My Petrol Lawnmower is hunting

If your petrol lawnmower engine repeatedly speeds up and slows down while running, this is known as engine hunting. Hunting is usually caused by an incorrect air-to-fuel mixture, often due to stale fuel, a blocked air filter, restricted fuel flow, or a dirty carburettor.

This guide explains the most common causes of engine hunting and the checks you can carry out to restore normal engine performance.

Quick Answer

A petrol lawnmower that is hunting is usually receiving too much air or not enough fuel, resulting in a lean air-to-fuel mixture.

The most common causes are:

• Old or stale petrol
• Dirty or blocked air filter
• Restricted fuel flow
• Blocked carburettor jets
• Air leaks in the intake system

In many cases, replacing old fuel and cleaning or replacing the air filter will resolve the problem.

Before You Begin

Before carrying out any inspection or maintenance:

• Switch off the engine and allow it to cool completely.
• Remove the spark plug cap to prevent accidental starting.
• Work in a well-ventilated area away from naked flames or ignition sources.
• Wear suitable personal protective equipment (PPE), including gloves and eye protection.
• Keep petrol in an approved fuel container and dispose of old fuel responsibly.

Tip: Before investigating a fault, consider the mowing conditions. Very long, thick or wet grass can place excessive load on the engine and may affect performance.

To reduce engine load:

• Cut long grass in stages.
• Avoid mowing wet grass where possible.
• Raise the cutting height when tackling dense vegetation.

Steps

Step 1 – Check the Fuel

Old petrol is one of the most common causes of engine hunting.

Ensure the mower contains:

• Fresh unleaded petrol.
• Fuel that is no more than 30 days old.

If the fuel is older than 30 days, drain the tank and refill with fresh petrol.

Step 2 – Replace Old Fuel

If stale fuel is suspected:

1. Remove the air filter cover by unclipping it or removing the retaining screw, depending on your mower model.
2. Remove the air filter.
3. Remove the fuel cap and carefully drain the old petrol into a suitable approved container.
4. Dispose of old fuel in accordance with local regulations.
5. Refit the air filter and air filter cover.
6. Fill the fuel tank with fresh unleaded petrol.

Start the engine and check whether the hunting has stopped.

Step 3 – Inspect the Air Filter

A blocked air filter restricts airflow and can upset the air-to-fuel ratio, causing unstable engine speed.

Remove the air filter cover and inspect the filter.

Sponge Air Filter

A clean sponge filter is typically yellow or grey.

To clean it:

• Wash in warm soapy water.
• Rinse thoroughly.
• Allow it to dry completely.
• Reinstall once fully dry.

Do not reinstall a wet sponge filter.

Paper Air Filter

A clean paper filter is normally white or off-white.

Paper filters should not be washed.

Replace the filter if it is:

• Dirty
• Damaged
• Oil-soaked

A clean air filter helps maintain the correct air-to-fuel mixture and improves engine performance.

Frequently Asked Questions

What causes a petrol lawnmower engine to hunt?

Engine hunting is usually caused by a lean air-to-fuel mixture. Common causes include stale fuel, a dirty air filter, restricted fuel flow, blocked carburettor jets or an air leak in the intake system.

Will old petrol make my lawnmower hunt?

Yes. Petrol begins to deteriorate after around 30 days. Old fuel burns less efficiently and can cause unstable engine speed, poor starting and engine hunting.

Can a dirty air filter cause engine hunting?

Yes. A clogged air filter can disrupt the correct air-to-fuel ratio, causing the engine speed to fluctuate.

Can I clean my lawnmower air filter?

Sponge air filters can usually be washed and reused once fully dry. Paper air filters should be replaced if they are dirty, damaged or contaminated with oil.

What if my lawnmower is still hunting after replacing the fuel and air filter?

If the problem persists, the carburettor may require cleaning, the fuel system may be restricted, or there may be an air leak. Further inspection or servicing may be required.