Sorry Will, but the regulator does not set fuel pressure. Fuel pressure is set entirely by the pressure line to the tank through the check valve. The regulator is just that, a fuel flow regulator. It is a demand regulator, somewhat like the Cline regulator. But in the YS, crankcase pressure and carburetor vacuum act together to open the regulator valve. Turning the screw in (clockwise) increases the spring pressure on the regulator valve, requiring more rpm (crankcase pressure) to open the valve, thus leaning the midrange mixture. Turning the screw out (counterclockwise) decreases the spring pre-load, letting more fuel flow at lower rpm, thus richening the mixture.

I had a very similar problem with my YS 63. Dave sent me a regulator plunger with the small head. It comes with a new silicon valve seat. Once installed, all is cured.

Maybe Dave will chime in here.