It should be possible to tune the power valve response to kick in the full-rich point to a particular manifold vacuum reading by altering the length and/or tension of the spring. A weaker spring will allow the mixture to stay lean longer by restricting fuel flow to the main jet only at a lower manifold vacuum. Once the vacuum drops below the point where it can hold the piston in the up (lean) position, then the piston will drop and force the power valve ball off its seat. That allows fuel to bypass the main jet and put the carb into its "full rich" mode. Stronger spring = richer mixture at higher manifold vacuum.
Jerry

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