Lawn Mower Blows White Smokes on Startup – Problem & Solution

Hello there! As a mechanic, I understand that having your lawnmower emit smoke during startup can be quite alarming. Don’t worry; I’m here to help you diagnose, troubleshoot, and fix this problem. Let’s begin by understanding how a typical lawnmower works, the possible causes for the smoke, and the consequences of the problem.

What are the Components of a Lawnmower?

Lawnmowers, especially gas-powered ones, use combustion engines similar to those in your car. The engine’s primary components include:

  1. Cylinder: The combustion chamber where fuel is ignited.
  2. Piston: A cylindrical piece that moves up and down within the cylinder.
  3. Crankshaft: Converts the piston’s linear motion into rotational motion, ultimately powering the blade.
  4. Valves: Inlet and exhaust valves control the intake of the fuel-air mixture and the venting of exhaust gases.
  5. Carburetor: Mixes fuel and air to create the proper remedy for combustion.
  6. Spark Plug: Produces a spark that ignites the fuel-air mixture, causing an explosion.

Possible Causes & Consequences of Lawn Mower Smokes On Startup

Smoke during startup can be due to several reasons, including:

  1. Incorrect fuel-to-oil ratio: Common in 2-stroke engines, an improperly mixed fuel can result in too much oil being burned, leading to smoke.
  2. Overfilled engine oil: Excess oil may get consumed during operation, producing smoke.

Engine oil is essential to maintain your mower engine’s optimal performance and well-being. If the level falls too low, it can cause irreparable damage and even ruin the engine. Most people know this, so they usually top up their mower oil regularly; however, I have observed that they tend to fill it up beyond what is required. Over-full oil is terrible for two reasons:

  1. Excess oil entering the combustion chamber causes smoke as it is burned.
  2. Too much oil in a small engine can be detrimental because many use splash lubrication, and an overfilled system may prevent proper lubrication.

How To Identify- Diagnosing an overfilled oil tank in your mower is easy: simply check the engine oil level. If you find it too full, you’ll need to remove some of the excess. The quickest and cleanest way to do this is with an oil siphon.

Solution- Alternatively, if your mower has an oil drain bung on the bottom of the engine or drains through the dipstick, you can tilt your mower over and use a shallow container to remove some oil. If you’re unsure how to do this, look at the video below for additional guidance.

Engine oil in the Exhaust System

This can occur if the lawnmower is stored on an incline, allowing oil to seep into the exhaust pipe.

Fixing the Problem 

To fix this problem, there are two steps you should take:

  1. Check your fuel-to-oil ratio: Make sure that your fuel and oil mix is in
  2. Dirty air filter: A clogged filter can produce a rich fuel mixture that generates smoke during combustion.
  3. Damaged seals or gaskets: Compromised engine components may cause oil leakage into the combustion chamber.

The consequences of the problem can range from reduced engine efficiency to severe damage to your lawnmower’s engine.

Troubleshooting Steps

  1. Check the fuel-to-oil ratio: For 2-stroke engines, ensure the fuel and oil are mixed at the correct ratio, usually specified in your owner’s manual.
  2. Inspect engine oil level: Check the dipstick and adjust the engine oil level as needed.
  3. Examine the air filter: Remove and clean or replace it if it’s dirty.
  4. Check for leaks: Inspect seals and gaskets for signs of leakage and replace them if necessary.
  5. Inspect the spark plug: Remove and clean it or replace it if it’s damaged.

Mower Stored Incorrectly

For some mowers, it is possible to fold the handlebar and store them by tilting upwards, as this type of engine is designed for that purpose.

However, when a regular mower is stored this way or tilted over on its side for cleaning the deck after use, engine oil can seep into the combustion chamber, producing smoke during start-up. 

Therefore, it is essential to store and handle the mower appropriately.

If you have been storing your mower upright or tilting it over on its side, this could cause smoke when starting up.

Solution- To prevent oil from entering the engine and spilling gasoline, tilt your mower so that the carburetor points upwards when storing or checking blade sharpness or cleaning the deck. Watch the video below for more information.

Additionally, we recommend regularly checking your spark plug to ensure it is clean and in good condition. This will help ensure the mower runs also and adequately prevent smoke on startup. 

If you are unsure how to do this, consult the instructions in your mower’s user manual or watch a helpful video online. Taking these precautions will help keep your mower running smoothly.

Wrong Oil Type

Not all oils are the same; they vary in thickness or weight. While regular engine oil suits most engines, high-performance grade oils are designed specifically for machines with tighter tolerances. 

A lighter oil type in an engine that requires heavier oil may slip past the seals and into the combustion chamber, leading to smoke emissions.

To ensure optimal performance and avoid voiding any warranties, it is essential to use the oil type and quantity specified by the engine manufacturer. 

