In many cases the fuel is heated before reaching the carburetor. This is done by coiling the feed pipe around the exhaust pipe or putting it in a jacket through which hot water circulates. Another device sends the mixture through a chamber heated by the exhaust, as shown in Figure 31. Figure 32 shows an arrangement in which the mixture passes through a jacket around one branch of the exhaust pipe. By means of a switch valve, A, more or less of the exhaust gases may be permitted to flow through this branch, so that the mixture may be heated to any desired degree. [Illustration] [Illustration: FIG. 31.—“HART-PARR” MIXTURE HEATER] All of these heating devices are so arranged that the heat is under the control of the driver, which permits him to heat the mixture as much as he judges to be necessary. Enough heat must be used to prevent the fuel from condensing; but too much heat will cut down the efficiency of the engine because it will cause so much expansion of the mixture that a cylinderful of it will not produce the maximum power. [Illustration: FIG. 32.—“TWIN CITY” MANIFOLD] Figure 33 shows the pump that is used in a force feed carburetor of the type shown in Figure 28. Its plunger is forced through an inward stroke by a cam, and makes an outward stroke as its spring returns it to position. The inlet and outlet openings of the cylinder are closed by ball check valves, the inlet check being open on the outward strokes, and the outlet check being open on the inward strokes. A pump of this sort requires no attention beyond seeing that the check valves work properly, and that there are no leaks. Figure 34 shows the connections between the fuel tank and the carburetor. Under the tank, 1, is a chamber containing a fine wire strainer, 4, through which the fuel must pass to reach the carburetor; any dirt that may be present is strained out, and collects in the cup, 2. Water in the fuel also settles here, and the cup is cleaned out by unscrewing the plug, 3. 5 is the shut-off cock; it should always be closed when the tractor is not working. [Illustration: FIG. 33.—FUEL PUMP] A complete fuel system is illustrated in Figure 35, showing the connections of the tanks, pumps, and carburetor. As dirt is injurious to an engine, the air that forms the mixture must be clean, so when a tractor works in a dusty field, it should be equipped with an air cleaner, of which there are three kinds. In one of these the air is required to pass through water, which washes it. A cleaner of this type is shown in Figure 36. The dusty air enters the central passage, and is forced to pass through the water in order to reach the outlet. Passage through the water and through the baffle plates frees the air of all its dust.