of hardened steel, or it may have a synthetic rubber section  which fits the seat. With no fuel in the float chamber, the float  drops toward the bottom of the chamber and allows the needle  valve to open wide. As fuel is admitted from the supply line,  the float rises (floats in the fuel) and closes the needle valve  when the fuel reaches a predetermined level. When the engine  is running, and fuel is being drawn out of the float chamber,  the valve assumes an intermediate position so that the valve  opening is just sufficient to supply the required amount of  fuel and keep the level constant.  [Figure 2-10]  If fuel is found  leaking from the discharge nozzle of the carburetor when the  engine is not running, the most likely cause is that the float  needle valve and seat is leaking and needs to be replaced. With the fuel at the correct level (float chamber), the  discharge rate is controlled accurately by the air velocity  through the carburetor venturi where a pressure drop at the  discharge nozzle causes fuel to flow into the intake airstream.  Atmospheric pressure on top of the fuel in the float chamber  forces the fuel out the discharge nozzle. A vent or small  opening in the top of the float chamber allows air to enter or  leave the chamber as the level of fuel rises or falls.  The main metering system supplies fuel to the engine at all  speeds above idling and consists of: to the difference in pressure between the atmosphere and  the intake manifold, air flows from the air intake through  the carburetor barrel into the intake manifold. The volume  of airflow depends upon the degree of throttle opening. As  the air flows through the venturi, its velocity increases. This  velocity increase creates a low pressure area in the venturi  throat. The fuel discharge nozzle is exposed to this low  pressure. Since the float chamber is vented to atmospheric  pressure, a pressure drop across the discharge nozzle is  created. It is this pressure difference, or metering force, that  causes fuel to flow from the discharge nozzle. The fuel comes  out of the nozzle in a fine spray, and the tiny particles of fuel  in the spray quickly vaporize in the air. Since the throttle valve controls the mass airflow through  the carburetor venturi, it must be considered a major unit  in the main metering system as well as in other carburetor  systems. A typical main metering system is illustrated in  Figure 2-12 . The venturi performs three functions: The fuel discharge nozzle is located in the carburetor barrel  so that its open end is in the throat or narrowest part of the  venturi. A main metering orifice, or jet, is placed in the fuel  The metering force (pressure differential) in most carburetors  increases as the throttle opening is increased. The fuel  must be raised in the discharge nozzle to a level at which it  discharges into the airstream. To accomplish this, a pressure  differential of 0.5 "Hg is required. When the metering  force is considerably reduced at low engine speeds, the fuel  delivery from the discharge nozzle decreases if an air bleed  (air metering jet) is not incorporated in the carburetor. The  decrease in fuel flow in relation to airflow is due to two factors: passage between the float chamber and the discharge nozzle  to limit the fuel flow when the throttle valve is wide open. When the engine crankshaft is revolved with the carburetor  throttle open, the low pressure created in the intake manifold  acts on the air passing through the carburetor barrel. Due  2-11 1.  The fuel tends to adhere to the walls of the discharge  nozzle and break off intermittently in large drops  instead of forming a fine spray. 3.  Limits the airflow at full throttle 2.  Decreases the pressure at the discharge nozzle 1.  Proportions the fuel/air mixture 5.  Throttle valve 4.  Passage leading to the idling system 3.  Main discharge nozzle 2.  Main metering jet 1. Venturi Figure 2-12.  Main metering system.  Main Metering System 14.7 p.s.i. Throttle valve Main metering jet Fuel discharge nozzle Main air bleed Venturi Idle feed passage Airflow