When you turn the ignition key, the lawn mower’s engine acts like it’s going to start, but it just won’t. This is most likely because it’s not getting the air, fuel, spark, or compression to ignite the fuel mixture.
Some items that can cause a riding mower to turn over but not start are a clogged air filter, wrong choke setting, dirty carburetor, bad fuel pump, plugged fuel filter, or a bad spark plug. Another likely reason is old gas.
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.
Riding Mower Turns Over But Won’t Start: Engine Cranks
1. Clogged Air Filter
So let’s start troubleshooting the problem by looking for items that can restrict airflow. The first item is the air filter.
This filter is required to protect the engine from dirt getting into the air intake and damaging the engine. Just a little bit of dirt or debris can shorten the engine’s lifespan.
With all of the dust and grass clippings that get tossed into the air when mowing, you must use an air filter. It needs to be cleaned regularly and replaced when it gets bad or damaged.
A clogged filter will reduce the amount of air that passes through the filter. When your engine doesn’t get air, it won’t start.
I recommend starting each mowing season with a new air filter and then cleaning it several times throughout the season. The air filter is an item you need to take time to check before operating your riding mower to ensure it is in good condition.
SOLUTION: Follow the instructions below to clean a paper air filter and foam pre-cleaner if your engine uses a pre-cleaner. Refer to your owner’s manual if your engine uses a different type of filter.
Clean a Riding Mower Paper Air Filter
- Remove the filter for the air filter housing. Be careful not to allow dirt to fall into the air intake.
- Use a clean dry rag to wipe out any dirt or debris left in the air filter housing.
- Tap your paper air filter against a solid surface to knock as much dirt out of the air filter as you can remove.
- Hold your paper filter up to a light source and look to see if you can see light shine through the element.
- Reuse a filter when you can see light.
- Replace the filter with a new one when you can’t see any light, the filter is extremely dirty, or is damaged.
- Install and reattach the air filter cover. (If your mower uses a foam pre-filter, clean it using the instructions below and install it before attaching the filter cover).
Clean a Riding Mower Foam Pre-Filter
Not all engines use a pre-filter or a foam-style pre-filter. Do not confuse this with a primary foam filter. A foam pre-filter is an extra filter used with a paper air filter to trap dirt. NEVER add oil to a pre-filter or you will damage the paper filter.
- Inspect the pre-filter. Replace it if you find any tears or if it has become brittle.
- Wash the foam filter with water and mild dish soap to remove dirt and oil.
- Rinse with water until the soap is removed and the water runs clear.
- Lay flat and allow to dry. Placing the filter outdoors in the sun will speed up the drying process.
- Once dry install the foam pre-filter with the paper primary filter and reattach the filter cover.
2. Wrong Choke Setting Affects Starting
You will find a choke lever or choke knob on your mower that will engage the choke. This is a part that contains a plate that opens and closes to control the amount of air mixed with gas to be burned in the engine.
A cold engine requires a rich fuel mixture with a high concentration of gas and less air to start. To do this, you need to place the choke in the on position which will close the choke plate.
Once the engine warms, the choke lever must be adjusted to the off position so the engine gets more air and continues to run. If the motor is already warm when you’re trying to start it, the choke must be off.
SOLUTION: When the choke lever is in the wrong position, your riding mower will have a hard time starting and may not start at all. Ensure the choke is in the right position.
If the choke is set correctly and you are still having airflow issues, check to make sure the choke plate is not stuck and the choke cable is moving freely. Use carburetor cleaner to help free a stuck choke plate and linkages.
You may even have to replace the choke cable if it is worn and you are unable to adjust it to work properly.
3. Fouled or Bad Spark Plug
Next, let’s look at the spark plug. You may have one plug or two plugs if you’re running a two-cylinder engine on your riding mower.
Remove a plug by using a 3/4″ or 5/8″ socket wrench. The size you need will depend on your engine model. Take a look at the firing end of the plug.
A spark plug that is damaged or very dirty can cause an intermittent or lack of spark. Without a spark, the engine won’t be able to start and run.
Things to look for in a bad spark plug are a dark-colored tip covered in soot, a wet spark plug, or broken porcelain. If you find any of these conditions, install a new spark plug.
