What causes a lawnmower to smoke? Causes and fixes


Lawnmowers are great machines that will seldom fail. Good maintenance will keep them working correctly for a long time, but all machines can break down at some point. It is always possible that while mowing your lawn, you notice some weird smoke coming out of the exhaust. You become worried that the lawnmower might have become faulty. This blog post will help you with the reasons why your lawnmower will smoke. Sometimes smoke might be regular. Excessive smoke might be an indication of some more severe problems and can have various causes. Important is the smoke color, as each will have a different reason and need another fix.

What causes a lawnmower to smoke, causes and fixes:

Smoke from lawnmowers may be white, blue, or black. There are distinct triggers behind each smoke type, and each needs another fix. Black smoke is usually caused when the fuel is not completely combusted, and some of it turns into black smoke. Issues with the air filter or spark plug can be the cause. Blue smoke is often caused by an oil overflow, an oil spill, a leaked crankcase, or a tilted engine during operation. White smoke occurs when the engine is burning oil that somehow got into the engine and is burned there.

What causes a lawnmower to smoke? Causes and fixes 1

Engine, lubrication, and cooling

Before diving deeper into the details of the causes and fixes, it will help to have a better insight into your lawnmowers working, especially the engine. The engine is the source where most smoke is produced. These are a few fundamentals involved.

● Working of engine

The engine on a lawnmower combusts the fuel and air mixture to convert energy from a fuel into the flywheel’s rotary motion and drive the blade. This procedure is broken down into four parts.

  1. Intake of air and fuel: Air and fuel are mixed in a precise amount. If this ratio is disturbed, the quality of performance is heavily affected. Intake occurs through a valve that is controlled by pushrods and cam mechanism. The air to fuel ratio and quality of the mix is controlled by the carburetor and related filters.
  2. Compression of fuel mix: The piston moves up inside the cylinder compressing the air-fuel mixture. The compression enables the power when fuel is burned.
  3. Combustion of fuel: When the piston is about to reach the topmost position, the fuel starts to burn. It happens either by a spark or due to enough rise in pressure. This blast pushes the piston down, transferring power into the flywheel.
  4. Removal of the burnt: Exhaust valve open, and all of the remnants are pushed out to muffler by the inertial effect of the already caused movement.

These steps might understand one side of the picture, but there is more to it. All moving parts require proper lubrication, and due to an elevated operating temperature, cooling is a significant concern.

● Lubrication

For lubrication, oil is used. Oil is introduced and pumped through orifices incorporated in the design. It must not enter the cylinder; for this purpose, O-rings are used around the piston. The presence of O-rings does not allow seepage of the oil film. Similarly, the gasket also serves as a seal.

● Cooling

Lawnmower engines can be air-cooled or have a coolant system. Air-cooled have just fin and extended surface that allows quick transfer of heat. At the same time, coolant is pumped through jackets present around the engine block.

Moving forward, we will learn more about the causes of smoke from the lawnmower. To know the causes of the smoke is classified based on color and then discussed in detail.

There are four main types of smokes:

  1. Black smoke
  2. Blue smoke
  3. White smoke
  4. White smoke in new lawnmowers

1. Black smoke: causes and remedies

● Cause 1: Incomplete combustion of fuel

A lawnmower blows black smoke while the fuel is not completely burnt. It is essential to know that proper and complete combustion of smoke in the engine is essential for smooth operation and health. In some cases, the air-fuel mixture that forms in the engine is too rich in fuel and does not have enough air to burn the fuel completely. As this occurs, the fuel that is not wholly burnt in the engine’s combustion chamber turns into black smoke. This condition is also named rich fuel.

– Fix: Adjust, Clean or replace the carburetor

If the incomplete combustion of fuel is, the cause behind your lawnmower’s black smoke is a problem with the carburetor. Cleaning or replacing the carburetor will fix it. It’s the carburetor’s role in the engine to create the right air-fuel combination, and that’s why a carburetor issue might lead to black smoke. Fixing the carburetor in such a way that the air-fuel mixture is appropriately proportioned is not tricky. What you can try first is to adjust the carburetor screw and see what happens. Check your user manual if there is more information on it. If adjusting the screw does not fix the carburetor problem, cleaning or replacing will be the next step.

