The 4300 power valve circuit includes a vacuum piston and a power jet and is used to supply the extra gas needed when powered up.
The vacuum piston is spring loaded and is forced down by the spring. The power jet is also spring loaded and tends to push up. The fuel is shut off at this point.
Vacuum is fed from the intake up through the carburetor and into the vacuum piston chamber. At idle, the vacuum is at the highest point and the piston is pulled up. As RPM is increased vacuum to the piston drops and the piston pushes down on the power jet, which opens up the flow of gas. This gas run through the power jet passage and mixes with the main well fuel.
4300 Power Jet
The power jet is removed using a 3/8" deep socket. The power piston is removed by scratching away any mushroomed metal around the surface of the piston, then using your finger flick the stem so that it bounces up. It will eventually break through any mushroomed metal and pop out of the hole. For frozen pistons you can apply some heat to the outside of the piston area. Be careful with the stem. They are easily broken and they are not available.
The vacuum piston needs to be free of binding.
Watch a video about the 4300 Power Jet: