Lawn Mower Blowing
Black Smoke

What We'll Cover

Lawn Mower Blowing
Black Smoke

What We'll Cover

Your Problem Will Likely Be Loose Battery Connections, Flat/Faulty Battery Or Faulty Starter Solenoid.

Causes

01. Choke On

02. Choke Stuck On

03. Blocked Air Filter

04. Faulty Carburetor

Most Common Cause

The most common reason for black smoke - Choke On.

 

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".

 

 

Rough Running


mower smoke
Mower Black smoke

Rough Running

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

Remove

Air Filter


mower air filter

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".

 

 

 

 

Choke

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.

 

 

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.


mower throttle lever mower carburetor

Manual/Auto Choke


Mower Choke lever Mower Choke lever

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.

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-burned 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

Carburetor

Carburetor


Mower carburetor