● Cause 2: Dirty or clogged air filter

Another potential source of black smoke may be a dirty air filter. This a day-to-day maintenance procedure that you should check for after every session of usage. 

– Fix: Clean the air filter

It’s easy to fix this problem. Only remove the air filter and wash it with water and soap. If it’s too dusty and can’t be cleaned appropriately, replace it with a new one.

● Cause 3: Carbon deposits on Sparkplug terminals

If your mower has been blowing black smoke and is not starting now, the problem must be with the spark plug.

– Fix: Clean the sparkplugs

Take the spark plug out and clean its terminals off any deposits. Use sandpaper or a file to remove the residues. If there is a buildup too big and thick to clean, replace the spark plug.

2. Blue Smoke

● Cause: Overfilling lubricant

Oil overfilling produces blue smoke. If your engine is blowing blue smoke, check the dipstick’s oil level, and if the oil level is too high, change it to the correct level.

– Fix: Drain oil

The dipstick is located on the reservoir. Clean the dipstick with a piece of cloth and remove the cap from the reservoir. After that, dip the dipstick for a moment, then take it out. The oil leaves a mark on the dipstick. If this mark is above the max limit mentioned on the stick, you’ll have to drain some of the oil. For draining the oil, consulting your mower’s manual will help. So, drain the oil and check the level with the dipstick. If the level is too low compared to the manual’s recommended level, keep adding small amounts of oil and keep checking with the dipstick until it is at the right level.

● Cause: Tilted lawnmower

If your lawnmower is working on a more steep, angeled terrain, this can lead to blue smoke as well.

– Fix: Let the lawnmower run on a flat location

The fix for the problem is easy. Just let the lawnmower run on a flat leveled area for some time. If the reason for the blue smoke was the angle, the smoke should go away. If it does not leave, it can be an oil problem.

3. White Smoke

● Cause: Blown Head Gasket

A lousy or blown head gasket often causes white smoke. Oil and coolant seep into the combustion chamber, causing the white smoke.

– Fix: Replace the head gasket

This fix will be more complex as the blown head gasket will probably need to be replaced with a new one. You can first check it, and if the gasket condition is okay, you could use a sealant paste and see if that works. But this is just a momentary approach. It is better to replace it. If you do not feel comfortable with these more complicated fixes, it can be wise to bring it to a mechanic.

4. Smoke in a new lawnmower

This smoke from a brand-new lawnmower is mainly due to oil or any cleaner residue leftover from the manufacturer’s side. There is nothing to worry about or take such an engine to the shop. If the smoke persists, then it is a sign to worry.

Some more conditions may be the cause of the smoke.

  • Two-stroke engine: In this engine, oil cannot be separated from the fuel rater added with it for regular operation. If the ratio of this additive combo is not as mentioned in the instruction manual, it may cause the lawnmower to smoke
  • Leaks in crankcase: Check the cracks or seeps in the crankcase. It might also be the cause of the smoke.
  • Breather tube: Breather tube is generally considered to be a part of carburetor but mostly neglected. If there is a blockage in the tube, it will cause smoke from the lawnmower.
  • Tilt angle: If the lawnmower is operated at very steep slopes, it will cause oil seepage into the engine block and cause smoke.

Related question

1. What if the smoke has a rubbery odor as well?

This problem might be caused by the timing belt being worn out, slip, or excessive rub.

2. Can the wrong grade of lube oil cause smoke?

Yes, in some instances, especially in 2-stroke engines. It would help if you tried to use only the grade of oil that is mentioned by the company.

Final Remarks

For people who have no understanding of the problems resulting in their mower blowing smoke, mower smoke may cause severe concern and anxiety. The blowing smoke of the mower is often caused by issues that are not too severe and can be resolved quickly. Let the burning mower run for a while in case of white and blue smoke. In most situations, after a few minutes, smoking will cease. Otherwise, obey the directions above. In the case of black smoke, what you need to do is ensure that the fuel is proper and that the carburetor and air filter are in the right condition.

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