Lawn Mower Blowing Black Smoke

Blowing 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 engine – the choke is set to “On”.

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”.

Black Smoke – Diagnosing the root cause of black smoke isn’t difficult, but if you need a little video help, check out mower repair video library, it walks you through the diagnosing process step by step, checking for choke, air filter and carburetor faults.

Choke Set To “On”

Many mower owners may not be fully familiar with the choke and how to use it or it could be a faulty choke system. (more on that below). In any event, a choke is only required when an engine is cold. When the engine warms, coke should be turned off. Using choke on a warm engine will cause black smoke and rough running. If you are unfamiliar with choke control, check out “How to start a mower?”. (More on choke stuck on below)

Choke control – Turn choke off as engine warms up.

Blocked Air Filter

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 that filter. 

Check Air Filter – Wet or dirty filter is a common cause of black smoke and it’s 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 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 and that causes black smoke, poor running and no starts and flooding. You can check out a video on how to unflood an engine here.
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”.

Faulty Carburetor

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 choke is off, your filter is clean but you are still blowing black smoke. Suspect a faulty carburetor.

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 cleaning”.

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.

Faulty Choke Test

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

As the engine warms a little, move the choke to the fast/run position. Check that choke is moving to the off (open) position, if not, adjust cable.

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.
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.

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, the carburetor is faulty.

Related Questions

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

Kohler engine blowing black smoke? Common reasons a Kohler blows black smoke, include:

  • Choke on
  • Choke stuck on
  • Blocked air filter
  • Faulty carburetor