Are Lawn Mowers Supposed to Smoke?

Approx Reading Time: 8 minutes

After using your lawn mower for a while, you will probably run into this problem in one way or another. Your mower starts emitting smoke – this can be very confusing, especially if it never happened before. But what is happening here?

Are lawn mowers supposed to smoke? Lawn mowers are not supposed to smoke continuously, but a small amount of smoke is not always a sign of a major problem. Smoking can occur when you overfill a gas or oil chamber. However, more severe issues such as a blown gasket, clogged air filters, or an incorrect fuel mixture can also cause a mower to smoke.

If your mower emits white, blue, or black smoke you should investigate further to determine the cause of the problem. This article is here to help you with that. 

What Causes White Smoke in a Lawn Mower?

Although a small smoke discharge may not signify a large problem, lawn mowers that continuously smoke have issues that must be addressed immediately. It’s not uncommon for mowers to emit white smoke after you start them up. The following list displays the possible causes of white smoke emissions:

  • Excessive moisture in the gas tank
  • Excess oil residue from the manufacturer (if using a new machine)
  • Spilled oil on the crankcase
  • Faulty spark plug
  • Blown head gasket

Excessive Moisture

If your lawn mower’s engine has difficulty starting and emits a white smoke when it cranks up, the problem may be excessive moisture. Storing a recently used mower in an enclosed space with high humidity can cause condensation to build up near the top of the tank.

A partially closed or damaged fuel cap can increase condensation in the tank, and over time, the water added to the gasoline will inhibit the starting process. Watered down fuel cannot ignite properly and will not improve until you change the gas.

To empty the old fuel, use a siphon pump or a turkey baster (better for push mowers with small tanks) to remove the gas. Dispose of the old fuel properly and add new fuel to the tank. 

Excessive Oil Residue

You may be concerned about your new mower after you noticed a puff of white smoke when you first started it up. This occurrence is normal for new mowers. Allow your mower to run idly until the smoke dissipates.

Sometimes the manufacturing process leaves small traces of oil residue on the engine, and the white smoke is only the oil being burnt off by the heat of the engine.

Spilled Oil

When you change the mower’s oil and fill up the oil chamber, sometimes, a small amount will spill into the crankcase. When the engine heats up above the oil’s boiling point, the oil burns off and creates white smoke.

Most new mowers come with a small oil pouch that includes the exact amount of oil for the mower, but when you change the oil, you use a larger plastic bottle to fill the tank. It’s easy to overfill the chamber, but after five to ten minutes of constant mowing, the oil should burn off, and the smoke will disappear.

Be sure to check the exact specifications from your mower’s manual concerning the appropriate grade of oil to use. If you use oil that is too heavy, the engine will blow white smoke until you replace the oil with the correct type.

Faulty Spark Plug

An overly carbonized spark plug can cause the engine to produce white smoke. Once you’ve turned off your mower, use a socket wrench to remove the plug. Scrutinize the contacts, and if there is carbon buildup, use medium-grit sandpaper or wire brush to remove the carbon.

A cracked plug can also be the cause of the smoke. If it’s damaged, replace the plug with a new one.

If the previous repairs do not affect reducing the smoke from the mower, the problem could be the dreaded blown gasket.

Blown Head Gasket

Over time, the heat and stresses placed on the engine’s gaskets can lead to cracking and eventual failure. This is not uncommon for old mowers, but if you have a newer model, check your warranty information before you contact a repair specialist.

You can determine if the gaskets are cracked or blown by disassembling part of the mower’s engine that exposes the head gaskets, or if you’re uncomfortable with repairs, contact a lawn mower repair shop. The gaskets are likely blown if you tried the first four suggestions and had no luck getting rid of the white smoke.

What Causes Blue Smoke in a Lawn Mower?

