THE INLET VALVE.--Now if this chamber G has at one side an extension, like L, Fig. 79, means may be provided for adding a valve to be controlled by the float. Within the extension is an upwardly-moving needle valve M, which is designed to close the duct which leads from the gasoline supply. Between the valve and the float is the fulcrum O, of a lever N, the short end of which engages with the upper end of the valve and the long end rests on the float H, as shown. The movement of the float above the predetermined point has the effect of seating the needle valve M, thus cutting off the inflow of gasoline until that in the chamber G is drawn out so that the float descends and again admits a fresh supply. [Illustration: Fig. 80. Carbureter Discharge Port.] Thus far we have the fuel oil control, together with the manner in which the primary air supply is introduced. We shall now go a step further, and illustrate the mixing chamber, discharge and throttle. THE THROTTLE VALVE.--Referring to Fig. 80 it will be seen that directly above the venturi tube described, is a space O. This is the mixing chamber, which has an outlet P to the left, which connects with the engine cylinders. Within this tube is a throttle valve Q, operated by the throttle lever on the steering wheel of the car. It is simply a disk which fits into the interior of the conduit and is adapted to be turned by a stem R, on which it is mounted. While the lower inlets K are designed to supply the primary air for carburetion, it is found necessary to admit a secondary supply, and this should be taken into the mixing chamber directly instead of passing the tube which conveys the oil. THE SECONDARY AIR SUPPLY.--The particular reasons for thus admitting the air may be explained as follows: When the engine draws in a supply of carbureted air, more or less of a vacuum is brought about in the mixing chamber O. The faster the engine runs the richer will the mixture become, because the additional suction draws in an increasing quantity of gasoline, but the throat of the tube does not change, and the requisite, proportionate quantity of air does not follow, so that the mixture has too much fuel for the air. AUTOMATIC ADMISSION OF SECONDARY AIR.--If the engine should be speeded up so twice the amount of oil is drawn into the mixing chamber, the additional suction will not, at the same time, draw in twice the amount of air. This necessitates a provision whereby the secondary air shall be admitted automatically only at times when the suction exceeds the normal requirement, or to prevent too rich a mixture, which is explained by reference to Fig. 80. [Illustration: Fig. 81. Carbureter Secondary Air Inlet.] The extension S, on the right side of the shell, has an opening T, with a seat to receive a weighted valve, like a ball U, preferably reinforced by a spring V, which is capable of having its pressure on the seat regulated by an adjusting screw W.