On some carburetors the extra air valve is set by the manufacturers, while on others it is adjustable by controlling the strength of the spring that holds it against its seat. The carburetor shown in Figure 23 has a spray nozzle adjustment of a very usual type. A rod is so arranged that its pointed end projects into the opening of the spray nozzle; by screwing it up or down the opening may be made larger or smaller, so that more or less fuel may flow out. The extra air valve is a flap valve that closes the air passage until the suction is great enough to lift it from its seat. Around the spray nozzle is a tube that connects the passage below the extra air valve with the passage above it; when the suction is too low to lift the extra air valve from its seat, any air flowing through the carburetor passes through this tube. The tube is so small that even a little air passing through it is enough to suck fuel out of the spray nozzle, and the spray nozzle is so adjusted that enough fuel comes out to make a proper mixture with that volume of air. This is the _low-speed adjustment_, which gives a mixture on which the engine will start and will run at its lowest or _idling_ speed. At this speed the engine produces just enough power to keep itself going. When the engine speeds up, and suction increases, the extra air valve is lifted off its seat, and a greater volume of air flows through the carburetor. The increased suction also draws more fuel out of the spray nozzle. If the greater amount of fuel were in proportion to the greater volume of air, there would be no change in the mixture, but this is not the case. As suction increases, the proportion of fuel drawn out of the spray nozzle becomes too great for the air, and the mixture becomes too rich. To overcome this, the extra air valve permits a still greater volume of air to pass, so that the proportions of fuel and air do not change. The chamber below the air passage in Figure 22 is the fuel cup, into which fuel flows from the tank. Inside the fuel cup is a ring of cork attached to a pivoted lever, on the other end of which is a needle valve that can close the opening through which the fuel enters the cup. As the cup fills, the cork floats on it, and in rising it moves the lever on its pivot. When the fuel reaches such a level that it is near the tip of the spray nozzle, the valve closes the opening and prevents more fuel from entering. [Illustration: FIG. 24.—“KINGSTON” CARBURETOR, MODEL E]