My Petrol Lawnmower is hunting

If your lawnmower engine repeatedly speeds up and slows down while running, this is commonly referred to as engine hunting.

This guide covers the most common causes and the checks you can perform to help resolve the issue.

What Is Engine Hunting?

Engine hunting occurs when the engine continuously increases and decreases its speed in a repeating cycle.

This is typically caused by a lean fuel mixture, meaning the engine is receiving too much air or not enough fuel.

Common causes include:

• Old or contaminated fuel
• Restricted fuel flow
• Dirty air filters
• Blocked carburettor jets
• Air leaks within the intake system

Initial Checks

Before inspecting the mower, consider the mowing conditions.

Grass Conditions

Very long, thick, or wet grass can place excessive load on the engine and may affect performance.

If possible:

• Cut long grass in stages.
• Avoid mowing excessively wet grass.
• Ensure the cutting height is set appropriately.

1. Check the Fuel

Old fuel is one of the most common causes of poor engine performance.

Ensure you are using:

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

If the fuel is older than 30 days, it should be replaced.

2. Replace Old Fuel

Step 1 – Remove the Air Filter

Locate the air filter cover on the side of the engine.

Depending on your model:

• Unclip the cover, or
• Remove the retaining screw.

Remove the air filter.

Step 2 – Drain the Fuel

Remove the fuel cap and carefully empty the old fuel into a suitable approved container.

Dispose of old fuel responsibly in accordance with local regulations.

Step 3 – Reassemble

Refit the air filter and air filter cover.

Step 4 – Refill with Fresh Fuel

Fill the tank with fresh unleaded petrol.

3. Check the Air Filter

A blocked air filter can disrupt the air-to-fuel ratio and contribute to engine hunting.

Remove the air filter cover and identify which type of filter is fitted.

Sponge Air Filter

A clean sponge air filter is typically yellow or grey in colour.

Cleaning a Sponge Filter

1. Wash the filter in warm soapy water.
2. Rinse thoroughly.
3. Allow it to air dry completely.
4. Reinstall the filter.

Do not reinstall a wet filter.

Paper Air Filter

A clean paper air filter is usually white or off-white in colour.

Paper filters cannot be cleaned effectively.

If the filter is:

• Dirty
• Damaged
• Oil-soaked

It should be replaced.