If you end up finding a wet plug, it’s best to find the reason why the plug is covered in fuel or oil so you can repair the problem.
SOLUTION: You can attempt to clean a plug that is in good condition and just a little dirty. Use a small wire brush to remove the carbon buildup.
Reinstall the spark plug after you ensure the electrode 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 by installing new spark plugs in your mower.
If your spark plug is in good condition, check the condition of the spark plug wire. Make sure it isn’t beginning to dry out and crack. This could cause it to ground out.
Next, check the ignition coil. If this fails, you won’t provide the power to create a spark.
4. Old Fuel or Low Fuel
You may not only have problems starting your riding lawn mower because of low fuel, but your problems may also be the result of bad fuel.
In case you don’t already know, gas doesn’t stay good forever. Ethanol added to fuel today causes it to go bad quickly. That is why engine manufacturers recommend consuming gas within 30 days of purchase.
Old gas leaves behind varnish and sticky deposits that cause fuel restrictions and component failures.
To ensure you run the right gas through your riding mower, always purchase fresh gasoline and consume it within a month. Stay away from gas with high ethanol levels.
Use gasoline with a minimum 87-octane rating and a maximum of 10% ethanol content.
SOLUTION: 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. Read more about the benefits of Sea Foam here.
If your mower starts, allow it 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.
5. Clogged Riding Mower Fuel Filter
Look for other reasons your mower isn’t getting fuel like a clogged fuel filter.
You will find an inline fuel filter installed between the fuel lines on your riding lawn mower. The filter is used to strain fuel that comes out of the fuel tank to keep dirt from flowing through the fuel system and to the engine.
The 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 lawn mower to turn over and not start.
SOLUTION: Replace an old or dirty fuel filter with a new fuel filter.
6. Clogged Fuel Line
Next, follow the fuel line from the fuel tank to the carburetor. Look for any kinks that may be restricting fuel flow causing the lawn 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.
SOLUTION: When you aren’t getting sufficient fuel flowing through a fuel line, shut off the fuel supply and remove the fuel line from the lawn tractor.
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.
7. Bad Mower Fuel Pump
You may have a fuel pump on your riding mower. Not all mowers use a pump so don’t worry if you don’t see one.
A pump is used if the carburetor is placed higher than the fuel tank. This pump is used to work against gravity and move fuel uphill.
If you do find a fuel pump, most will be a vacuum-style fuel pump. This type of pump uses the vacuum of the crankcase to move fuel.
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, ensure 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)
SOLUTION: 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 your mower.
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.
8. Dirty Carburetor on a Riding Lawn Mower
The carburetor’s function is to mix gas with air to form combustion in the engine. When the carburetor fails to work, your lawn mower will turn over and not start because it isn’t getting enough gas.
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.
SOLUTION: 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. You can find more detailed information for cleaning a carburetor here.
9. Clogged Mower Fuel Tank Cap
The fuel tank must vent to equalize the air pressure inside that tank to the atmospheric air pressure. On a lawn mower, the vent is located in 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. The lawn mower will turn over, but not start because of the lack of gas.
This is more often a problem when you’ve been running your mower for a while, it shut down, and won’t restart.
To determine whether or not your gas cap is the problem, loosen the cap and attempt to start the mower. If it starts, the gas cap may be the cause.
To further confirm the cap as being the problem, continue to let the mower run while tightening the cap. If it begins to sputter, shuts down, and won’t start again until you loosen the cap.
SOLUTION: Replace a bad or clogged gas cap with a new one.
10. Poor Engine Compression
Once you have verified you are getting fuel, air, and spark, take a look at the engine. You can perform a compression check or leak-down test to confirm the engine has compression and there aren’t any internal engine issues with the valves, piston rings, or other items.
You’ll need the right gauges and the specifications for your engine model. Note: many small engines have an automatic compression release. A leak-down test is a better test for these types of engines.
I recommend taking your mower to your dealership with factory-trained support or a reputable small engine mechanic in the area for any engine diagnostics and repairs.
More Help With Your Riding Mower Problems
When you own a riding mower long enough, chances are you’re going to experience a number of problems. Because of this, I have put together a list of common problems with links to help troubleshoot the problem.
Check out my article with a list of common problems that develop in a riding mower here.