Abstract: An improved fuel valve for an air-vapor mixer device for the fuel supply system of an internal combustion engine with which it is in thermal contact is disclosed. The valve operates in a fuel metering orifice and is composed of an acetal copolymer resin having a high coefficient of expansion. When the valve is cool, the metering orifice provides a richer mixture of fuel to air for cold starting and idling. As the engine warms up, the fuel valve expands through thermal contact with the engine and as a result of its high coefficient of expansion decreases the effective size of the orifice to lean the mixture and thereby provide better fuel economy and reduced pollutants. For mid-range control the fuel valve has a section of generally frustoconical configuration provided with circumferential steps that lean out mid-range mixtures for average operating conditions and produce an optimal admixture of fuel and air.