Lawn Mower Blowing
Black Smoke

Bowing black smoke like a train, spluttering and no power, this mower is useless, what's going on?

Why is my lawn mower blowing black smoke? The most common reason for black smoke from a lawn mower is because the choke is set to on, but there are other possible reasons:

  • Choke stuck on
  • Blocked air filter
  • Faulty carburetor

When your mower starts and warms up, move the throttle lever off the choke position. Choke is only needed for cold starts. Black smoke is a sign that the engine is getting too much fuel or not enough air, check out "How to start a lawn mower".

Mower Black smoke

Rough Running

Diagnosing the root cause of black smoke isn't difficult, but if you need a little video help, definitely check out my mower repair video library, it walks you through the diagnosing process step by step, checking for choke, air filter and carburetor faults. Of course all the relevant repairs are covered too.


The library covers a ton of other common mower repairs also.

It's packed with 25 years experience, tips, tricks and hacks and the best part, you'll save a packet on mower repair bills.


Check The Air Filter

A blocked air filter is also a common cause of black smoke and easy to check. Clean air is as important as fuel. Filtering the air before it enters the engine prevents grit from damaging the carburetor and engine components, it also settles the air.


Filter Check

The filter should be checked and cleaned every 25 hours and more often in dusty environments, replace every 100 hours. Most engine manufactures make it easy for owners to quickly check the air filter. Filter covers are usually held on by plastic clips or simple wing nuts.


The easy way to check if the filter is the cause of the black smoke, remove the filter and if the smoke clears, replace the filter. Go ahead and remove the filter, run the engine, if the smoke clears, replace the filter. 


Mower filter check


Filter Types

Common filter types are pleated paper element, fiber elements and foam. Some air filters will have a pre-filter, usually foam, wrapped around the main filter, its function is to catch larger debris.


Filter Cleaning

Paper and fiber elements can be cleaned using compressed air, or banging them on the ground works reasonable well. If these filters are oil or fuel soaked they will need to be replaced as this blocks air flow.


Foam elements can be washed in soapy water, B&S coat their foam air filters with engine oil, this helps trap the fine dust particulars. Never oil paper filters. Check out "Mower tune-up guide".


What Does The Choke Do?

As you know the function of a choke or priming bulb is to enrich the fuel mixture, so a cold engine starts smoothly. Your mower engine will be fitted with one or the other.


The choke enriches the mix by restricting the amount of air entering the carburetor. The priming bulb does it by injecting extra fuel.


Air/Fuel Ratio

Gas engines run best when the ratio of air to fuel is 14.7 to 1. Meaning 14.7 parts air to 1 part fuel, also known as air fuel ratio (AFR). Using the choke or priming bulb counteracts the lean condition caused by the dense cold air, on a cold start.


Choke Test

Manual Choke - Most mowers will typically have a lever to control choke, if a choke flap is sticking in the on position, the mower will run rough and blow black smoke. Cables will need adjustment from time to time.


Auto Choke - More recent engines from Briggs & Stratton and Honda offer a thermostatically controlled automatic choke system. It's a simple set up - a thermostat, positioned beside the muffler pushes open the choke flap progressively as the engine heats.


The system is a great idea no doubt, no more fiddling around with choke levers or priming bulbs. This type can also stick in the on position, causing black smoke.


Primer Bulb - This type set up won't cause black smoke, if you have a primer bulb type carburetor and it blows black smoke even with the air filter removed, then the carburetor is faulty.


mower throttle lever mower carburetor

Manual / Auto Choke Off

Manual - As the engine warms a little, move the choke to the fast/run position. Check that it's moving to the off position, if not adjust cable.

Auto choke - As the engine warms the choke flap should open automatically.

Check that it's moving to the off position, if not check for binding links, or faulty thermostat.

What Does The Carburetor Do?

The carburetors job is to meter the gas and mix it with the air precisely at 14.7 parts air to one part gas. The black smoke is un-burnt gas, the carburetor is supplying more gas to the engine than it can handle. 


Faulty Carburetor

So to keep this simple, ​if your filter is clean and the airway to the carburetor is clear, the choke flap is in the open/off position and your mower is still blowing black smoke. 


You can try cleaning the carburetor, but too much gas is a sign of a faulty carburetor, I would go ahead and replace it, check out "Carburetor clean out".


mower auto choke


Related Questions

Lawn mower blowing white smoke and leaking oil why? The most common reason is too much oil in the engine, remove the excess oil and run the engine until the oil clears.

Kohler engine blowing black smoke? Common causes include:


  • Choke on
  • Choke stuck on
  • Blocked air filter
  • Faulty carburetor
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.