fuel from the fuel tank into its carburetor. Mower fuel pumps are made up of a spring and diaphragm.
It’s common for debris to enter the fuel pump and clog it up. It can block the fuel pump and cause it to stop functioning.
A quick way to test a lawn mower’s fuel pump is to remove the outflow pipe from the pump and crank the engine. If you crank the engine, the fuel pump will leak fuel. 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 flow into the fuel tank. This hole can block the flow of air into the fuel tank. This will prevent gasoline flowing through it.
Take off the fuel cap and inspect 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.
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.
A small jet is used to vaporize the fuel and allow it to be ignited by a carburetor. Contrary to popular belief, liquid gasoline doesn’t burn; it’s the fumes from gas that do.
If the tiny jet becomes blocked, which can occur if it’s clogged with fuel, or other junk, it won’t produce the necessary vapor to start the engine. To check for a blocked or dirty carburetor, remove it to inspect its interior.
People often overlook the safety features that prevent the mower from starting. This isn’t a result of a malfunction but a feature that keeps you out of danger.
The seat, brake, and blade engagement button (these buttons are for riding mowers) can all be used to stop the engine.
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.
Electric 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. These are the most common problems that prevent an electric mower’s starting.
Corded electric lawn mowers come with a long cable to connect the mower to a power socket. These cables can become very heavy when they are being dragged around the yard.
You can inspect the cables visually to verify their integrity. Alternatively, you can use a multimeter to test the voltage coming out of the cable once it’s powered up.
Poor electrical connections
An electric lawn mower’s connection can often develop corrosion. This is often due the moist environment in which the mower spends most time.
The connection can be found at the point where the cord/battery is connected to the mower, or more often, where cable connects with 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. You can also use a multimeter to check the connections.
Brushes that are damaged
Electric motors require power transfer between the terminals/stator and the spinning shaft. To transfer power, electric lawn mower motors use spring loaded brushes. They rub against the internal rotation.
The friction caused by spinning causes brushes to wear and must be replaced. This is the only maintenance that an electric lawnmower needs.
Once you have gained entry to the motor, you should be capable of removing the terminal mounting screw to allow for the brushes to fall off. With some motors, you don’t even need to do this since the brushes are just clipped in for easy removal.
Once the brushes have been taken out, you can inspect the brushes and determine if the brushes are long enough to reach the rotor. They are often marked to indicate when they should replace.
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? Let me tell you how I can fix this problem.
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, you can use a small wrench set. It is important that you disconnect the battery first, then connect it positively.
Next, you’ll need to set your charger setting. This tells the charger the battery’s voltage and specification, such as lead acid.
Both chargers will indicate when a battery has been charged. Not all chargers will turn on. Be sure to learn how your charger works before you use it.
How long does it take for your lawn mower battery to charge? 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
- Set of Small Wrench
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, remove the spark plug using a socket wrench. Next, place the new spark plug into the lawnmower.
It shouldn’t be necessary to test the gap between your new spark plug and its original. It should be properly gapped.
Next, insert the new sparkplug loosely in the engine. Next, use your socket wrench to tighten the plug. You are done!
Tools & Parts to Replace the Spark Plug
- New Spark Plug
- Plug Wrench
Replacing the filter
The clip can be removed from the air filter cover. Next, place the filter on one side. Next, take out the filter/s that you don’t need and dispose of them in the trash.
Use a clean, dry cloth to wipe the interior. To remove any gasoline or oil residue, you can use WD-40. Check your filter for oiliness.
Specific oil helps keep the filter flexible and prevents them from drying out and cracking. However, oil shouldn’t be used on paper filters, only on sponge filters. You can finish the process by spraying oil cans with oil to oil your filter.
With the air filters prepped, you can go ahead and insert them into the lawn mower’s filter housing. The last step is to attach the filter housing cover.
Tools & Parts to Change the Lawn Mower’s Air Filters
- New Filters
- Cleaning Cloth
- Filter Oil Spray – When You Need It
Replacing the fuel filters
First, you’ll want to switch off the fuel using the fuel cut-off valve. This is to prevent fuel escaping from the filter.
Next, take a pair of pliers and remove the spring clips that connect the fuel lines to the fuel filter. Slide them along the fuel line until the clips are free of the filter.
Next, twist and hold the fuel lines while you twist the filter. This will loosen the lines. The filter can be removed to remove the fuel lines. You should be careful as fuel can escape from the filter and line.
Next, attach the fuel lines to the 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.
Next, use your pair of pliers to reposition the spring clips so that they rest over the lines. Grab everything you can with your pliers.
