If your petrol lawnmower is blowing black smoke from the exhaust, the engine is usually running with too much fuel and not enough air. This is known as a rich fuel mixture and is most commonly caused by a dirty or blocked air filter restricting airflow to the engine.
This guide explains how to inspect, clean, or replace the air filter to help restore normal engine performance.
Quick Answer
A petrol lawnmower that is blowing black smoke is usually suffering from a rich air-to-fuel mixture.
The most common cause is a dirty or blocked air filter, although other causes can include:
- Overfilled engine oil
- A sticking choke that remains engaged
- A carburettor fault allowing too much fuel into the engine
- Incorrect carburettor adjustment (where applicable)
Start by inspecting the air filter, as this is the quickest and most common solution.
Before You Begin
Before carrying out any maintenance:
- Switch off the engine and allow it to cool completely.
- Remove the spark plug cap to prevent accidental starting.
- Work in a clean, well-ventilated area.
- Wear suitable personal protective equipment (PPE), including gloves and eye protection.
Locate the air filter housing on the side of the engine. Depending on your mower model, the cover may be secured by a clip or a retaining screw.
Steps
Step 1 – Remove the Air Filter Cover
Locate the air filter cover on the side of the engine.
Depending on your mower model:
- Unclip the cover, or
- Remove the retaining screw.
Carefully remove the cover and take out the air filter.
Identify whether your mower uses a sponge air filter or a paper air filter.
Step 2 – Inspect the Air Filter
Examine the filter for signs of:
- Dirt and dust build-up
- Grass clippings or debris
- Oil contamination
- Tears or other damage
A dirty or damaged air filter restricts airflow, causing the engine to run rich and produce black exhaust smoke.
Step 3 – Clean a Sponge Air Filter
A clean sponge air filter is usually yellow or grey.
To clean the filter:
- Wash it in warm soapy water.
- Rinse thoroughly with clean water.
- Allow it to air dry completely.
- Reinstall the dry filter.
- Refit the air filter cover securely.
Important: Never reinstall a wet sponge filter, as this can reduce engine performance and cause running issues.
Step 4 – Replace a Paper Air Filter
Paper air filters cannot be cleaned effectively.
Replace the filter if it is:
- Dirty
- Oil-soaked
- Torn or damaged
- Heavily discoloured
Fit a new replacement filter and securely refit the air filter cover.
Step 5 – Test the Engine
Reconnect the spark plug cap and start the engine.
If the black smoke has stopped, the air filter was likely restricting airflow.
If black smoke continues after replacing the air filter, further inspection of the choke, carburettor or engine oil level may be required.
Frequently Asked Questions
Why is my petrol lawnmower blowing black smoke?
Black smoke usually means the engine is running with too much fuel and not enough air. The most common cause is a dirty or blocked air filter restricting airflow.
Can a dirty air filter cause black smoke?
Yes. A blocked air filter reduces the amount of air entering the engine, creating a rich fuel mixture that produces black exhaust smoke.
Can I clean my lawnmower air filter?
Sponge air filters can usually be washed, dried completely and reused. Paper air filters should be replaced if they are dirty, damaged or contaminated with oil.
What if my lawnmower still blows black smoke after replacing the air filter?
If replacing the air filter does not resolve the problem, check that:
- The choke has fully disengaged after starting.
- The engine oil has not been overfilled.
- The carburettor is operating correctly and is not supplying excessive fuel.
Further servicing may be required if the fault persists.
Is it safe to use a lawnmower that is blowing black smoke?
It is not recommended. Running the engine with a rich fuel mixture can increase fuel consumption, reduce performance, foul the spark plug and may eventually damage engine components if left unresolved.