In the carburetor shown in Figure 24, the principal air passage is past the spray nozzle, and all air goes by this passage when the engine is running at low speed. The extra air inlet consists of a number of holes through which air can pass without going past the spray nozzle. On each of these holes is a ball; when the suction is low the balls completely close the holes. When speed increases, the suction becomes great enough to lift the balls off the holes, and the extra volume of air that is necessary is permitted to enter. By making the balls of different weights, it can be seen that the volume of air admitted for any speed is under good control. Like the carburetor shown in Figure 23, this carburetor is of the _float feed_ type; that is, the flow of fuel to it is controlled by a valve that is operated by a float. Either of these two carburetors may be adjusted for gasoline or for kerosene, but the adjustment that is right for one is wrong for the other. Thus, if an engine is started on gasoline, with the intention of running on kerosene, the carburetor must be readjusted when the change is made. This is unsatisfactory, so a double carburetor is sometimes used, as shown in Figure 25. This consists of two carburetors of the kind shown in Figure 24, having a single mixture outlet, one being adjusted for gasoline and the other for kerosene. Either of them can be connected with the mixture outlet by means of a switch valve. [Illustration: FIG. 25.—“KINGSTON” CARBURETOR, DUAL MODEL] In order to run on kerosene or distillate it is necessary to apply heat for the reason that these oils do not evaporate readily at ordinary temperatures. Gasoline, on the other hand, evaporates readily, and a cold engine can be started on it. Tractors that run on kerosene or distillate are therefore started on gasoline and run on it until they are hot enough to vaporize the heavier oil. A carburetor that will run on either gasoline or kerosene is shown in Figure 26. The main air inlet is at E, which leads the air around the spray nozzle and into the chamber G. The mixture flows to the cylinder by the passage B. The control of the fuel at working speeds is by the high-speed adjustment, which is a needle valve screwing into the spray nozzle. Above this is another needle valve that adjusts the flow of fuel for slow speed. Extra air enters through the opening A, which is closed at slow speed by a valve held against it by a spring. This valve bears against one end of a pivoted lever, the other end of which is attached to the slow-speed needle valve; when the extra air valve opens it moves the lever and the slow-speed needle valve is lifted to permit the flow of a greater volume of fuel from the spray nozzle. [Illustration: FIG. 26.—“E-B” CARBURETOR]