PART THROTTLE OR MAIN METERING SYSTEM (Figure 14)
The purpose of the part throttle or main metering system is to provide fuel during the transition period from off-idle to wide-open throttle operation. Under power operation, it is supplemented by the power system. It will be noted that the quantity of fuel being discharged from the idle and secondary idle discharge holes has diminished due to the wider throttle valve opening resulting in a decrease of intake manifold vacuum acting on these holes.
As the throttle valve opening increases, fuel begins to flow through the main metering system due to an increase in air velocity through the venturi system. This causes a drop in pressure in the main or primary venturi which is increased many times in the secondary venturi. Because a low-pressure area is now acting at both points (main and idle circuits), fuel will flow in the following manner.
Tracing the path of fuel, we see that the fuel flows from the float bowl through the main metering jet into the main well area. The size of the main metering jet determines how much fuel can flow through. The fuel will now divide: Some will flow through the idle circuit and some through the main metering circuit.
Idle Fuel Flow:
The fuel flowing through the idle circuit will move up the idle tube to the top of the passage where it meets the first idle air bleed. Here air is drawn in and mixed with the fuel. This mixture travels across the cross channel where it is bled a second time by air from the second idle air bleed. From there the lighter mixture of air and fuel travels down the idle passage past the calibrated idle restriction and discharges into the bore of the carburetor through the idle and secondary idle discharge holes.
Main Metering Fuel Flow:
Fuel flows through the main metering system due to the low pressure in the secondary venturi being transmitted to the tip of the main well tube. Pressure within the fuel bowl which is greater than venturi pressure forces fuel into the main well area through the metering jet. The fuel then travels up through the center of the main well tube. Air entering through the main well air bleed will mix with fuel through the uncovered holes at the top of the main well tube. This action serves to break up or help atomize the fuel in the main discharge circuit the same way as the idle air bleed serves to break up fuel particles in the idle discharge passage. The mixture of air and fuel now continues up the main well tube to the top of the venturi cluster assembly. Because of the low pressure or suction acting on the fuel at this point, the fuel is made to flow down the mixture passage where it is further mixed with air being drawn through the open end at the top of the mixture passage. The fuel continues flowing to the secondary venturi and on into the intake manifold.
It will be noted at the base of some secondary venturies a brass cage containing a series of small holes is sometimes used. This cage in effect also helps to break up liquid droplets of fuel changing them into a fine mist state for easier burning. With the increased throttle opening, there is an increase in the velocity of the air through the venturi system. This causes a further drop in pressure resulting in a larger quantity of fuel flow through the main metering system and a corresponding drop or lessening of fuel from the idle system to the point of complete stoppage.
As the throttle opening is progressively increased causing more fuel to be drawn through the main well tubes, the fuel level in the main well drops. As this fuel level drops, more calibrated holes in the main well tubes become exposed. When this occurs they become air bleeds, thus mixing progressively more air with the increase of fuel passing through the main well tubes. Thus, although the nozzle suction is increased by increasing the throttle opening, the fuel mixture to the engine remains constant throughout the part throttle range. If this action (lowering of the fuel level) in the main well did not occur, the mixture would be overly rich, resulting in a stumbling engine during the part throttle range.
POWER SYSTEM (Figure 15)
The purpose of the power system is to provide the extra quantity of fuel required when more power is desired or extreme high speed driving is to be maintained. The Rochester 2-barrel carburetor uses a vacuum-operated power system. During the period of high-speed driving when fuel needs are extremely great, the amount of fuel which could normally enter the engine through the metering jet is not sufficient to satisfy engine demands. Therefore, other means of supplying additional fuel to the engine under these conditions must be used. This is accomplished by use of the power system.
Designed into the power system is a spring-loaded power piston and stem assembly located directly above a power valve assembly. During idle and normal cruising speeds when intake manifold vacuum is strong, this vacuum which is transmitted through a vacuum passage to the top of the power piston assembly will hold the power piston upward, keeping the stem of the power piston assembly away from the power valve assembly. The power valve is now in a closed position with no fuel permitted to flow through. The only fuel now reaching the engine is that which is flowing through the main metering jet.
Under extreme power demands, intake manifold vacuum will weaken and drop to a point where the tension of the power piston spring will overcome the strength of the existing vacuum acting on the top of the power piston causing the power piston and stem to be pushed downward contacting the power valve assembly. The power valve assembly will now be held open allowing fuel to flow through a passage, containing a calibrated restriction, and on into the main well area supplementing the fuel which could normally enter the main well area through the metering jet. The combining action of fuel from these two places is enough to satisfy engine demands. The combination of fuel through the power valve assembly and the main metering jet will now flow up the main well tube into the mixture passage and be drawn into the area of high vacuum or suction at the secondary venturi.
Idle Fuel Ceases:
It will be noted that the fuel has discontinued flowing through the idle passage. This is because the throttle valves are now in a wide open position and the low pressure area is no longer concentrated in the vicinity of the idle discharge holes. Due to the large volume of air rushing into the carburetor past the primary and secondary venturi, the low pressure or vacuum is now confined solely to this area.
