You’re frustrated with your mower because it just won’t start. Is it time to buy a new mower? Before you go to this extreme, troubleshoot and try to fix the problem using the details below.
A lawn mower turns over and won’t start when it isn’t getting air, spark, or fuel due to a plugged air filter, incorrect choke setting, bad spark plug, plugged fuel filter, clogged fuel line, bad fuel pump, dirty carburetor, or old gas.
Remove the spark plug wire prior to performing repairs on your mower. Follow the specific safety precautions for your type of mower as provided in the 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.
9 Reasons a Lawn Mower Cranks But Won’t Start
Stuck Choke or Incorrect Choke Setting
When you are trying to start a cold engine, it will be difficult to start when the choke isn’t engaged. The choke is used to restrict the amount of air through the carburetor throat.
This is so the engine receives more fuel than air to form combustion in a cold engine. When the engine warms up, the choke lever must be adjusted to the off position so the engine gets enough air to continue to run.
Like a cold engine won’t start without the choke engaged, a warm engine won’t start with the choke engaged. Having the choke set correctly is required so your engine starts after it turns over.
If the choke is set correctly and you are still having airflow issues, check to make sure the choke plate is not stuck. Use carburetor cleaner to help free up a stuck choke plate.
Also, check that the choke cable is moving freely. Use a lubricant like Sea Foam Deep Creep or a similar product to free up the linkages. Replace a damaged or worn choke cable.
Plugged Air Filter
Another airflow restriction you may have that prevents the mower from starting after it turns over is the air filter. This is an important part required to keep dirt from entering the air intake and permanently damaging the engine.
When the air filter isn’t regularly checked, cleaned, or replaced, it can develop a buildup of dirt and debris that prevents a sufficient amount of air to get to the engine.
I recommend starting each mowing season out with a new air filter and then cleaning it several times throughout the season. Of course, if you are using the mower more than the average homeowner, you will need to clean and replace the filter more frequently.
Follow the instructions below to clean common air filter types. If you are unsure what type of filter you have and its cleaning instructions, find the information in the operator’s manual.
You can find more information on air filters in Guide to Lawn Mower Air Filters: Differences & How to Clean Them.
Steps to Clean a Push Mower Air Filter
Take caution when working near the engine as the engine can be hot.
- Turn off the engine and remove the spark plug boot to prevent the engine from starting
- Remove the engine shroud which is usually held on with a thumb screw.
- Remove the filter being careful not to knock dirt into the intake. Remove any dirt remaining from the air filter housing with a dry cloth.
- Cleaning a Paper Filter – Tap the filter to knock the dirt from the filter. Hold the filter up to the light. If you see areas of the filter where light does not shine through or the filter is damaged, you must replace it.
- Cleaning a Foam Filter – Wash the filter with dish soap and water. Squeeze excess water out of the filter and lay it flat in the sun to dry. Once the filter is dry, use filter oil to lubricate the filter so it is lightly covered and not dripping with oil. If the sponge filter is excessively dirty or has any tears, you need to replace it.
- If your mower is using a foam pre-cleaner, you must replace it if it is torn, excessively dirty, or becomes dry and brittle. You can clean the pre-cleaner with warm soap and water and then lay it flat to dry in the sun.
- Replace the air filter and pre-cleaner (if your engine uses a pre-cleaner).
- Reinstall the engine shroud.
Steps to Clean a Paper Air Filter
Take caution when working near the engine as the engine can be hot.
- Turn off the engine and remove the spark plug boot to prevent the engine from starting
- Remove the engine shroud which is usually a cap held on with a couple of clips or bolts.
- Remove the filter being careful not to knock dirt into the intake. Remove any dirt remaining from the air filter housing with a dry cloth. Do not use an air compressor to blow out your air filter.
- Tap the filter to knock the dirt from the filter. Hold the filter up to the light. If you see areas of the filter where light does not shine through or you find the filter is damaged or extremely dirty, you must replace your air filter.
Dirty Spark Plug
Remove the spark plug using a 3/4″ or 5/8″ socket wrench. The size you need depends on the engine model used on your mower. Inspect the condition of the plug.
A spark plug that is damaged or very dirty can cause an intermittent or lack of spark required for the engine to start and run. When looking at the spark plug, check for a burnt electrode, broken porcelain, or dark-colored tip.
If you find any of these conditions, install a new spark plug. If the plug is in good condition and just a little dirty, clean it with a wire brush.
Reinstall the spark plug after you ensure the gap is correct. Then securely attach the spark plug wire.
Because a good spark plug is essential to a good performing lawn mower, I recommend starting each season out with a new spark plug.
Old Fuel
Gas doesn’t stay fresh for very long. It actually can begin to break down as quickly as 30 days after purchase. Old gas leaves behind varnish and sticky deposits that cause fuel restrictions and component failures.
