Why is My Lawnmower Sputtering?

A cranky lawn mower will not do a good job cutting grass. If your lawnmower is coughing, chugging, and sputtering, it may be time for a little routine maintenance to get it back on its feet once again. There is good news! With a bit of troubleshooting, there is a simple solution to a sputtering mower most of the time.

How a mower engine works

A mower engine is like a car engine, only smaller. Much like a car, a lawn mower has a carburetor that mixes fuel and air before it gets to the engine. The fuel is ignited by a spark plug. This is why it is called a combustion engine. The engine rotates so that the mower blades engage and rotate.

Reasons for a sputtering lawn mower and how to fix it

Problem: Moisture in the fuel line

There should not be any moisture in the fuel line or the fuel tank. Moisture can get into the fuel line from condensation, a loose gas cap, or the fuel container. No matter how the moisture gets in, it will cause problems. The moisture may block the ignition process and cause sputtering.


You can prevent this problem by checking the gas tank for moisture before pouring the gas into the mower. Make sure the gas cap is on tight, so no moisture gets in.

If there is already moisture in the fuel line or gas tank, drain the contaminated fuel out of the mower and replace it with new fuel. Be careful to dispose of old fuel properly.

Problem: Dirty air filter

A dirty air filter is the most common culprit for a sputtering mower. A clogged or dirty air filter can disrupt that necessary blend of fuel and air to the combustion chamber. You may even see your lawnmower sputtering and smoking.


Some mowers have disposable paper filters. Owners should change the filters every year. A reusable foam filter can be cleaned with dish soap and water, then wrung dry. It’s a good idea to remove the air filter every couple of weeks and shake it out or blow it off to keep the air flowing smoothly.

Problem: Malfunctioning spark plugs

The spark plug is the source of the spark that ignites the fuel and air mixture. A worn out or malfunctioning spark plug will make the mower difficult to start, and it may cause sputtering.


There are two parts of the spark plug to check. First of all, check the firing tip. If the tip is dirty or bent, clean it, and reshape it with a metal brush. Check the porcelain housing on the outside of the spark plug. If it is cracked, the spark plug needs to be replaced.

Problem: Grass build-up

Grass can build up in the mower deck, especially with tall or wet grass. The grass will build up in the mower deck instead of exiting the mower, either to the lawn or to a bag. All this clumped-up grass can cause sputtering.


Clean out the grass! Remove the spark plug before you turn over your mower. Scrape away the built-up grass from the bottom of the mower. Turn it over and replace the spark plug.

Problem: Fuel issues

Dirty Gas Can

The cheapest fuel option at the pump is usually at least 10% ethanol. Ethanol may work well for your car, but your mower probably won’t like it. Ethanol tends to burn quicker and may melt plastic parts in the mower.

Ethanol will also go bad over time. If the fuel has been sitting in your mower or gas can for a while, it could cause your mower to sputter.


Drain all the old fuel from the lawnmower into a container. Make sure the fuel lines and carburetor are emptied, too. Replace the fuel with ethanol-free gas.

Problem: Dirty fuel filters

The fuel filter allows the gas to flow to the engine. If the filter is blocked or clogged, the stream of fuel is disrupted.


Change the fuel filter! Make replacing the fuel filter part of your yearly mower maintenance routine.

Problem: Dirty carburetor

Do you have a dirty carburetor? Over time, the carburetor becomes home to layers of gunky deposits as a byproduct of combustion.


Clean the carburetor regularly. Use a quick spray of carburetor cleaner spray before or after you mow.

Problem: Malfunctioning carburetor

If your carburetor is clean, your fuel is perfect, and all your filters are clear, but the mower is still sputtering, there may be other problems with the carburetor. The precise blend of fuel and air produced by the carburetor is important to keep your mower running. If the ratio is off, the mower will have issues.


Carburetor repair may require expert help. Unless you’re experienced with small engine repair, take your lawnmower to a trusted professional for a repair.

Problem: Bad gas cap

The gas cap on a mower is designed to let in a small amount of air, but not too much. Too much air can cause a vapor lock in the gas tank, leading to that annoying sputtering sound.


This is an easy one! Just replace the gas cap.

Frequently Asked Questions

Repairing a Lawn Mower

Why does my lawn mower pulsate?

A pulsing, vibrating, or surging mower is probably due to one of the reasons mentioned above. Check the air filter first. Sometimes, a clean air filter is all you need for a smoothly running engine.

Why is my lawn mower not running smoothly?

If the mower starts well but then sputters and dies, the issue is probably not the spark plug. Check the filters, and make sure you are using the right fuel.

How do you fix a lawn mower that sputters?

As you can see from the list above, there are quite a few reasons a lawnmower might sputter. Start by changing the air filter and switching to an ethanol-free fuel. If the mower is still sputtering, make sure the deck, fuel filter, spark plugs, and carburetor are clean.

What do I do if I put bad gasoline in my lawn mower?

Drain all of the old fuel out of the mower. Detach the fuel lines. Make sure to remove the fuel from the carburetor as well. You could use a siphon hose to remove the fuel. Replace the bad fuel with ethanol free gas.

Final Thoughts

Get your mower running smoothly again! Most of these fixes are quick, simple, and inexpensive. Keep your mower running smoothly in the future with regular cleaning and maintenance.