It is usual for a carburetor to be fitted with a _strangler_, or _choke_, which makes it easier to form a mixture at slow starting speed. When an engine is cold, the fuel evaporates slowly; and, furthermore, when an engine is cranked by hand its speed is so low that the suction in the carburetor is not sufficient to draw out enough fuel to form a mixture. The strangler is a valve similar in every way to the throttle, but placed between the main air inlet and the spray nozzle. When it is closed and the engine is cranked, very little air can enter the carburetor; the suction is therefore very great. Far more fuel than usual is then sucked out of the spray nozzle, and of this greater amount enough reaches the cylinder to form a combustible mixture. The engine will start, but as soon as it does so, the strangler must be opened so that the normal amount of air enters. If this is not done, the excessive suction will draw so much fuel out of the spray nozzle that the mixture formed will be too rich to burn. CHAPTER V CARBURETORS 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. [Illustration: FIG. 23.—“KINGSTON” CARBURETOR, MODEL L]