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Motorcraft 4300 Power Piston
The power piston is made of brass (at least the piston itself is) and fits into the float bowl top. See #26 in the diagram. The purpose of the power piston is to supply extra fuel when you are in the power mode. At idle and low speeds, vacuum is at the highest level and pulls the power piston up and off of the power jet (closing it). The power jet is located in the bottom of the float bowl. As you gain power, vacuum starts to drop and the spring on the power piston rod forces the power piston down and pushes the pin on the power jet open. This allows more fuel to feed into the carburetor throat.