off its seat far enough to provide more fuel than is needed  for idling. This potentially overrich mixture is regulated by  the idle valve. At idling speed, the idle valve restricts the fuel  flow to the proper amount. At higher speeds, it is withdrawn  from the fuel passage and has no metering effect. main control diaphragms. The purpose of the regulator unit is  to regulate the fuel pressure to the inlet side of the metering  jets in the fuel control unit. This pressure is automatically  regulated according to the mass airflow to the engine. The carburetor fuel strainer, located in the inlet to chamber  E, is a fine mesh screen through which all the fuel must pass  as it enters chamber D. The strainer must be removed and  cleaned at scheduled intervals. Vapor vent systems are provided in these carburetors to  eliminate fuel vapor created by the fuel pump, heat in the  engine compartment, and the pressure drop across the poppet  valve. The vapor vent is located in the fuel inlet (chamber E)  or, on some models of carburetors, in both chambers D and E. The vapor vent system operates in the following way. When  air enters the chamber in which the vapor vent is installed,  the air rises to the top of the chamber, displacing the fuel  and lowering its level. When the fuel level has reached a  predetermined position, the float (which floats in the fuel)  pulls the vapor vent valve off its seat, permitting the vapor  in the chamber to escape through the vapor vent seat, its  connecting line, and back to the fuel tank. Referring to  Figure 2-24 , assume that for a given airflow  in lb/hr through the throttle body and venturi, a negative  pressure of  1 ⁄ 4 psi is established in chamber B. This tends  to move the diaphragm assembly and the poppet valve in a  direction to open the poppet valve permitting more fuel to  enter chamber D. The pressure in chamber C is held constant  at 5 psi (10 psi on some installations) by the discharge  nozzle or impeller fuel feed valve. Therefore, the diaphragm  assembly and poppet valve moves in the open direction until  the pressure in chamber D is 5 1 ⁄ 4 psi. Under these pressures,  there is a balanced condition of the diaphragm assembly with  a pressure drop of  1 ⁄ 4 psi across the jets in the fuel control unit  (auto-rich or auto-lean). If nozzle pressure (chamber C pressure) rises to 5 1 ⁄ 2 psi, the  diaphragm assembly balance is upset, and the diaphragm  assembly moves to open the poppet valve to establish  the necessary 5 3 ⁄ 4 psi pressure in chamber D. Thus, the  1 ⁄ 4  psi differential between chamber C and chamber D is re- established, and the pressure drop across the metering jets  remains the same. If the fuel inlet pressure is increased or decreased, the fuel  flow into chamber D tends to increase or decrease with  the pressure change causing the chamber D pressure to do  likewise. This upsets the balanced condition previously  established, and the poppet valve and diaphragm assembly  respond by moving to increase or decrease the flow to re- establish the pressure at the  1 ⁄ 4 psi differential. If the vapor vent valve sticks in a closed position or the vent  line from the vapor vent to the fuel tank becomes clogged, the  vapor-eliminating action is stopped. This causes the vapor to  build up within the carburetor to the extent that vapor passes  through the metering jets with the fuel. With a given size  carburetor metering jet, the metering of vapor reduces the  quantity of fuel metered. This causes the fuel/air mixture to  lean out, usually intermittently. If the vapor vent valve sticks open or the vapor vent float  becomes filled with fuel and sinks, a continuous flow of  fuel and vapor occurs through the vent line. It is important  to detect this condition, as the fuel flow from the carburetor  to the fuel supply tank may cause an overflowing tank with  resultant increased fuel consumption. To check the vent system, disconnect the vapor vent line  where it attaches to the carburetor, and turn the fuel booster  pump on while observing the vapor vent connection at the  carburetor. Move the carburetor mixture control to auto-rich;  then return it to idle cutoff. When the fuel booster pump is  turned on, there should be an initial ejection of fuel and air  followed by a cutoff with not more than a steady drip from  the vent connection. Installations with a fixed bleed from the  D chamber connected to the vapor vent in the fuel inlet by a  short external line should show an initial ejection of fuel and  air followed by a continuing small stream of fuel. If there  is no flow, the valve is sticking closed; if there is a steady  flow, it is sticking open. The fuel flow changes when the mixture control plates  are moved from auto-lean to auto-rich, thereby selecting  a different set of jets or cutting one or two in or out of the  system. When the mixture position is altered, the diaphragm  and poppet valve assembly repositions to maintain the  established pressure differential of  1 ⁄ 4 psi between chambers  C and D, maintaining the established differential across the  jets. Under low power settings (low airflows), the difference  in pressure created by the boost venturi is not sufficient to  accomplish consistent regulation of the fuel. Therefore, an idle  spring, shown in  Figure 2-24 , is incorporated in the regulator.  As the poppet valve moves toward the closed position, it  contacts the idle spring. The spring holds the poppet valve  2-17