On the suction stroke of the engine, air is drawn through the upper left hand opening, past the partially open auxiliary air valve =A=, past the needle valve =D=, through the mixing chamber =C=, and into the engine through =R=. The suction of the engine produces a partial vacuum in the mixing chamber =C= which causes the gasoline to issue from the nozzle =D=, in the form of a fine spray which is taken up by the air passing through the passage =H=, and is taken into the engine through =R=, thoroughly mixed. The amount of mixture entering the engine, and consequently the engine speed is regulated by the throttle valve =K=, operated by the lever =P=. In order that the amount of spray given by the nozzle =D= be constant it is necessary that the level, or height of the gasoline in the nozzle be constant. The level is maintained by means of the float =F=, which opens, or closes the gasoline supply valve =H=, opening it and allowing gasoline to enter when the level is low, and closing the valve when the level is high. The carburetor is connected to the gasoline supply tank, by pipe connected to the inlet =G=, through which the gasoline flows into the float chamber =B=. The float chamber carries a small amount of gasoline on which the float =F= rests. The richness of the mixture is controlled by opening or closing the nozzle needle valve =E=, which passes through the center of the nozzle =D=. The float =F= surrounds the nozzle in order to keep the level of the liquid constant when the carburetor is tilted out of the horizontal by climbing hills, or by the rocking of the boat when used on a marine engine. A drain cock =T= is placed at the bottom of the float chamber for the purpose of removing any water, or sediment that may collect in the bottom of the float chamber. At low speeds, the auxiliary air valve =A= lies tight on its seat, allowing a constant opening for the incoming air through the space shown at the bottom of the valve. When the speed of the engine is much increased, the vacuum is increased in the mixing chamber =C=, which overcomes the tension of the air valve spring =O= and allows the valve to open and admit more air to the mixing chamber. The action of the auxiliary air valve keeps the mixture uniform at different engine speeds, as it tends to keep the vacuum constant in the mixing chamber. When the engine speed increases, the flow of gasoline is greater, and consequently more air will be required to burn it; this additional air is furnished by the automatic action of the valve, and when once adjusted, compensates accurately for the different engine speeds. The gasoline is generally supplied by a tank elevated at least six inches above the level of the fluid in the float chamber; although in some cases the gasoline is supplied by air pressure on a tank situated below the level of the carburetor.