Because of this, it’s important to purchase fresh gas and consume it within 30 days. Stay away from gas with high ethanol levels. Only use gasoline with a minimum 87 octane rating and a maximum 10% ethanol content.
Read more in This is the Type of Gas Lawn Mowers Use. I will share more about the right gas to use in a lawn mower with a 2-cycle or 4-cycle engine.
If you find old gas in the fuel tank, drain the fuel tank using a fuel siphon pump. Fill the tank with fresh fuel in addition to a fuel stabilizer like Sea Foam to help clean the fuel system and reduce moisture.
Allow the engine to run so the gas and fuel additive works its way through the system. If you aren’t able to get it started yet, continue reading to troubleshoot other fuel-related items that can cause your starting problem.
Plugged Fuel Filter
A fuel filter is used to keep dirt out of the fuel system. Most mowers use an inline fuel filter placed between the fuel lines. It strains fuel as it comes out of the fuel tank.
The fuel filter can become plugged when it isn’t replaced regularly. This will prevent a good flow of fuel from being able to pass through the filter and may cause the mower to turn over and not start.
If you don’t find an inline fuel filter on your mower, chances are you have a filter located at the bottom of the fuel tank. You must drain the fuel tank to replace this filter.
Replace an old or dirty fuel filter with a new fuel filter.
Clogged Fuel Line
Next, follow the fuel line coming out of the fuel tank to the carburetor. Look for any kinks that may be restricting fuel flow causing the mower to turn over, but not start.
If you don’t find any kinks, check for a fuel restriction in the fuel line. Do this by first shutting off the fuel flow using the fuel shut-off valve or fuel pinch-off pliers.
Check each section of the line by removing the end of the line furthest from the fuel tank and placing it in a container to check for sufficient flow after turning the fuel supply on.
When you aren’t getting sufficient fuel flowing through a fuel line, shut off the fuel supply and remove the fuel line.
Spray carburetor cleaner into the line. This is used to attempt to loosen the clog. Then, blow compressed air through the line to dislodge and remove the clog. Repeat using the carburetor cleaner and compressed air until the clog is removed.
Reinstall the fuel line once the clog is removed. Replace it with a new fuel line if the restriction is not removed or you find the fuel line is aged and beginning to crack.
Bad Fuel Pump
A lawn mower uses a fuel pump when the carburetor is positioned higher than the fuel tank. The pump is required to work against gravity to move fuel uphill to the carburetor.
Most mowers that require a fuel pump use a vacuum fuel pump. This style of pump uses the vacuum off the crankcase to get fuel to the carburetor.
When the fuel pump cracks or fails to work correctly you will have to replace it. If you don’t see physical cracks or fuel leaking, you can take some steps to check the condition of the fuel pump.
Before you test the pump, make sure you are getting fuel to the inlet port on the pump. (If you are not, check for a blockage in the fuel line or fuel filter)
Once you have confirmed fuel flow to the pump, remove the fuel line from the carburetor and place it in a container. Check your pump is working correctly by starting your fuel flow and starting the engine.
You should have a steady or pulsating flow of fuel coming out of the fuel line. If you do not, you need to replace the fuel pump.
If your mower has an electronic fuel injection pump, obtain a pressure reading using a fuel pressure gauge. Refer to your operator’s manual for fuel pressure specifications. Replace the fuel pump if the pressures are lower than the specification required by the engine manufacturer.
Dirty Carburetor
The carburetor’s function is to mix gas with air to form combustion. When the carburetor fails to work, your mower will not start.
Too often, the main culprit of a carburetor not functioning properly is old gas. Old gas leaves behind a varnish that may plug the fuel jet or cause the internal components to stick.
When you find the carburetor isn’t working, you’ll need to attempt to clean it, replace any faulty parts, or replace it with a new one.
Before you tear apart your carburetor, do this first:
- Confirm you are getting good fuel flow to the carburetor.
- Remove the air filter.
- Spray carburetor cleaner into the air intake and start the mower. If it turns over and starts using the carburetor cleaner, but then begins to run sluggish and possibly shut off once the carburetor cleaner is burned, chances are your carburetor is dirty. *Don’t use starter fluid*
- Proceed with disassembling the carburetor and cleaning it or replace it with a new one.
Plugged Fuel Vent
The fuel tank must vent to equalize the air pressure inside that tank to the atmospheric air pressure. You will find the the tank vents through the gas cap.
When the vent becomes plugged and no longer allows air to pass through the cap, the fuel tank forms a vacuum. This vacuum keeps gas from getting to the carburetor.
To determine whether or not your gas cap is the problem, loosen the cap and attempt to start the engine. If it starts, the gas cap may be the cause.
To further confirm the cap as being the problem, continue to let the engine run while tightening the cap. If it begins to sputter, shuts down, and won’t start again until you loosen the cap, you must replace the cap with a new one.