When a return-to-normal cruising speed is desired and the throttle is progressively closed, the extra fuel from the power system is no longer required. The vacuum in the intake manifold now begins to increase or build up once again. As the strength of the vacuum increases, it is again transmitted through the vacuum passage to the top of the power piston assembly. When the vacuum reaches a predetermined value, it will overcome the tension of the power piston spring, pulling the power piston and stem assembly upward allowing the power valve to once again close preventing any further flow of fuel. A small spring contained inside the power valve assembly keeps the valve in a closed position.
Vacuum Bleed-Off Port:
There is a hole or vacuum bleed off port drilled through the wall of the power piston cylinder. This vacuum bleed off port is necessary for the following reason: Because vacuum in the vacuum passage is acting on top of the power piston, if it were not for the bleed off port, it would be possible for the air in the float chamber to be drawn out past the power piston and cylinder wall. This removal of air would lower the air pressure in the float chamber seriously affecting the calibration of the carburetor. As previously stated, the carburetor operates under the theory of pressure differences. This means that fuel is made to flow according to the amount of pressure or vacuum acting on the exit hole of that particular passage in the carburetor. Therefore, the action of the vacuum bleed off port allows air to be drawn from the top section of the carburetor where no harm is done instead of allowing air to be withdrawn from the float chamber itself.
Additional Air Bleeds:
Notice as the throttle valves are held in the wide open position, the level of fuel in the main well area has dropped to its lowest point. This action uncovers additional holes in the main well tube which then serves as additional air bleeds. Although the volume of fuel has increased, so has the volume of air,thereby maintaining a constant mixture ratio to the engine throughout this power range.
Two-Step Power Valve:
Some Rochester models use a "two-step" power valve assembly. The first step unseats the spring loaded plunger and fuel is metered by the plunger for light power requirements. For the second step, the plunger is completely bottomed and fuel is metered entirely for full power operation by the calibrated restriction in the passage leading to the main well area.
Other Carburetor Models:
Due to the many different models among carburetor manufacturers, the basic design of the parts contained in the power system will vary in appearance.
Figure 16 Figure 17 Figure 18
Some power systems as used in Holley, Rochester and Stromberg carburetors will be vacuum-operated using the piston, stem and valve arrangement as described earlier (figure 16). Others of the same basic type (Holley carburetors) will use a diaphragm in place of the piston (figure 17). Vacuum working against the diaphragm operates the unit in the same manner.
Other systems as used in late model 2 and 4-barrel Ford and Holley carburetors will also be vacuum-operated but through the use of a self-contained power valve assembly (figure 18). This type has a diaphragm built into the power valve unit and is connected directly to the plunger controlling open and closed positions in the following manner.
The manifold vacuum acting on one side of the diaphragm at idle and normal load conditions is strong enough to hold the diaphragm inward closing off the plunger opening by overcoming the tension of a built-in power valve spring.
When high-power demands place a greater load on the engine and intake manifold vacuum drops below a predetermined point, the power valve spring overcomes the reduced vacuum acting upon the diaphragm opening the valve. Fuel flows through the valve until engine demands decrease resulting in an increase of vacuum once again closing the valve by means of diaphragm movement.
While most carburetor power systems use the separate power valve method as described above, practically all Carter carburetors are designed to incorporate a movable metering rod (or step-up rod) working up and down in the metering jet varying the area of the jet thus controlling the amount of fuel flow through it. The specific design and application of the metering rod method which Carter uses will vary according to their different model carburetors, but for purposes of illustration the following is the basic manner in which it operates. Refer to figure 19.
Fuel for part-throttle and full-throttle operation is supplied through the high-speed circuit. The position of the step-up rod in the main metering jet controls the amount of fuel permitted to reach the discharge nozzles. The step-up rod, having different size steps at the base, is connected to a spring-loaded vacuum piston and positioned in the metering jet according to the strength of the intake manifold vacuum applied to the base of the vacuum piston.
During part-throttle operation, manifold vacuum overcomes the strength of the spring under the piston pulling the piston and rod assembly down. The large diameter of the metering or step-up rod is now held in the main metering jet. Fuel is now being metered around the large diameter of the rod.
Under power demands or wide-open throttle, manifold vacuum drops to the point where the tension of the step-up piston spring being stronger than vacuum forces the piston and rod upward. The smaller diameter of the step-up rod now being located in the main metering jet permits a larger quantity of fuel to flow by.
The design of Carter carburetors will vary in that only one metering rod is used for single-barrel carburetors - two rods for 2 and 4-barrel carburetors. Some models will use a mechanically operated metering rod to coincide with throttle valve movement, plus a vacuum-operated step-up piston and rod assembly for power demands. Step-up rods do not require an adjustment upon reassembling carburetor. Metering rods require an adjustment, and complete instructions with accompanying illustrations are included with each Echlin carburetor tune-up kit.
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