THE MOTORCYCLE GASOLINE ENGINE All motorcycle engines are built on the four stroke cycle principle, and they are made with one, two, four and eight cylinders.' The one and four cylin der engines are of the vertical type and the two and the eight cylinder engines are of the V type, as shown in Fig. .40.
The Fuel Supply System.—The gasoline system or fuel system, as it is called, consists of two chief parts, and these are (1) the tank, which contains the supply of gasoline, and (2) the carburetor, which regulates the amount of gasoline used, breaks it up into a spray as it is needed and mixes it with air to make the fuel mixture.
The Gasoline Supply Tank.—In motorcycles the tank is always set above the engine and the gasoline is fed into the carburetor by gravity.
Why a Carburetor is Used.—Instead of the sim ple gasoline valve which is used on stationary en gines, all mobile gasoline engines employ a more complicated but better device for the purpose, called a carburetor.
The reason a gasoline valve cannot be used is that it delivers the same amount of fuel to the cylinder each and every stroke unless it is regulated by a governor, while with a carburetor the amount of gas oline needed for varying loads can be regulated to a nicety without a governor, which, indeed, could not be used because the speed of the engine must be variable at all times and under the control of the operator, How a Simple Carburetor Works.—A in Fig. 4/ shows a diagram of a carburetor in its simplest form, so that you can understand it. It is made up of (1) a float chamber; (2) a float valve; (3) a nozzle; (4) an air inlet pipe, and (5) a mixing chamber.
The float chamber is connected to the supply pipe which leads to the tank of gasoline. It has a needle valve in it and this is fixed to the hollow metal float ; when the float is down the gasoline from the tank flows into the float chamber and as it fills the float rises until the needle valve cuts off the supply.
There is an air vent in the float chamber so that it will not get air-bound and thus prevent the gaso line from running into or out of it. It also has a priming pin, with which you can press the float down and fill the float chamber full of gasoline, or flood it, as it is called, when you want to start the engine.
The float chamber is connected to the nozzle with a pipe and when the gasoline is drawn through the nozzle by the suction stroke of the engine it forms a spray as shown at A. But to break up the gasoline into minute particles and make an explosive mixture of it, air must be drawn up and mixed with it at the same time.
To give the gasoline the required amount of air a large air-pipe, which is open at one end, encloses the spray-jet at the other end. This pipe is bent and is constricted, that is, it is made smaller just above the jet and this gives the air a higher pressure when it is drawn through the pipe because it also forms a kind of a nozzle.
At a point just above the constriction the tube is enlarged to form a space for the air and gasoline to mix in and hence this part of the carburetor is called a mixing chamber. An inlet pipe connects the mix ing chamber of the carburetor with the cylinder of the engine as shown at B. Where two or more cyl inders are to be fed at the same time the pipe branches out and it is then called a manifold.
How a Regular Carburetor is Made and Works.— While a real carburetor is made and works exactly like the simple one I have described, it is built much more compactly and it has one or more auxiliary air valves to regulate to a nicety the amount of air needed for high and low speeds. The construction of the automatic auxiliary air valve is also shown at A and B. It is simply a valve working against a com pression spring; when more air is needed the suction of the piston opens it and when enough air has been drawn in the spring closes it.
The main, or primary air inlet of the carburetor has a butterfly valve in it, so that the air can be shut off to make it easy to start the engine and to make a rich or a lean fuel mixture, as desired. Finally, a throttle valve is set in the top of the mixing cham ber so that the amount of fuel mixture which goes to the cylinders can be regulated and the speed of the engine controlled. A throttle rod runs to the handle-bar of the motorcycle so that the rider can work the throttle.