Lawn mowers are very simple machines. They’re designed to start right up and get the job done when you need it.
But, if your mower is getting old or you’ve neglected to maintain it properly, you can run into real trouble. A rough running or sputtering lawn mower is one of the most common problems people run into with their lawn equipment.
Thankfully, since these machines are relatively simple, the solution to a sputtering lawn mower is often very simple. Keep reading for a closer look at how lawn mowers work and some simple ways to get yours running again.
How Does a Lawn Mower Engine Work?
If you want to figure out why your lawn mower is sputtering or running poorly, it helps to understand how the engine works. As complex as it may seem from the outside, a lawn mower motor is actually very simple.
Your mower’s engine is basically just a smaller, much simpler version of the motor in your car. Unlike your car, a lawn mower just has a single cylinder and spark plug.
Air and fuel are pulled into the lawn mower engine with the carburetor and ignited in the combustion chamber by the spark plug. Each time this happens, the engine’s cylinder is compressed. This motion turns the crankshaft which then turns the mower’s blade. If you have a self propelled lawn mower, the crankshaft also turns the drive wheels when engaged.
Why is My Lawn Mower Sputtering?
Old Fuel
If your mower isn’t running right, start with the simplest solution. Fuel is one of the simplest things to check, so start by determining how old your gas is.
Believe it or not, gasoline does have a shelf life. If you’re trying to start your mower for the first time this spring and are using fuel you bought last summer, think again. Old gas can make your lawn mower run poorly and create extra build-up internally that will give you more problems in the future.
Old Fuel: The Solution
This one is pretty easy. Drain the old gas out of your mower into an appropriate container and refill the mower with new gas
If bad fuel was the problem, your lawn mower should run better right away.
The Wrong Fuel
Most fuel available at gas stations contains about 10% ethanol. Ethanol is a fuel made from corn that burns just fine in most modern cars, but not in smaller carbureted engines.
I have personally seen gas with ethanol destroy small engines with repeated use.
The Wrong Fuel: The Solution
While it’s rare, you can find gas stations that sell “pure” gas with no added ethanol. If you can find a station near you that does this, make sure to fuel up your mower there.
If you don’t have a station like this near you, most home improvement stores sell fuel specifically designed for small engines. While it is more expensive, this type of fuel will help your mower’s engine last much longer.
Dirty Air Filter
Your lawn mower needs air and fuel to run properly. If either of these is restricted, the mower will not run properly.
Over time, a lawn mower’s air filter will get dirty. As the filter gets dirtier, airflow becomes more restricted.
Dirty Air Filter: The Solution
A quick fix for this problem is to remove the filter from the mower and shake off the excess dirt and debris. This will often help your mower run better for a short time.
If the filter is old and excessively dirty, it probably just needs to be replaced.
Lawn mower air filters are usually located beneath a cover that can be quickly removed with no tools, making this a very simple repair.
Worn Out or Dirty Spark Plugs
One of the most common causes of a sputtering lawn mower is worn out or dirty spark plugs. Spark plugs corrode and wear out with use, and will not work as well as the wear gets worse.
Worn Out or Dirty Spark Plugs: The Solution
To fix this problem, you’ll need to check the spark plug in your lawn mower.
Use a spark plug socket to remove the spark plug safely. With the spark plug removed, check it for excessive build-up and wear.
If the spark plug looks overly dirty, go ahead and replace it. Spark plugs are very inexpensive and simple to replace on a lawn mower.
Dirty Carburetor
A dirty carburetor is another common cause of a sputtering or poorly running lawn mower. Small engine carburetors can get dirty pretty quickly especially when they’re used in less than clean environments — like your yard.
A dirty carburetor makes it difficult for the engine to get the right fuel and air mixture. This can make your mower sputter and run poorly.
Dirty Carburetor: The Solution
This problem can be solved very easily with carburetor cleaner. Carburetor cleaner comes in an easy-to-use aerosol can and is usually designed to spray directly into the carb while the engine is running.
The air filter — and sometimes the air filter housing — will need to be removed first, then simply spray some carburetor cleaner into the open carb while the mower is running. The mower may run a little roughly while it burns up the cleaner, but should then smooth out once the carburetor is clean.
Grass Build-Up in the Mower Deck
If you let your grass get too long or are trying to mow wet grass, dirt and grass can build up quickly in the mower deck.
The mower deck is the area under the mower where the blade rotates. If too much grass builds up in the deck, the mower’s engine will have a hard time turning the blade, causing your mower to sputter and possibly stop running completely.
Grass Build-Up in the Mower Deck: The Solution
If you suspect that grass is building up in the lawn mower deck while you are mowing, try stopping and back up a few steps. Simply backing up will often break the grass build-up free and allow your mower to mulch the grass or bag it. You can also try leaning on the handle and lifting the front of the mower slightly while backing up.
If backing up doesn’t work, shut the mower off and lift the front of the mower or turn it on its side to clean off stubborn build-up.
Dirty Fuel Filter
A dirty fuel filter can restrict the flow of gas to the engine, which could lead to a sputtering lawn mower.
Dirty Fuel Filter: The Solution
Check and change your mower’s fuel filter regularly to prevent problems. The fuel filter is usually found right on the lawn mower’s fuel line, making it very easy to change.
Bad Gas Cap
While it’s not very common, a loose or bad gas cap could also make your lawn mower sputter.
Bad Gas Cap: The Solution
Check your lawn mower’s gas cap regularly to make sure it still fits properly. If the gas cap is loose, you should be able to find a new one very inexpensively.
Your Turn
If you’re wondering “why is my lawn mower sputtering?” the answer is often to simply make sure your mower is clean and properly maintained. Routine maintenance, like changing spark plugs and filters, and a little cleaning will keep your lawn mower running smoothly for many years.