If fuel is leaking from the primer bulb on your petrol lawnmower, the most common cause is a stuck carburettor float valve. When the float valve sticks open, fuel can overfill the carburettor and flow back through the primer system, causing fuel to leak from the primer bulb.
In many cases, this can be resolved without replacing any parts by carrying out a simple bleed and tap procedure.
Quick Answer
If your petrol lawnmower is leaking fuel from the primer bulb:
- Locate the carburettor.
- Loosen the angled float chamber nut.
- Allow fuel to drain for 20–30 seconds.
- Tighten the nut securely.
- Gently tap the carburettor float chamber.
- Restart the engine and check if the leak has stopped.
If the primer bulb continues to leak after completing these steps, the carburettor may require cleaning, servicing or replacement.
Before You Begin
Before carrying out this procedure:
- 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, sparks and other ignition sources.
- Wear suitable personal protective equipment (PPE), including protective gloves and eye protection.
- Place a suitable container beneath the carburettor to collect any fuel that drains during the procedure.
Petrol is highly flammable. Wipe up any spilled fuel before attempting to restart the engine.
Steps
Step 1 – Locate the Carburettor
Locate the carburettor on the side of the engine.
It is positioned between the air filter housing and the engine block and is approximately the size of a fist.
Step 2 – Loosen the Float Chamber Nut
Locate the two nuts on the carburettor float chamber:
- One on the bottom of the float chamber.
- One positioned at approximately a 45° angle.
Using a suitable spanner, carefully loosen the angled nut.
Do not remove the nut completely.
Step 3 – Bleed the Carburettor
Allow fuel to drain from the loosened nut for approximately 20–30 seconds.
This relieves pressure inside the carburettor and helps remove excess fuel from the float chamber.
Step 4 – Tighten the Float Chamber Nut
Once the fuel has drained:
- Tighten the angled nut securely.
- Ensure it is fully sealed.
- Do not overtighten.
Step 5 – Tap the Float Chamber
Using the handle of a screwdriver, a spanner or a similar tool:
- Gently tap the float chamber several times.
This can help free a float valve that has become stuck in the open position.
Avoid striking the carburettor with excessive force.
Step 6 – Restart the Lawnmower
Reconnect the spark plug cap if it was removed.
Prime the engine if required and start the lawnmower using the normal starting procedure.
Inspect the primer bulb and carburettor to confirm that the fuel leak has stopped.
Frequently Asked Questions
Why is fuel leaking from my primer bulb?
Fuel usually leaks from the primer bulb because the carburettor float valve has become stuck, allowing the float chamber to overfill and forcing fuel back through the primer line.
What does the bleed and tap procedure do?
Draining a small amount of fuel reduces pressure inside the carburettor, while gently tapping the float chamber can help free a sticking float valve.
Will this fix every primer bulb fuel leak?
No. If the float valve, seals or carburettor are worn or damaged, the carburettor may need to be cleaned, repaired or replaced.
Is it safe to use the lawnmower if fuel is leaking?
No. Fuel leaks present a fire hazard and the lawnmower should not be operated until the leak has been repaired.
Why has the float valve become stuck?
A float valve can stick due to stale fuel, dirt or debris inside the carburettor, or after the lawnmower has been stored for an extended period without use.
What should I do if the leak continues?
If fuel continues to leak after completing the bleed and tap procedure, inspect the carburettor for contamination or damage. The float valve or carburettor assembly may require servicing or replacement.