Most of the time, a mower that sputters but still runs is not typically due to a damaged engine as long as you take care of your problem right away. If you don’t address the problem, your engine can develop significant damage.
Lawn mower sputtering is often due to an insufficient mixture of air and fuel required for combustion due to a clogged air filter, dirty carburetor, bad gas cap, moisture in the fuel tank, and old fuel.
It can also be caused by a bad spark plug, overworking the engine, and a dirty engine.
I’ll share different causes that can make your mower sputter along with how to fix them.
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.
Reasons Your Lawn Mower Sputters But Runs
1. Dirty or Clogged Air Filter
Your lawn mower engine needs air to run. The air filter is designed to filter out debris and other contaminants from entering the engine’s air intake. A clogged filter will limit the air that gets to the engine.
With all the grass and dirt that can get kicked up into the air while mowing, it’s a good idea to check your air filter frequently as the filter can get plugged rather quickly in these conditions.
You’ll want to clean your clogged or dirty air filter or replace the filter if it is in bad condition. Running a dirty air filter can cause extensive engine damage and cause your engine to not only sputter but overheat as well.
FIX: Remove your air filter by first removing the cover from the air filter housing. The cover is typically held on with one or two thumb screws.
Next, remove the filter from the housing. Be careful to not knock any loose dirt into the air intake. Wipe out any dirt remaining in the housing with a dry cloth.
Cleaning instructions vary by the type of filter you have.
- Paper Air Filter
- Tap your air filter against a solid surface to knock any loose dirt out of the filter.
- Hold the filter up to the light. You can continue to use the air filter if you are able to see good light shine through the paper.
- Replace your filter if you cannot or if it is damaged, very dirty, or covered in oil.
- Do not use an air compressor to blow out your air filter. This can actually cause more harm than good as the compressed air can force dirt into the small holes in the paper and further clog them.
- Foam Air Filter
- If your filter is excessively dirty, has dark spots, or is torn, you will want to replace it with a new air filter.
- If it is not, proceed with cleaning your filter by washing it with mild dish soap to remove as much dirt as you can.
- Rinse the filter with water until all the soap is removed.
- Lay flat to dry. Laying it out in the sun will speed up the drying process.
- Once your filter is completely dry, lightly saturate it with clean engine oil. Don’t use too much oil. You don’t want the oil to be dripping off the filter.
Read more about air filters in my article, “A Guide to Lawn Mower Air Filters”.
2. Bad or Old Fuel
Most lawnmowers with 4-cycle engines use regular gas with an octane rating of 87 or greater. It must contain no more than 10% ethanol. Ethanol can create problems in a small engine.
Gas is often the root cause of a mower’s running problem. You can have sputtering issues if you are using the wrong type of gas or if your gas is old.
Gas has a shelf life of 30 days before it begins to break down and does not run efficiently. Always purchase fresh gas and consume it within 30 days.
FIX: Drain old fuel from the tank into an approved container.
- Push mower: You can either tip the mower to drain the gas or remove the fuel line from the carburetor and drain it into a container.
- Riding mower or zero-turn: Using a fuel siphon pump is the easiest way to drain the fuel tank.
Mix fresh gas with a fuel additive like Sea Foam or STA-BIL to reduce moisture and help clean the fuel system. Then add the mixture to the fuel tank.
Start the mower and let the gas and additive work their way through the fuel system.
3. Moisture in the Fuel System
The ethanol in your gas attracts moisture and eventually begins to separate with the moisture and ethanol mixture sinking to the bottom of your tank.
To identify if this may be causing your sputtering problem, inspect the gas for signs of water. A flashlight works well for this. Shine your flashlight into the tank and look for gas and water separation.
To prevent excess water from getting into your fuel system, keep your fuel tank cap securely in place and store all gas indoors.
Steps to prevent moisture buildup:
- Add a fuel additive to your gas like Sea Foam or STA-BIL. You can read more about the positive effects of using a fuel stabilizer here.
- Store the mower inside or covered it to keep it out of the rain.
- Store all fuel in a dry location.
- Make sure the seal on the fuel cap is in good condition.
FIX: If you find water in your fuel tank, drain fuel from the tank into an approved container. Fill the tank with fresh gasoline with a fuel additive like Sea Foam or STA-BIL to reduce moisture and help clean the fuel system.
Start the mower and let the gas and additive run through the system until it no longer sputters or smokes from water.
4. Clogged Fuel Filter or Fuel Lines
Fuel filters and fuel lines can become clogged by dirt or old fuel. When bad fuel and ethanol break down, it can leave a gummy deposit that can travel through your fuel system getting clogged in your fuel lines and filter.
FIX: Replace the clogged dirty fuel filter or clogged fuel lines. You can check for fuel restrictions by using your fuel shut-off valve to start and stop fuel flow.
If your lawn mower doesn’t have a fuel shut-off valve located at the bottom of the fuel tank, go ahead and use a clamp on your fuel hose to stop the flow.
Check the different sections of your fuel hose by stopping the fuel flow, placing the end of the fuel hose you are checking into a container, and turning the fuel flow back on to see if you are fuel is running through the fuel hose.
Once you have isolated a section of the hose that has a blockage, remove that section of the hose from your lawnmower and clean it:
- Spray a carburetor cleaner into the hose to help loosen the obstruction.
