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 gasoline 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. 41 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.

FIG. 41-A. CROSS SECTION OF A SIMPLE CARBURETOR

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 mixing chamber of the carburetor with the cylinder of the engine as shown at B. Where two or more cylinders 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 compression 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 chamber 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.

A cross-section of a Stromberg carburetor is shown at A in Fig. 42, and it is fitted with many little refinements such as a needle valve adjusting screw, a gasoline strainer, drain cock, hot-air horn,

FIG. 42—A. CROSS SECTION OF A STROMBERG CARBURETOR

etc. A view of the carburetor complete is shown at B.

The Electric Ignition System.-The only type of ignition system used on motorcycle gasoline engines is the jump-spark.

As in stationary gasoline engines there are two kinds of jump-spark systems used on motorcycle engines, and these are (1) the jump-sparkcoil system, and (2) the high tension magneto system.