If fuel is leaking from the primer bulb on your petrol lawnmower, it is usually caused by the carburettor float valve becoming stuck. When this happens, fuel can overfill the carburettor and flow back through the primer system, causing it to leak from the primer bulb.
In many cases, the problem can be resolved 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 carburettor float chamber drain nut.
- Allow fuel to drain for 20–30 seconds.
- Tighten the nut securely.
- Gently tap the float chamber to free the float valve.
- Restart the engine and check whether the leak has stopped.
If fuel continues to leak, 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 outdoor area away from naked flames and other sources of ignition.
- Wear suitable personal protective equipment (PPE), including gloves and eye protection.
- Place a suitable container or absorbent cloth beneath the carburettor to catch any fuel that drains out.
Petrol is highly flammable. Clean 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 and is approximately the size of a fist.
Step 2 – Loosen the Float Chamber Nut
Locate the angled drain nut on the carburettor float chamber.
Using a suitable spanner:
- Loosen the angled nut slightly.
- Do not remove the nut completely.
Fuel will begin to drain from the carburettor.
Step 3 – Bleed Excess Fuel
Allow fuel to drain for approximately 20 to 30 seconds.
This helps relieve excess pressure and may free a sticking float valve.
Step 4 – Tighten the Drain Nut
Once the fuel has drained:
- Tighten the drain nut securely.
- Do not overtighten, as this may damage the carburettor.
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 may release 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.
Start the engine following the normal starting procedure.
If your lawnmower uses a primer bulb, prime the engine as required.
Inspect the primer bulb and carburettor for any further signs of fuel leakage.
Frequently Asked Questions
Why is fuel leaking from my primer bulb?
The most common cause is a stuck carburettor float valve, which allows excess fuel to build up inside the float chamber and flow back through the primer system.
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 free a float valve that has become stuck.
Will this fix every primer bulb fuel leak?
No. If the float valve is worn, damaged or contaminated with debris, the carburettor may need to be cleaned, repaired or replaced.
Can I continue using my lawnmower if fuel is leaking?
No. Fuel leaks present a fire risk and should always be repaired before operating the lawnmower.
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 mower has been stored for an extended period.
What should I do if the primer bulb still leaks after completing these steps?
If the leak continues, inspect the carburettor for contamination or damage. The float valve, float or carburettor seals may require servicing or replacement.