- Then, use compressed air and blow it into the line to clear the line so fuel will begin to flow.
- Reattach the unrestricted fuel hose.
Quick note: If your fuel hose appears dry or cracked, go ahead and replace it with a new fuel hose now before they develop fuel leaks. Read more about common places to check for fuel leaks in this article.
5. Dirty Carburetor
The carburetor is the component on your lawn mower that regulates the air and fuel mix your engine requires to create combustion.
When it is dirty, your lawn mower isn’t getting the right amount of fuel to air mixture and can be sputtering.
FIX: Take your carburetor apart to clean the deposits that have built up. You can find instructions on cleaning a carburetor here.
There are many small parts to the carburetor including the float needle, springs, and gasket. If any of these pieces are damaged, you will have to rebuild the carburetor or replace it.
6. Bad Spark Plug Issue
Your spark plug exists to create the spark your engine requires to ignite the air and fuel mixture. A dirty or bad spark plug can cause an intermittent spark and your engine may sputter or possibly quit running.
FIX: Remove your spark plug(s) to check for carbon buildup on the tip. Clean with a wire brush.
If the spark plug is excessively dirty, the porcelain is cracked, or the electrode is damaged you must replace the spark plug. Make sure the spark plug is gapped to the manufacturer’s specifications or your mower may continue to sputter.
7. Bad Gas Cap
So why would a gas cap cause your mower to sputter? It’s because the fuel tank vents through the gas cap. This is to allow air to pass through the cap and into the tank to displace gas that is consumed.
When the gas cap is bad and is no longer venting, a vacuum forms in the tank that restricts fuel flow to the carburetor.
FIX: An easy way to check this is to loosen the cap to allow air into the tank. If you have a fuel tank vent problem, the engine will run better and not sputter after the cap is loosened.
If the engine appears to be running better with the cap loosened, retighten the cap, and run the mower to see if the engine starts to sputter again. Confirm the problem by loosening the cap again and listening to the engine performance.
Some people attempt to clean the fuel cap. I prefer to replace it.
8. Dirty Mower Deck Clogged with Grass Clippings
Putting additional stress on your engine can put it under load and make it seem like the engine is sputtering. A clogged mower deck can cause your engine to work harder because your blades need to spin through the debris.
Adding dull mower blades to a dirty mower deck can also magnify the issue putting more strain on the engine.
FIX: Access the bottom of the deck. Use a deck scraper to scrape it. If you don’t have a scarper, a putty knife or wire brush works well too.
Tips to reduce buildup under your mower deck:
- Avoid mowing wet grass. Wet grass tends to clump and stick to the bottom of the deck.
- Use a deck spray to prevent sticking. While this isn’t a miracle product, it can help reduce the amount of grass that sticks. It works best when combined with mowing dry grass.
9. Engine Speed Too Low
Another reason an engine can be under too much load is when the engine speed is too low. Engaging the mower blades takes a lot of engine power and you may experience mower sputtering when you aren’t running it at full throttle.
FIX: Make sure you are running the mower at full throttle when mowing. Most push mowers these days don’t have a throttle control to adjust the RPMs.
If the engine doesn’t seem like it’s giving you the power it once did, it may be time to bring your mower to a small engine repair shop to be tested.
10. Ground Speed Too Fast for Mowing Conditions
If you’re trying to mow tall, thick, or wet grass, the engine may bog down and sputter because you’re putting more load on the engine than cutting dry grass.
FIX: Assess your mowing conditions and slow down to match the conditions.
11. Dirty Engine
The small engine uses air to stay cool. It’s important to keep the air intake and the cooling fins clean. The cooling fins are used to dissipate heat.
FIX: Use a small bristol brush and clean around the engine to remove debris and dirt. Be careful when using water around the engine. You don’t want a lot of water pressure.
Item That Can Seem Like Your Lawn Mower is Sputtering
There are a couple of other reasons that may seem like your lawn mower is sputtering. I’m going to go over these other things just in case you may be having this problem.
A Bad Safety Switch
You have a safety switch that is part of the operator presence control system where the switch is a safety feature to ensure the mower shuts off in certain circumstances when it can’t recognize the operator.
If your switch is not operating right, it may cause your mower to shut off. For example, if you are running your mower deck, and the mower doesn’t sense you in the seat, it can shut down.
This could mimic mower sputtering when a safety switch doesn’t work correctly, and every time you hit a bump the lawn mower seems like it sputters because the switch isn’t making a good connection.
FIX: Check your safety switches and replace them if you find a faulty switch.
Perform Regular Maintenance to Prevent Sputtering
You should perform regular maintenance on your lawn mower. This will catch many of the items that can cause your mower to sputter.
Regular maintenance includes checking your engine oil, filters, and safety switches, scraping the mower deck, and more.
A fuel restriction is the most likely reason a lawn mower sounds like it’s running out of gas. This is often due to old gas, a dirty carburetor, or a plugged fuel filter. A clogged air filter, fouled spark plug, or overworking the engine may also cause this symptom.
The most likely cause for a lawn mower sputtering white smoke is burning oil. This can be from tipping the mower or adding too much oil to the crankcase.
An engine that runs rich consumes a higher concentration of gas-to-air ratio causing the mower to blow black smoke.