The apparatus that forms the mixture is in two parts, one being the carburetor that proportions the fuel to the quantity of air drawn into the cylinder, and the other the mixing chamber, or manifold, that connects the carburetor with the valve chamber. The mixing chamber has no adjustments; it is a passage, often a pipe, that is shaped to fit the conditions, and according to the ideas of the manufacturer. When kerosene and distillate are used, the mixing chamber must be heated, so it is frequently built into the exhaust manifold, which is the pipe that conducts the burned gases away from the engine. In some cases it gets heat from the water jacket of the engine, a water jacket formed around it being connected with the cooling system.

The carburetor, on the other hand, has adjustments that must be understood in order to run the engine economically. The understanding of these adjustments is simplified if it is remembered that the object of the carburetor is to maintain a correct proportion of fuel to the volume of air that passes through it.

All tractor carburetors operate on the same principles, and the principles are applied in much the same way. If these principles are understood, and there is an understanding of what the parts of a carburetor are for and what they do, there should be no difficulty in adjusting and caring for any kind of a carburetor that may be offered.

The main body of the carburetor is the tube through which the air passes. This is a casting, and cannot be adjusted or altered. Into this passage projects the spray nozzle, which is usually provided with an adjustment to control the amount of liquid that may flow out of it. When no adjustment is provided, the spray nozzle is made removable, so that a nozzle with an opening of any desired size may be inserted.

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.

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.

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.

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.

This carburetor is intended for an engine of which the speed does not change greatly. Its only adjustment is the spray nozzle, and this is altered to correspond with changes in engine speed.

If an engine is clean and in good condition, it will run as well on kerosene as on gasoline, although the heating effect of kerosene is greater. When an engine is carbonized, as is usually the case, a condition known as preignition will occur unless it is prevented. Carbon from unburned fuel or from lubricating oil will deposit on the piston head and the parts of the combustion chamber, and particles will become heated to the glowing point, when they will set fire to the fresh mixture during the compression stroke and before the proper time. The effect is to make the engine lose power, and it also gives rise to a sharp metallic knocking. By reducing the temperature in the cylinder during the compression stroke this condition can be prevented. This can be done by adding water vapor to the mixture, and kerosene carburetors are therefore built with a water attachment. As can be seen in Figure 28, this is a water cup and spray nozzle like those for the fuel. When the engine knocks, and shows that preignition is occurring, water is turned on, and, being carried into the cylinder, keeps the mixture from being heated to the point of ignition before the proper time.

Figure 29 shows the attachment of this carburetor to an engine which, in this case, is horizontal. To start the engine, gasoline is injected into the carburetor, as shown; this will give a sufficiently good mixture for the purpose, and enough heat for running on kerosene is thus obtained.

The carburetor shown in Figure 30 is similar, but has a bowl and spray nozzle for gasoline, to use in starting. It is also provided with a heating jacket through which hot water or hot gases may circulate.

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.

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.

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.

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.

In the cleaner shown in Figure 37, the air is passed through loose wool, which filters out the dust. Another type of cleaner works on the same principle as a cream separator; the air is given a whirling motion, which throws the dirt out at the sides, and it is collected in a glass jar.

These air cleaners must be emptied frequently, for if they are not kept clean it cannot be expected that they will do their work.

A tractor engine is built to develop its maximum power at a certain speed; if it runs at greater speed, it will not operate efficiently, and there will be unnecessary wear of its parts. These engines are therefore usually fitted with governors which hold them at their most efficient speed. A governor operates by centrifugal force.

Anything in motion tries to move in a straight line; if it is forced to move in a circle, it will exert force in trying to move away from its center. It is this that is called centrifugal force. It is centrifugal force that holds water in a pail that is being swung around the head, and that makes the pail fly off if it is released.

In applying this principle to a governor, weights are attached to a plate and made to revolve; springs hold them together, but in spite of this, centrifugal force throws them outward. In moving, they act on a rod that operates the throttle; as the speed increases, the move outward more and more, and it is a simple matter of adjustment to cause them to close the throttle when the speed reaches a desired point.

A governor and its connections are shown in Figure 38. The weights, R, are L-shaped, and pivoted at the angle to a plate driven by the engine. The shaft that drives the plate also supports a collar, P, that is loose on it and can slide endways; the collar rests against the short bar of the L-shaped weights. The other end of the collar touches the lever, E, which is moved when the collar moves. As the lever is connected with the throttle, a movement of the collar will control the position of the throttle.

When the shaft revolves, the long arms of the L-shaped weights tend to fly outward; this moves them on their pivots, and the short arms thereupon force the collar to slide on the shaft, which moves the lever and operates the throttle. The speed at which the throttle will begin to close is determined by the setting of the spring that holds the weights in.

Governors and governor connections are shown in Figures 39 and 40.

The governor shown in Figure 41 is enclosed in a housing that can be locked or sealed. This prevents the unauthorized changing of the adjustment.