This is because they have tested different oils in their engines before recommending a particular one. Ultimately, it would be best if you always stuck with what the manufacturer recommends to get the best out of your engine.

Solution- Make sure to use the correct type and viscosity of oil as specified by the manufacturer. Again, if you’re unsure about what kind of oil to use, refer to the user manual of your lawnmower or contact customer service. They will be able to provide you with the correct information.

Worn Valve Stem Seals

Valves are mechanical components that enable your engine to breathe by opening and closing in a specific order. As the valves have moving parts, they require lubrication to function correctly.

However, oil from the shaft might enter the combustion chamber without a valve stem seal. The seal slides over the shaft and prevents this from happening.

Unfortunately, hard seals can be an issue. This is the most common problem associated with valve stem seals.

Hard Seals

As engines age, their seals become more challenging and allow oil to pass by. Replacing the seals can require an engine partial strip and is not a simple process. To avoid it, use a seal conditioner – an oil additive that makes seals softer and more elastic.

However, worn or damaged seals must be replaced regardless of the treatment. We’ve been using Lucas oil treatment for many years and can recommend it – it’s available on

Paying attention to the condition of your engine seals and giving them the proper care is essential for maintaining your engine’s performance over the long term.

Worn seals

Over time, frequent valve operation can cause wear and tear on the seals. This is caused by increased movement in the valve stem, which causes the seal to expand and allows oil to leak past it.

How to Change Valve Stem Seals?

Before replacing the valve stem seals, we must take precautions to prevent the valves from dropping into the cylinder when removing the valve keepers.

A helpful mechanic tip is to remove the spark plug and insert a soft rope into the cylinder while keeping ahold of one end.

After these steps are taken, we can remove the valve cover, rockers, push rods, valve keepers, cap, and spring.

  • Rotate the crankshaft clockwise to press the rope into the combustion chamber, keeping the valves in place when you remove their keepers.
  • Once that is done, remove the valve rockers, pushrod, keepers, cap, and spring from each valve. Loosen the old seals by levering them up and away from the stem before sliding them out.
  • Finally, fit the new seals in place.

Revert the process to rebuild.

Identifying Serious Problems & Seeking Professional Help

If you’ve followed the troubleshooting steps above and the problem persists, it may be time to seek professional assistance. A skilled mechanic can help identify more complex issues like damage to the piston rings or cylinder walls, problems within the carburetor, or a broken fuel injector.

Preventative Measures & Maintenance Tips

To avoid future smoke problems, make sure to:

  1. Follow the recommended fuel-to-oil ratio for 2-stroke engines.
  2. Store your lawnmower on a level surface to prevent oil leakage.
  3. Routinely check and maintain the engine oil level.
  4. Clean or replace the air filter as needed, typically every 25 hours of operation

Related FAQs

Why My new lawn mower smokes on startup?

The smoke from your new lawn mower could be due to several reasons. It could be caused by oil leaking past the valve stem seals or piston rings, resulting from accumulated dirt and debris in the engine.

Additionally, it may be due to an incorrect type or viscosity of oil being used in the engine. To fix this, look for any signs of oil leakage and replace the valve stem seals if necessary.

Make sure to use the correct type and viscosity of oil as specified by the manufacturer, and also check to see if there is an accumulation of dirt or debris in the engine that needs cleaning out. If all else fails, it may be time to seek professional help from a qualified mechanic.

Will a bad spark plug cause a lawn mower to smoke?

Yes, a bad spark plug can cause a lawn mower to smoke due to the incomplete combustion of fuel. This is because it isn’t igniting the fuel efficiently, so not all of it is being burned off. The excess unburnt fuel will then be released as smoke from the exhaust pipe.

Is it normal for a lawn mower to smoke after putting oil in it?

Yes, it is normal for a lawn mower to smoke after putting oil in it. This is due to the oil coming into contact with hot metal parts of the engine and burning off. However, the smoke should not last very long and should clear up on its own. If the smoke continues for longer than expected, you may have an issue with the engine and should get it checked out.

What type of oil should be used in a lawn mower?

Using 10w-30 or 5w-30 motor oil in your lawn mower is recommended, depending on the manufacturer’s instructions. Synthetic oils are also suitable for some engines. Avoid using thick oils like SAE 30, as these can cause the engine to smoke and not run properly. Always check your lawn mower’s user manual for the specific type of oil required.


Mower smoking on startup is a common problem many lawn mower owners face. To address this issue, it’s essential to identify the cause before implementing any solutions. 

This could include checking for worn or broken seals, using the wrong type of oil, and ensuring that the fuel-to-oil ratio is correct. If these steps are followed, and the smoke persists, it may be time to seek professional assistance.

Additionally, it’s essential to take preventative measures and maintenance tips like properly storing the mower, routinely checking engine oil levels, and replacing/cleaning air filters when necessary. With the proper care, your lawn mower should work reliably for years.

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