Stewart Metering Pin Carburetor
The carburetor shown at Fig. 47 is a metering type in which the vacuum at the jet is controlled by the weight of the metering valve surrounding the upright metering pin. The only moving part is the metering valve, which rises and falls with the changes in vacuum. The air chamber surrounds the metering valve, and there is a mixing chamber above. As the valve is drawn up the gasoline passage is enlarged on account of the predetermined taper on the metering pin, and the air passage also is increased proportionately, giving the correct mixture. A dashpot at the bottom of the valve checks flutter. In idling the valve rests on its seat, practically closing the air and giving the necessary idling mixture. A passage through the valve acts as an aspirating tube. When the valve is closed altogether the primary air passes through ducts in the valve itself, giving the proper amount for idling. The one adjustment consists in raising or lowering the tapered metering pin, increasing or decreasing the supply of gasoline. Dash control is supplied. This pulls down the metering pin, increasing the gasoline flow. The duplex type for eight- and twelve-cylinder motors is the same in principle as model 25, but it is a double carburetor synchronized as to throttle movements, adjustments, etc. The duplex for aeronautical motors is made of cast aluminum alloy.