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Fig. 66.-Sectional View of Kingston Carburetor, Which Uses Venturi Type Concentric Mixing Chamber.

Q. What is the Venturi tube mixing chamber?

A. A Venturi tube mixing chamber of conventional design is clearly outlined at Fig. 66. In this it will be noticed that the mixing chamber is constricted at a point approximately in line with the top of the jet. This insures a high velocity of air past the top of the spray nozzle even at low engine speed. With a straight mixing chamber the speed of the enter ng air stream at low engine speed might not be sufficient to pick up enough fuel to form an explosive mixture. Constricting the tube at the proper point means that the air velocity will always be sufficient to draw a full supply of liquid from the spray nozzle.

Q. What is an automatic carburetor and why is it needed? A. Whenever a Venturi tube construction is employed to secure greater air velocity at low speed it is apparent that at high. engine speed the air velocity might be great enough to draw more fuel than was actually needed into the engine cylinder. This would mean that the excessively rich mixture provided at high engine speed would cause overheating and waste fuel and a comparatively thin mixture at low engine speed would interfere with prompt starting. In actual practice these conditions should be reversed. The rich mixture is only necessary for starting while a much

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Fig. 67.-Typical Concentric Float Carburetor With Poppet Valve Controlling Auxiliary Air Supply.

thinner mixture or one containing a larger proportion of air can be used to advantage at high engine speed. The automatic form of carburetor, which is the type generally used to-day, provides for the introduction of an auxiliary supply of air through a separate opening that will dilute the excessively rich mixture obtained at higher speed and make it more suitable as a combustible gas.

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Fig. 68.-Two Jet Carburetor of Simple Design.

Q. How may auxiliary air be admitted to the carburetor? A. The auxiliary air passage may be controlled by any form of automatic valve. In the design shown in Fig. 67, the air is admitted by means of an ordinary spring seated mushroom or poppet valve. The spring tension is so proportioned that the valve will open only on medium and high engine speeds, at which times the suction is

greater than that prevailing at low speed. The auxiliary air passages are sometimes controlled by means of reeds which open progress ively as more auxiliary air is needed or by a series of balls which close the auxiliary air ports. The strength of the reeds or the weight of the balls may be varied so the air passages will open progressively and admit more air as the demands increase.

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Fig. 69.-Latest Holley Carburetor Has Fixed Air Intake and No Auxiliary Air Valve.

Q. What is a multiple jet carburetor?

A. A simple form of multiple jet carburetor is shown at Fig. 68. In a device of this character, two or more spray nozzles are used instead of a single jet. The arrangement is usually such that the

primary nozzle is used at low speed while the secondary nozzle is brought in action at higher speed when more fuel is needed. In some types the arrangement is such that the primary nozzle acts only at low speed while the secondary nozzle supplies gasoline only at high speed. In other multiple jet carburetors, the nozzles are brought into action progressively when the throttle is opened to

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Fig. 70.-Sectional View of Planhard Carburetor Showing Fuel Supply Regulating Needle.

such a point that the primary nozzle, which has a small spraying orifice, cannot supply fuel enough; then the secondary nozzle is brought into action and contributes its quota of liquid to compensate for the augmenting demand of the engine.

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