Fuel Delivery and Air Induction: Description and Operation
Fuel Inlet System
The fuel inlet system maintains the fuel level in the carburetor bowl so the fuel metering systems can deliver the proper mixture to the engine. As the fuel level in the bowl drops the float drops, allowing additional fuel to enter the bowl through the fuel inlet fitting located in the main body. Only enough fuel is admitted to replace the amount used.
A dual-pontoon float made of non-porous material monitors and maintains the proper fuel level in the fuel bowl. The Viton-tipped inlet needle seats directly in the fuel inlet fitting, and is permanently retained by a cap assembled to the fitting. The fuel inlet fitting and inlet needle are serviced as an assembly. Fuel flows past the needle and seat and enters the bowl through four holes in the valve seat body. Because the valve seat body extends only a short distance into the fuel bowl, the discharging fuel is shrouded by the main body casting and the inlet does not require a separate fuel baffle. A fuel bowl filler is used to help maintain fuel level during sudden stops and maneuvers.
Main Metering System
As engine speed increases, air flow through the carburetor increases, causing lower pressure in the venturi. Present in the fuel bowl above the fuel, near atmospheric pressure causes the fuel to flow to the lower pressure created at the venturi. This venturi action is increased by the use of dual venturi boosters. This creates a stronger venturi effect which in turn provides more precise response to changes in air flow through the carburetor.
Fuel flows from the fuel bowl through the main jet into the main well. A small amount of air is admitted by the high speed bleed. This air is carried down the main well tube to a series of small holes where it excapes and mixes with fuel in the main well. The resultant mixture of fuel and air, which is lighter than solid fuel, responds faster to changes in pressure in the venturi and is more readily atomized when discharged into the air stream.
The main metering system is calibrated to deliver a lean mixture for improved part-throttle economy and exhaust emissions. When more power is needed, a vacuum-operated power system enriches the air/fuel mixture.
Idle System
During idle and low speed operation, air flow through the carburetor is not sufficient to cause fuel to be discharged through the main discharge passage. For this reason, a separate idle system is part of the construction of the 1946. The idle system is capable of fine adjustment, and can supply the engine with an appropriate air/fuel mixture at idle and can supplement the main metering system during critical off-idle operation.
Fuel entering the idle system flows from the fuel bowl through the main metering jet into the main well. An angular idle well connects with the main well, and an idle tube is pressed into this well. Fuel flows up the idle tube and is mixed with air at the top of the passage just below the idle air bleed in the air horn. The fuel air mixture then flows down the idle channel and into the throttle body where the transfer slot and curb idle discharge port are located. With the throttle plate at curb idle, the fuel and air mixture flows down the idle channel where additional air is introduced through the portion of the transfer slot which is above the throttle plate. The air/fuel mixture is then discharged below the throttle plate at the curb idle discharge port. Idle fuel mixture is regulated by the adjustment of the idle mixture screw. Maximum fuel flow at idle is limited by the size of the restriction in the idle tube.
During low speed operation, as the throttle plate opens, it begins to expose the idle transfer slot to manifold vacuum. As the slot is exposed, the fuel flow through the slot increases to meet engine demand.
As the throttle is opened further, and engine speed increases, the air flow through the carburetor also increases. The increased air flow creates a low pressure in the venturi, and the main metering system begins to discharge fuel.
Gradient Power Enrichment Systemm
A vacuum-operated enrichment system is used to provide the fuel mixture enrichment necessary for part throttle and wide open throttle operation. The Enrichment system consists of an enrichment valve assembly containing a needle, installed in the floor of the fuel bowl near the center of the main body and a vacuum piston installed in the air horn.
A vacuum passage leads from the top of the piston bore through a vacuum hose connection with choke pulldown assembly. When the manifold vacuum is high, the vacuum piston is raised to the top of its cylinder and the spring above the piston is compressed. When manifold vacuum drops to a predetermined level, the spring overcomes the vacuum and pushes the piston down, permitting fuel to pass by the needle, through the calibrated enrichment valve channel restriction and into the main well. This fuel supplements the fuel being discharged through the main metering system jet.
External Fuel Bowl Vent System
The model 1946 has an electrical bowl vent solenoid. The system allows fuel vapors to flow to the carbon canister through the external fuel bowl vent, when the engine is Off. With the ignition On the valve closes the external vent and opens the internal vent tube to the carburetor air horn and into the engine intake.
The fuel bowl vent can be serviced without removing the carburetor from the vehicle.
Accelerator Pump System
When the throttle plate is opened suddenly, the air flow through the carburetor increases almost immediately, but there is a brief lag before the fuel begins to flow enough to provide the desired air/fuel ratio. The accelerator pump system operates as the throttle is opened, supplying the fuel necessary to maintain the correct mixture until the main metering system can catch up to the air flow.
The fuel level is above the normal raised position of the pump piston, causing fuel to enter the pump cylinder through the pump cup stem clearance hole when the pump is lifted to a refill position. As the throttle is again opened, the pump link, operated through a system of levers and a drive spring, pushes the pump piston down, seating the pump cup against the face of the stem. Fuel is forced through a passage, around a check ball and weight, and through the pump discharge jet which is drilled in the main body.
When the pump is not in operation, vapors or bubbles forming in the pump cylinder can excape through the stem clearance hole of the floating piston cup and past the pump stem into the fuel bowl.
Automatic Choke System
The automatic choke system provides the richer air/fuel mixture required for starting and operating a cold engine.
A bimetal spring inside the choke housing begins to exert closing pressure on the choke valve at a temperature below approximately 75~ F. As temperature drops, closing pressure on choke plate increases. As the engine warms up the electric choke cap transfers heat to the spring and reduces the closing force on the choke plate.
When the engine starts, two auxiliary systems also act to open the choke plate against the pressure of the spring: First, manifold vacuum is applied to a modulated choke diaphragm assembly through an externally-routed manifold vacuum hose. A link connected to the diaphragm pulls the choke open a set amount to let the engine run without flooding out. The amount of choke opening depends on the temperature of the spring, since the closing force of the bimetal is opposed by a modulating spring within the pulldown motor. This type of system is thus called a modulated choke pulldown system. In addition to choke pulldown, choke opening is assisted by air rushing into the carburetor and acting against the choke plate, which is offset on the shaft.
The increased engine speed required during cold idle is accomplished by a throttle stop cam positioner. The cam is positioned by a link connected to the choke plate shaft.