The Holley 1940 power system consists of a power valve that sits at the bottom of the float bowl. A vacuum piston sits in the bowl top which is fed with vacuum that comes up from the throttle body.
At idle or low power, the intake vacuum is at it’s highest, which pulls the power piston up and the valve stem on the power valve also moves up shutting off the fuel flow from the jet. At higher speeds, the vacuum drops and so does the power piston, which presses down on the power valve and allows more fuel to flow from the float bowl into the main well.
The power valve comes in a set with the jet, stem and spring.
Be sure the stem is straight up and down. Also be sure the power piston move up and down freely with no catching.