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Why is My Lawn Mower Smoking? 7 Possible Reasons

lawn mower and weeds in the lawn

It is common to see smoke pour out of your lawn mower when you start it up, especially if you haven’t started it in a long time. The smoke can be alarming. How much smoke is too much smoke? Why is my lawn mower smoking? Is something broken? These are common questions that run through any owner’s mind when this happens.

There are many reasons why your lawn mower might be smoking. Most of the reasons are entirely harmless and have simple explanations or solutions. Here are seven possible reasons your lawn mower might be smoking.

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The 7 Possible Reasons Why Your Lawn Mower Smoking

1. Oil Reservoir Was Overfilled

lubricant oil can
Image By: Skitterphoto, Pixabay

One of the most common causes of smoke coming from a lawn mower is simply an overfilled oil reservoir. When pouring new oil into a lawn mower, it is easy to accidentally overshoot the fill line and put too much in. If this happens, do not worry, many oil reservoirs have a line that siphons off excess oil in the case of an overage. The smoke is the result of the excess oil being burned off and is harmless. The smoke should only last a couple of minutes at most if it is the result of too much oil.

2. Gas or Oil Spilled On Engine

Another common cause of smoke coming from a lawn mower is simply spilling gas or oil onto the engine. This happens all the time when people go to refill their machines. When you are pouring the liquid into the often tiny holes, there is a chance that oil and gas will slosh out of the can and hit the engine. If you start the engine right after spilling, the lawn mower is going to burn off the gas and oil from the surface, which can sometimes cause smoke. This is also harmless as it is just some spilled liquid being burned off when the engine starts.

3. Lawn Mower Is Used On Steep Slopes

cropped man mowing lawn with lawn mower
Image Credit: Pexels, Pixabay

Your lawn mower might produce a lot of smoke if you mow on steep hills a lot. Some people have very sharp gradients in their yards that need to be mowed. When a lawn mower is used on a hill, the liquids run downward and can pool in areas and run out of their respective reservoirs. This will cause the lawn mower to burn off the excess liquids as smoke, similar to what happens if you overfill the tanks.

4. Bad Oil Seals

One of the more serious causes of lawn mower smoke is bad engine seals. When the seals in the engine erode and crack, it can cause oil to leak into the combustion chamber, which will create smoke. Corroded engine seals are a problem because they will cause oil to continuously drip into your engine, which is bad for its long term health.

It will also cause the oil to slowly burn away, leaving your engine without the important lubricant it needs to function properly. If your lawn mower is producing smoke for longer than a couple of minutes and produces smoke regularly, then you might need to have your seals checked.

5. Clogged Air Filter

person replacing air filter of lawnmower
Image Credit: The Toidi, Shutterstock

Surprisingly, a clogged air filter can cause a lawn mower engine to smoke. When air cannot properly flow to the engine, it can throw off the fuel balance, which can cause smoke to form during use. Clogged air filters can also let debris fall into the engine, which will burn up and cause smoke. If you have small amounts of smoke coming from your engine intermittently and you did not spill any liquid on it, you should check your air filter for signs of dirt and grass.

6. Damaged Engine Components

Sometimes the engine components themselves can get damaged and cause smoke to form. Persistent smoke could be an indicator of a cracked crankshaft. If air gets into the crankshaft, it can cause the moving parts inside the engine to start smoking. There could be a crack or leak in the fuel lines. You might also have erosion around the pistons in the engine, which can cause oil to seep and burn. A damaged engine will often require the services of a professional small engine repair mechanic to fix.

7. Dirty or Ill-Adjusted Carburetor

Red Old mower in the garden
Image Credit: David Schwimbeck, Shutterstock

A dirty carburetor can also be a source of engine smoke coming from a lawn mower. If a carburetor is filled with grime, that build up can flake off and burn, which then causes smoke to appear. A poorly adjusted carburetor can also cause this problem. You can clean the carburetor with carburetor spray that you can find at your local auto shop or home improvement store. Consult your owner’s manual to find out how to adjust your carburetor so it is in pristine working condition.

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There are three different colors of smoke that often come out of an engine, blue, white, and black. Each color is the result of a different issue that could be taking place within your lawn mower’s engine.


Blue smoke or bluish smoke is the result of oil or fuel being burned in the engine. It is not uncommon to see puffs of blue smoke come out of your lawn mower during operation. As long as you are not seeing a constant stream of bluish smoke emanating from your lawn mower, you should be in good shape. Blue smoke is the most common color of smoke that will regularly appear around your lawn mower.

lawn mower on grass
Image Credit: Skitterphoto, Pexels


White smoke is often an indicator that your lawn mower is burning oil. In many cases, this is not a severe problem. It is not uncommon for oil to spill or seep from one area of the engine into the other. If you do not see white smoke regularly, you have nothing to worry about. You only need to worry about white-colored smoke if it occurs all the time or comes out in a long persistent stream that lasts more than five minutes.


Black smoke often denotes a fuel issue. Old or dirty fuel can cause black, oily smoke to appear. Black smoke also happens when you leave the choke on for too long. If the engine is not getting enough air, it will throw off the fuel mix, and it will start burning too much fuel, which will cause black smoke to build up. This can also happen if the air filter is dirty or clogged. Not enough air can lead to black smoke coming from your lawn mower.

Budget Lawn Mowers

When Should You Be Concerned About A Smoking Lawn Mower?

Belching, smoking, and sputtering are all common occurrences with small gas engines. In most cases, these behaviors are normal and completely harmless. You do not have to worry about smoke coming from your engine unless the smoke lasts for longer than five minutes or if your engine is producing an abnormal amount of smoke. Small puffs, initial clouds, and small short-lived streams of smoke are all common.

If your engine is producing a massive cloud of smoke that fills your face or garage when you turn it on, something could be wrong. Similarly, if you start up the lawn mower and start using it for ten, fifteen, or twenty minutes at a time and the smoking does not stop during regular use, then there could be a problem.

If you run into an issue where your lawn mower won’t stop smoking, you might need to take it into a repair shop to get it checked out. Leaving an engine problem unresolved can severely decrease the lifespan of your lawn mower.

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You should now have the tools to properly identify and diagnose what your lawn mower’s engine smoke means. In most cases, you should be happy to find out that the smoke is not a sign of a serious problem, but even if it is, you should be able to spot the warning signs before the problem becomes more severe. All small engines smoke a little bit. It is all about determining why and adjusting from there.

Featured Image Credit: Pixabay


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