The float needle valve ensures a regulated fuel supply to the carburettor. It is controlled by the carburetor float. Above a certain level, defined by the carburetor manufacturer, the float needle seals the inlet hole of the needle valve seat. The rubberized cone tip of the needle ensures this. If the float level drops, the float needle opens an annular gap and the fuel can run on again. How large this annular gap is, has a decisive influence on the filling speed of the float chamber.
If the needle valve is too small, the fuel supply may no longer be sufficient under full load. If the valve is too large, the engine can be flooded with gasoline, resulting in increased fuel consumption, poor running characteristics and other unsightly results.