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Lawnmower Smokes on Startup – This is why…

By: Author John Cunningham. Published: 2022/08/22 at 1:14 pm

A smoking mower is a pain, not to mention bad for your health and the environment. Fixing the issue is usually a simple fix. I’m a mechanic, and you’ll learn a few tricks to take care of the problem the easy way.

A lawnmower engine commonly smokes on startup because the oil level is over full. Other common causes include:

  • Mower stored incorrectly
  • Wrong oil type
  • Worn oil stem valve seals

In this post, you’ll learn to diagnose why your mower smokes on startup and what you can do to fix it right now!

We’ll begin with the most common reasons first.

Engine Oil Level Over Full

Oil dipstick check

Engine oil is super important for the performance and the health of your mower engine. A low oil level will cause some irreparable damage and is a common engine killer. 

Of course, most folks know that and top up their mower oil level regularly. The thing is, in my experience, they very often overfill the oil level. 

Over-full oil is bad for two reasons:

1 – The excess oil makes its way inside the combustion chamber and burns to cause smoke.

2 – An over-full oil level is bad for a small engine because most small engines commonly employ splash lubrication, and overfilling such a system actually prevents proper oil lubrication.

How to diagnose: Easy, check your engine oil level. See the infographic below. 

How to fix it: If you find your oil level is too full, to fix it, we’ll need to remove some. I use an oil siphon to remove the excess oil; it’s the fastest and cleanest option. 

That said, you can drain some oil by following your mower’s oil drain procedure. Older mowers usually have an oil drain bung on the bottom of the engine. More modern engines drain the oil through the dipstick; you’ll need a shallow container, and you’ll need to tilt your mower over. It’s covered in the video below.

If you haven’t changed your oil this season, it makes sense to drain it all out and replace it with fresh oil. I’ve covered the whole procedure here in “Mower tune-up.”

Mower Stored Incorrectly

Mower tilted

While some mowers may be stored ergonomically by folding their handlebars flat and storing them by tilting them upwards, you’ll find those types of mowers have an engine designed especially for that purpose.

Storing a regular mower in that way or, indeed, even tilting a mower over on its side to clean the deck after use may cause engine oil to sneak into the combustion chamber, which burns off the next time you start the engine-causing engine smoke.

If you have been storing your mower in this way or tilting your mower over in this way, it’s very likely this is the cause of the smoke on startup.

The fix – It’s safe to tilt your mower over to store it, check the blade, clean the deck, etc., but you’ll need to tilt it carburetor side upwards.

Doing so helps prevent oil from entering the engine and also gas spills. Check out the video below.

Wrong Oil Type

Oil is oil, right? Well….yes and no. Oil is, as you know, graded by weight; broadly, weight refers to how thick the oil is. And while most engines will be pretty happy with a regular engine oil grade, they may not like light oil. 

Such high-performance grade oils are designed for higher-performing engines with tighter tolerances. Using them in a mower engine with wider tolerances allows the oil to slip past sealing surfaces and into the combustion chamber, which, as you know, is where it’s burnt and results in smoke.

All engines will have a preferred oil type that engine manufacturers have tested, and so should ideally be followed.

Failing to use the correct oil type and quantity may void your engine warranty.

How to fix: If you think this may be your problem, go ahead and drain your oil and replace it with the correct oil type; check the infographic below for the recommended oil type. Job Done!

Lawn mower oil chart

Worn Valve Stem Seals

What are valve seals? Valves, as you know, are mechanical components that open and close sequentially to allow your engine to breathe. As they are moving parts, they obviously need lubrication. 

Therein presents a problem: while the valve opens a passageway to the combustion chamber, its shaft must be lubricated. Oil from the shaft could easily enter the combustion chamber if not for the valve stem seal.


The seal, as its name suggests, slips over the shaft and prevents oil from entering the combustion chamber.

So what’s the problem with the seals? There are a couple of common problems, the most common of which is hard seals. 

Hard Seals – The seals simply get hard with age or when the engine oil isn’t changed regularly. When they get hard, they allow oil to sneak past.

Replacing the seals is the more usual repair. It’s not a difficult job, but it does require an engine partial strip.

Before going that road, it may be worth trying a seal conditioner. It’s an oil additive that will help if your seals are simply hard. However, it won’t solve a worn or damaged seal issue; they’ll need to be replaced.

Here’s a link to the treatment I’ve been using for years; it’s called Lucas oil treatment, and it’s sold and delivered by

Worn seals – General wear and tear of seals is generally caused by wear in the valve sleeve itself. The greater movement in the valve stem causes the valve seal to widen, allowing oil to slip past.

The only fix here is to replace the seals.

How to replace Valve Stem Seals

To replace the valve stem seals, we’ll need to remove the valve cover, valve rockers, push rods, valve keepers, valve cap, and spring.

But before we do any of that, we’ll need to prevent the valves from falling into the cylinder when the valve keepers are removed.

Here’s a handy mechanics secret – Remove the spark plug and stuff a soft rope into the cylinder, not forgetting to keep hold of one end.

  • Now go ahead and rotate the crankshaft clockwise so as to squash the rope into the combustion chamber. The rope pushing on the valves keeps them in place when the keepers are removed.
  • Now you are free to remove the valve rockers, pushrod, keepers, cap, and spring.
  • Remove the seals by prying them upwards and sliding them off the valve stem.
  • Fit the new seals

Reverse the process to rebuild.

Note valve lash will need to be adjusted, and I’ve covered that in a post previously – “Adjusting Valve Lash,” or you can check out the video below.

You may also find these pages helpful:

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