To test the mower, turn on the fuel switch again. It may take some more pulling to get the fuel through, then fill the filter.
Tools & Parts to Change the Lawn Mower’s Fuel Filters
Installing a Fuel Pump on a Ride Mower
Although there are many options for lawn mower fuel pumps, the basic principle is the same. Let me show you how to connect an OHV to a standard fuel tank. Both zero turns and riding mowers have OHVs.
First, switch off the fuel cut-off valve. After you have removed the fuel lines, this will prevent fuel leaking from them. Locate the fuel pump, and remove the spring clips from the three pipes. To remove the spring clips, you will need to use a pair of pliers.
Next, slide off three of the fuel pump’s pipes. 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.
Next, use your socket tool to remove the bolts holding your mower’s pump together. The bolts must be removed before you can remove the old pump.
Place the lawn mower pump on the mower. Next, insert the bolts that were previously removed. To tighten the bolt, use your socket wrench. These pumps are made out of plastic and may crack.
Once the new pump is mounted on the pipes, attach them using your numbers. Next, slide the spring clips into their proper positions. If you carefully examine your pipes, you’ll see that they have a slight ding in their original positions.
This is it. This is it. Now, you can turn the fuel on again and start your mower. To allow fuel to flow, you will need to turn the mower on.
Tools & Parts to Change the Lawn Mower’s Fuel Pump
- Marker Pen
- Socket Wrench Set
- New Fuel pump
Clean out the fuel tank
Use a pin or needle to open a fuel tank cap. Use the tool you have and push it through to remove the dirt. There probably isn’t going to be much, just enough to cause a problem
Tools & Parts to Unclog a Fuel Cap
Replacing the Carburetor
Cleaning a carburetor is a difficult task. But if you’re up for the task, I’ll cover how you can swap your old one for a new one. Prices for replacement carburetors vary depending on the engine.
Removing the Lawn Mower’s Old Carburetor
First, remove and replace both the filter and the cover of the air filters. Next, place them on the one slide. Next, use your socket wrench and remove the bolts that attach the backplate of the mower’s air filter housing. These bolts pass through carburetors and keep them in place. After the bolts are removed, the carburetor should be freed.
Next, turn off the fuel using the fuel cutoff. Next, remove the spring clip that holds the fuel line onto the carburetor using your pliers. After the clip has been removed, you can remove fuel line.
Next, you’ll need to remove the governor/throttle linkage from the carburetor. This can be done by gently twisting the brackets. 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. Next, attach the fuelline to the carburetor and place the springclip. Next, thread the bolts through the engine. Continue with the second bolt, making sure everything is in the correct order.
The bolts should be tightened. Finally, reinstall the cover. That’s it – see, it’s not too difficult. Once the carburetor is installed, you can turn the engine on and switch the fuel back on.
Tools & Parts to Replace a Carburetor
- Socket Wrench Set
- New Carburetor
- Gaskets will be required
Electric Mower Fixes
That’s the gasoline powered mower out the way. Now we can move on to electric lawn mowers. If your electric lawn mower doesn’t start, let’s see if these fixes can get it working again.
It’s easy to replace the cord of an electric mower. There’s not much you can do with a damaged cord. You can repair it, but this will require you cutting a section of the cable and making it shorter. I recommend replacing it with a brand new one.
Cleaning the Connections
First, take the connectors off the terminal. Next, use some sandpaper on the corroded areas. After you have removed all corrosion, spray the metal with a contact cleaning agent. Allow the cleaner to sit for a few minutes, then brush it with an old toothbrush.
Next, use a towel to clean the area you’ve been working on. Once you are satisfied with the final product, attach the connector. Once the process is completed, you can attach the connector to the terminal again. If the connector feels a little loose, you can try to tighten it with a pair of pliers.
Make sure everything is connected correctly.
Tools & Parts to Clean Electrical Contacts
- Contact Cleaner
- Cloth Cleaning
Change the brush
Last on my list is changing the bushes on your electric mower. 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 can be used to remove the top. Then you’ll need to remove the old brushes. If the brushes are still clipped in, you can remove them from the clipper and slide in the new ones. For the rest of you with replaceable brushes, you’ll need to remove the terminals.
Use your screwdriver for removing the screw that holds the terminal. Once the screw has been removed, you can slide out the brush and insert the new one. You should repeat this procedure for both the positive or negative terminals. After the new brushes have been installed, attach the cover to the terminals and replace it.
Tools & Parts Replacement for Motor Brushes
- New Brushes, Version 2