A vacuum operated power system is used in the carburetor to provide additional fuel for high speed operation or increased road load. A direct manifold vacuum passage within the carburetor to the engine intake manifold connects to the power piston. Under heavy engine load, the manifold vacuum drops, decreasing the vacuum pull on the power piston and the piston is forced downward by a spring above the power piston. The power piston spring is calibrated to force the power piston downward at a given manifold vacuum (approx. 9"Hg).
The downward motion of the power piston unseats the spring loaded ball in the power valve assembly. Fuel passes around the ball in the base of the main well support. A calibrated power restriction meters the fuel prior to joining the fuel from the main metering jet. As the manifold vacuum rises above a specific point, the power piston is drawn to the up position and the spring loaded ball of the power valve closes. There is no adjustment required for the power system.
The vacuum break passage which is drilled from the bore of the air horn to the power piston chamber serves to relieve any vacuum build-up around the piston. This vacuum, if unrelieved, will draw vapors from the float bowl past the piston and down the vacuum passage into the manifold, resulting in an overly rich mixture.
Power valve plug, spring, and checkball are illustrated in this image, parts 14, 15, 16
When a 1 barrel carburetor sits for any length of time (years), the power piston will most likely end up frozen in the carburetor. Penetration oil probably won't get it out, but heat will.
Clamp the carburetor in a vise, apply heat to the ouside of the power piston well. As you heat gently wiggle the power piston. Once you get the carburetor hot enough, the piston will start to move and you should them be able to pull it out.
Buff the power piston which is brass to get the dirt and grime off. Polish the inside of the piston well with crocus cloth. You can also use silicon spray lubricant and a brush to clean the well. The point here is that the power piston should move in and out freely. Only vacuum pulls the piston in.