A mower that expels blue smoke may have similar problems as a mower that blows white smoke. Sometimes a larger amount of oil burning off the engine will cause blue smoke rather than white smoke. The following list displays the possible causes of blue smoke emissions:

  • Overflowed oil
  • Wrong oil type
  • Incorrect oil mixture
  • Blown head gasket

Overflowed oil

You may see blue smoke billowing from your mower after you topped off the oil or recently changed it. Oil leaked onto the crankcase can cause white or blue smoke depending on the quantity spilled and the type of oil used.

For instance, if you spill half of a teaspoon of oil on the engine, you’ll probably see white smoke. Spilling a quarter cup of oil or more onto the engine can result in blue smoke. 

In addition to burning oil on the crankcase, your spilled oil may burn off in the muffler if you recently tilted the mower on the muffler side. In both the crankcase and muffler situations, the solution is to continue to run the mower until the smoke disappears.

Wrong Oil Type

Sometimes the wrong oil grade will cause the mower to blow blue smoke. If a previous owner or lazy family member substitutes a heavier grade of oil for the correct type, the engine can smoke. 

Check the owner’s manual or manufacturer’s website to find the correct oil for your mower. Discard the old oil properly and fill the chamber with fresh oil. For less than thirty dollars, the best tool for oil removal is an extractor pump. When you mow with the new oil, you shouldn’t see any blue smoke.

Incorrect Oil Mixture

This section only applies to two-stroke engines that require an oil/gas mixture. Each two-stroke engine requires a specific oil to gas ratio, and if the mixture is too oil heavy, the engine can emit blue smoke.

Refer to your owner’s manual for the exact ratio of oil and gas and drain the gas tank to remove the old mixture. Fill the tank with the correct mixture, and you shouldn’t see a recurrence of blue smoke.

Blown Head Gasket

The wear and tear from years of summer mowing can eventually damage the head gaskets. As previously mentioned, the mower can emit white smoke from a blown gasket, but in some cases, the smoke can appear blue.

Blue smoke usually means that the gaskets are more damaged than those that produce white smoke. However, unless you have experience with small engine repairs, you’ll have to take the mower to a repair shop to fix the engine. 

Estimates of gasket repairs range from $80 to $300. For a small push mower, it’s often a better deal to purchase a new mower.

What Causes Black Smoke in a Lawn Mower?

Unlike the conditions experienced with blue or white smoke, black smoke from the lawn mower usually means too much fuel is being burned off. The following reasons can cause this scenario.

  • Blocked Air Filter
  • Incorrect Air/fuel mixture
  • Wrong gap on spark plug

Blocked Air Filter

If your mower is burning too much fuel and producing black smoke, the problem may be as simple as a clogged air filter. The filter sits in front of the carburetor, and if the filter is dirty, the carburetor cannot mix the gas and air properly.

On most models, you can remove the filter by opening a plastic clasp or removing a bolt. Replace it with a new filter according to the manufacturer’s specifications.

Incorrect Air/Fuel Mixture

Another reason for black smoke emissions is the incorrect air/fuel mixture from the carburetor. Some carburetors can be adjusted with a small screw. To adjust the carburetor, keep the mower running while you twist the screw to the right or left.

If the engine stalls, restart it and adjust the screw in the opposite direction. When the engine’s sound improves as you adjust the screw, you shouldn’t see any black smoke.

Wrong Spark Plug Gap

Sometimes, an aging spark plug can cause your mower to blow black smoke. If the plug’s gap is too large, the plug will not start the mower correctly and will sometimes sputter and produce black smoke. Instead of adjusting the gap size, you should purchase a new plug.

Spark plugs only cost a few dollars, and it’s better to replace an old plug than repair it and wait for it to fail.

Closing Remarks

When your mower emits smoke, it may seem like a disaster waiting to happen, but most of the time it’s a simple problem that’s inexpensive to repair. 

However, if your repairs fail to improve your smoking lawn mower, take it to a professional. You may have a blown gasket that will continue to smoke up your yard and draw complaints from your next-door neighbors.

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!