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Q. How many methods of mixing chamber placing relative to float chamber prevail?
A. There are two common arrangements of the float and mixing chambers as outlined at Fig. 64. In one, the float chamber is carried at one side of the mixing compartment, while in the other the float chamber is concentric with and surrounds the mixing chamber.
Q. What are the advantages of the concentric design?
A. When a float bowl is carried at the side of the mixing chamber the level in the jet will vary except when the device is absolutely
level. If the float chamber is in front of the mixing chamber when climbing a grade, it will tend to cause flooding, because it is tilted higher than the spraying nozzle top. When descending a hill the conditions are reversed and the fuel level in the spray nozzle will be too low and not enough liquid will be sprayed into the mixture. With the concentric design, where the spray nozzle is carried at a central point of the carburetor, no reasonable amount of tilting will alter the level of liquid at that point appreciably. The concentric construction is preferred because it tends to promote uniformity of mixture proportions under all conditions of car operation.
Fig. 65.-Complete Fuel Supply System of Chalmers Car, Showing Gasoline Tank Under the Seat and
Piping Leading to Carburetor. Sectional View of Carburetor Outlined in Upper Right-Hand Corner.
Q. What is the Venturi tube mixing chạmber?
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 entering 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
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.
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