To obtain the proper mixtures required for maximum engine power under heavy loads a vacuum operated power system is used.
The power system is located in the primary side of the carburetor. A vacuum channel from the top of the power piston is exposed to manifold vacuum beneath the throttle valves. The vacuum in this channel varies directly with manifold vacuum which is normally high in the idle and main metering ranges. The vacuum is sufficient to hold the power piston in the up position against the force of a calibrated spring. However, as the throttle valves are opened, the vacuum drops.
When the manifold vacuum drops below approximately 9"Hg, the calibrated power piston spring forces the piston down. When the piston drops down, it unseats the spring loaded power valve. This permits additional fuel to flow from the float bowl through the calibrated power restrictions and into the main well.
The additional fuel supplements that already flowing through the main metering jets (on the primary side) providing a richer mixture for power demands. This power mixture continues to be supplied as long as the manifold vacuum remains below approximately 9"Hg. When the manifold vacuum again increases sufficiently, the force of the power piston spring is overcome and the piston is drawn up, returning the carburetor to normal mixtures.
The power piston cavity in the carburetor air horn is connected to the air horn bore by a vacuum break hole. The purpose of this hole is to prevent the transfer of vacuum acting on the power piston from also acting on the top of the fuel in the float bowl. Any additional vacuum acting on the fuel in the float bowl would affect carburetor calibration.
It is also in this range that the secondary side of the carburetor provides additional air and fuel to the engine for increase power. For high speed or power operation, the throttle linkage engages the secondary throttle valves and opens them completely in the remaining few degrees of primary throttle travel. In this range, manifold vacuum acting on the secondary side of the carburetor is multiplied at the main and boost venturi and draws fuel from the float bowl through the calibrated main metering jets into the main wells. The fuel then passes through the main well tubes and is bled in a manner similar to that described previously in the operation of the primary main well air bleeds. It is then drawn to the tips of the main well tunes (nozzles) and passes through the mixture passage to the boost venturi and is discharged into the intake manifold. The lower idle air bleeds (where used) also supply fuel throughout the power range in a manner similar to that described under the main metering system operation.
The auxiliary valves provide a means for controlling secondary bore openings according to air velocity at wide open throttle. During the period in which the secondary throttle valves are opened and air flow is not high enough in the secondary bores to open the auxiliary valves, additional fuel is needed for the air which by-passes around the auxiliary valves. This additional fuel is supplied by down tubes which extend from the mixture channel in the venturi cluster arm or bowl, to the low pressure point below the closed auxiliary valves.
When the air flow is high enough to open the auxiliary valves, the down tubes no longer feed fuel as the low pressure point is now in the boost venturi. With this feature, the correct air/fuel mixture can be supplied at any point during secondary throttle valve operation.
The auxiliary valves are normally held closed by a calibration spring. The tension of the spring is set so that the valves will open, only when the engine demands more air and fuel for power operation.
The auxiliary valves are factory calibrated and cannot be adjusted in the field.
Warning! the power piston stem is easily broken.
The power piston is held in the well by staking the edge of the hole.
Start with an awl and scrape around the piston circle to remove excess metal.
Press down on the stem and then let it spring back up. The piston will bang against the staking.
Do this over and over until the Piston comes out.
If the piston is frozen, heat the outside area. This will almost always break it loose. If that doesn't do it, then discard the carburetor as it is probably too corroded.