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.
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.
Fig. 26.—“E-B” Carburetor
This carburetor is started on gasoline. When the engine is hot, a switch valve is operated to permit the burned gases from the engine to flow through the carburetor; they pass through the pipe C, D, and as the chamber G is directly in their path it becomes intensely heated. The carburetor can then be switched to kerosene. A side view of this carburetor is shown in [Figure 27].
These carburetors are all of the float feed type, and are used on engines of which the speed is variable. A carburetor that is fed by a pump is shown in [Figure 28]. This is a simple tube with a fuel cup cast on one side of it. Fuel is pumped to the bowl, and the proper level is maintained by an overflow through which excess fuel passes back to the tank.