So you’re mowing along as usual when your mower starts feeling sluggish. It’s like it doesn’t know whether it should run or maybe it should give up and stop. You may immediately jump to the conclusion your engine is going bad which can be expensive.
Good news! Most lawn mower sputtering is not typically due to a damaged or hurt 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.
A lawn mower sputtering is due to an insufficient mixture of air and fuel required to form a combustion in the engine. This can result from a fuel or air restriction, a dirty carburetor, or water in the fuel system.
I’ll share different causes that can make your mower sputter along with how to fix them.
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Follow all safety instructions provided in your equipment operator’s manual prior to 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 or Runs Rough
Plugged Lawn Mower 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.
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 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.
Solution: 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 light shine through the paper.
Replace your filter if you cannot. 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 filter 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”.
Bad or Wrong Fuel or Water in the Mower’s Fuel System
Most lawn mowers with 4-cycle engines use regular gas with an octane rating of 87 or greater. It must contain no more than 10% ethanol.
You can have sputtering problems 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. 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, use a flashlight to check the tank for fuel 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.
I like adding a fuel additive called Sea Foam Engine Treatment to my fuel to stabilize gas and prevent moisture buildup. You can read more about it in “Why Use Sea Foam Fuel Additive in a Lawn Mower“.
Solution: If you find water in your fuel tank, drain fuel from the tank into an approved container. Fill the tank with fresh gasoline that includes Sea Foam.
Start the mower and let the gas and Sea Foam run through the system until it no longer sputters or smokes from water.
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.
Solution: 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 mower doesn’t have a fuel shut-off valve located at the bottom of the fuel tank, go ahead and use clamps on your fuel hoses to stop fuel 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 lawn mower.
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 my article.
Dirty Lawn Mower 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.
Solution: 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.
Bad Spark Plug
Your spark plug exists to create the spark your engine requires to ignite the air and fuel mixture to form a combustion. A bad spark plug can cause your engine to sputter or not run at all.
Solution: 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.
Other Items Can Seem Like Your 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 sputtering when a safety switch doesn’t work correctly, and every time you hit a bump the mower seems like it sputters because the switch isn’t making a good connection.
Solution: Check your safety switches and replace them if you find a faulty switch.
Increased Load on Your Engine
Putting additional stress on your engine can put it under load and make it seem like it is sputtering. A clogged mower deck can cause your engine to work harder because your blades need to spin through debris in the deck.
Solution: Scrape the mower deck. Reduce buildup under your mower deck by not mowing grass while it is wet. Reduce your ground speed when mowing thick, heavy, or wet grass.
Perform Regular Maintenance on Your Lawn Mower
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. For a list of items you need to complete on your lawn mower check out one of these articles:
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a lawn mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For a list of the most common lawn mower problems and items that can cause them, check out my guide “Common Lawn Mower Problems: Solved!“