The Ford 1 barrel carburetor used on all six cylinder engines consists of two main assemblies, the upper body assembly and the lower body assembly.
The upper body assembly contains the major metering components of the carburetor. The main and idle fuel, power valve, float chamber vent and fuel inlet systems, are all part of the upper body assembly.
The function of the carburetor is to mix fuel and air, in the proper proportion, and to supply the mixture in the proper quantity to the engine for all engine operating conditions. The engine speed is regulated by controlling the quantity of fuel and air delivered to the cylinders. Operation is based on the principle of pressure differences, or vacuum. Air is drawn into the carburetor air horn by vacuum created through movement of the engine piston. As the air passes through the carburetor on its way to enter the cylinders, lower pressure is created at the fuel discharge outlets of the carburetor. The fuel bowl is vented to the atmosphere and or the air cleaner. The higher air pressure exerted on the fuel in the bowl will force fuel to travel up through the fuel discharge channels and out into the air stream passing through the carburetor. The fuel and air are mixed at this point and distributed into the engine cylinders for burning.
The fuel inlet system of a carburetor maintains a predetermined fuel level within the fuel bowl. If the level of the fuel within the bowl is below the specified setting, a lean fuel air mixture will result. A rich fuel air mixture will occur from a high fuel level. The entire calibration for the carburetor is disturbed if the fuel level is not set as specified.
Idle Fuel System
The idle system functions when the engine is operating at lower rpm. It supplies the fuel air mixturre when the air flow past the carburetor venturi is insufficient to operate the main metering system.
The range of the idle system will extend into the operation of the main metering system. Fuel will flow from the main well up the idle well and through the calibrated idle fuel channel restriction. Filtered air from the carburetor air horn enters the idle air bleed restriction and mixes with the fuel. The air bleed restriction also serves as a vent to prevent syphoning of fuel at high speeds or when the engine is shut off. The fuel air mixture then passes down through an idle channel restriction and is transferred to the idle channel in the lower body assembly. The fuel air mixture passes down the idle channel past two idle transfer holes, to the idle mixture adjusting screw. The idle transfer holes act as additional air bleeds at normal idle. The fuel air mixture passes through drilled holes in the adjusting screw and is discharged below the throttle plate. The amount of mixture to be discharged is determined by the position of the idle screw in the lower body passage.
During off idle operation, when the throttle plate is moved past the idle transfer holes, each hole begins discharging fuel as it is exposed to the lower air pressure (manifold vacuum). Continued opening of the throttle plate increases engine rpm and air flow through the carburetor. The greater air flow past the booster venturi causes a pressure drop in the venturi great enough to bring the main fuel metering system into operation as the idle fuel metering system tapers off.
Main Metering System
The main fuel metering system supplies the fuel required for engine operation during the cruise or part throttle range. The system begins to function when the air flow thorough the carburetor venturi creates a sufficient vacuum to start fuel flowing in the main system. The vacuum at the discharge nozzle will increase as the air flow increases. The faster the engine operates, the more fuel will flow through the main fuel system.
Fuel entering the main jet, located at the bottom of the main well, flows up toward the main nozzle. Inserted in the main well is the main well tube. Air from the high speed bleed channel enters the main well tube through a calibrated restriction at the top of the tube. The air passes through holes spaced along the tube, mixing with the fuel flowing up the main well. The fuel and air mixture, being lighter than solid fuel, responds faster to changes in venturi pressures. The mixture continues flowing up the main well to the anti-syphon bleed. More air is introduced at the anti syphon bleed to the fuel and air mixture, which is then discharged from the main nozzle. The fuel is mixed with the filtered air moving past and through the booster venturi.
The anti syphon bleed also acts as a vent to prevent syphoning of fuel at low engine speeds.
Accelerating Pump System
Smooth acceleration requires a momentary increase in the supply of fuel. The air flow through the carburetor responds almost immediately to any increase in carburetor throttle valve opening. The fuel within the metering passages will lag momentarily in its response to the pressure difference created by this increased air flow. This lag in fuel response will cause a temporary leanness in the fuel air mixture that results in a hesitation in engine acceleration. A mechanicaly operated accelerating pump system supplies added fuel to provide a richesr fuel air mixture for this brief period of time.
The accelerating pump, located on the side of the lower body assembly, is actuated by linkage connected to the throttle shaft. When the throttle is opened on acceleration, the diaphragm forces fuel from the acceleratiing pump chamber into the discharge channel. The inlet ball check closes to prevent a reverse flow of fuel. Fuel under pressure forces the discharge ball check and the weight off its seat, allowing fuel to pass up to the discharge nozzle. The fuel is sprayed from the nozzle into the air stream above the main venturi.
When the throttle plate is closed on deceleration, a return spring forces the diaphragm back, drawing fuel through the inlet chaannel. The inlet ball check opens, allowing fuel to pass into the chamber while the discharge ball check closes, preventing entry of air.
Power Fuel System
When the engine is required to deliver more power to meet an increased road load demand or wide open throttle operation, the carburetor must deliver a richer fuel air mixture than supplied during the operation of the main fuel system. When the engine is running under a high power demand, intake manifold vacuum is low. The vacuum below the carburetor throttle plate approimates intake manifold vacuum. The carburetor power valve will open when the manifold vacuum drops below a pre-determined value. The fuel air mixture is thus automaticaly enriched to meet the increased engine power demands.
Manifold vacuum is transmitted from an opening below the throttle plate through a channel to the upper body assembly and to the top of the power valve piston. At idle and normal engine speeds the manifold vacuum is great enough to hold the power valve piston up.The power valve rod is connected to the piston. The foot of the rod controls the spring loaded power valve ball check. With the piston held up by manifold vacuum, the ball check closes the power system inlet chaannel.
A power valve spring is located on the rod. The spring is shim calibrated to overcome the vacuum above the piston when manifold vacuum drops below a predetermined level. Upon demand for more power, the manifold vacuum drops below this level. The spring tension moves the rod down and allows the power valve ball check to open. Air pressure above the fuel bowl forces fuel to flow through the power system channel, adding to fuel in the main fuel system, enriching the fuel air mixture.
As the demand for power decreases and manifold vacuum increases, the vacuum above the piston overcomes the spring tension. The piston and rod move up, and the ball check closes the power system channel.
Fuel Bowl Vent SystemThe fuel bowl requires venting to provide proper operation for the various systems. Fuel vapors may form in the fuel bowl when a hot engine is stopped, idling, or operating at very low speeds. By venting the fuel bowl to the atmosphere, engine performance is improved. At higher engine speeds, venting to the carburetor air horn prevents calibration changes due to normal air cleaner contamination.
A valve connected through linkage to the throttle shaft, and located in a bore over the fuel bowl, is at the inward position during closed or part throttle operation. In this position, the valve allows venting only to the atmosphere.
At normal or wide open throttle operation, the rod moves outward sealing the external vent and opening the vent to the carburetor throat.
An Econoline vehicle requires a vent system that will draw fuel vapors away from the engine compartment. The carburetor used on Econoline vehicles has an external vent tube and an open passage to the air cleaner. Both vents are always open on this carburetor.
The dashpot is used on automatic transmission vehicles and is used to hold the throttle open slightly when decelerating to give the fuel time to burn off.