Why is one carb overflowing fuel?
Finding fuel pouring out of one carb bowl is a sure sign of a float or needle valve problem. Each carburettor controls its own fuel level through a float assembly and needle valve. When that valve sticks open, the bowl overfills and fuel begins to leak from the overflow or into the engine itself.
This is usually caused by old fuel turning to varnish, which gums up the float pivot or needle. It can also happen if the tip of the float needle has hardened or the seat O-ring has perished. On older bikes, even a slight misalignment of the float tab can set the fuel level too high and cause flooding.
Many riders try to fix it by tapping the bowl with a screwdriver handle, and while that might stop it temporarily, it doesn’t remove the underlying cause. The only real solution is to strip the carb, clean the float assembly ultrasonically, and replace any worn parts.
At The Carb Shack, we rebuild carbs daily for this exact issue. Every unit is ultrasonically cleaned, inspected under magnification, fitted with new float needles and seals, and the fuel level reset to the manufacturer’s specification. We also leak-test all bowls before reassembly so you can be sure the issue won’t return.
If one of your carbs is constantly overflowing or dripping fuel, don’t leave it — it’s a fire risk and can dilute your engine oil with petrol. Book your carbs in for professional cleaning and rebuild with The Carb Shack and sort it properly.
