My Petrol Lawnmower is leaking fuel from the primer bulb

If fuel is leaking from the primer bulb on your petrol lawnmower, it is usually caused by the carburettor float valve becoming stuck. This allows fuel to back up through the carburettor and escape from the primer bulb.

This guide explains how to carry out a simple bleed and tap procedure that may free the float valve and stop the fuel leak.

Quick Answer

If petrol is leaking from the primer bulb on a lawnmower fitted with an SV150 or SE475 engine, the carburettor float valve has most likely become stuck.

In many cases, you can resolve the problem by:

• Bleeding fuel from the carburettor float bowl.
• Tapping the float chamber to free the float valve.
• Restarting the engine and checking that the leak has stopped.

If the leak continues, the carburettor may require cleaning or repair.

Before You Begin

This guide only applies to models with SV150 engine or SE475 engine, please refer to instruction manual.

fitted with either:

• SV150 engine
• SE475 engine

Before carrying out any work:

• Switch off the engine and allow it to cool completely.
• Remove the spark plug cap to prevent accidental starting.
• Work in a well-ventilated outdoor area away from naked flames or sources of ignition.
• Wear suitable personal protective equipment (PPE), including gloves and eye protection.
• Keep a cloth or suitable container ready to catch any fuel released during the procedure.

Steps

Step 1 – Locate the Carburettor

Locate the carburettor on the side of the engine.

It is positioned between the engine and the air filter housing and is approximately the size of a fist.

Step 2 – Loosen the Float Bowl Nut

Locate the angled nut on the bottom of the carburettor float bowl.

Using a suitable spanner:

1. Loosen the angled nut slightly.
2. Do not remove it completely.

Fuel will begin to drain from the float bowl.

Step 3 – Bleed the Fuel

Allow fuel to drain from the loosened nut for approximately 20 to 30 seconds.

This helps flush the float chamber and may free a sticking float valve.

Once complete, tighten the nut securely.

Step 4 – Tap the Float Chamber

Using the handle of a screwdriver, a spanner or a similar non-sharp tool, tap the float chamber several times.

The light impacts help free the float valve if it has become stuck in the open position.

Avoid striking the carburettor excessively, as this may cause damage.

Step 5 – Test the Lawnmower

Reconnect the spark plug cap and start the engine.

Inspect the primer bulb while the engine is running.

If the float valve has returned to its correct position, fuel should no longer leak from the primer bulb.

If the leak persists, the carburettor may require cleaning, servicing or replacement.

Frequently Asked Questions

Why is fuel leaking from my primer bulb?

Fuel leaking from the primer bulb is usually caused by a float valve inside the carburettor sticking open. This allows excess fuel to flow back through the primer circuit.

What does the bleed and tap procedure do?

Bleeding the carburettor removes excess fuel from the float bowl, while gently tapping the float chamber can free a float valve that has become stuck.

Will this repair every fuel leak from the primer bulb?

No. If the float valve is worn, damaged or contaminated with debris, the carburettor may need to be dismantled, cleaned or replaced.

Can I continue using the mower if fuel is leaking?

No. Fuel leaks present a fire hazard and should always be repaired before operating the lawnmower.

Which lawnmowers does this guide apply to?

This procedure applies to petrol lawnmowers fitted with the SV150 or SE475 engine.