Power valve adjustment screw is covered by Soft lead plug easily removed with small screwdriver

Under that led plug is a 1/8 Allen head screw

Screw is held tight by some sort of Loctite heat will be required to loosen up the Loctite 15-20 seconds usually does it/ but if doesn't turn really easy , more heatapply more heat without heat it will strip.

Removal of that led plug will not result in a vacuum leak

In your tuners notebook keep track of the direction and how many turns you adjusted that screw so you can get back to where you started. As old as that spring is by now it must have lost at least some tension.

it's a taper type arrangement.

Fits in this brass piece

White is spring ,green is Allen head screw red is Allen wrench. Turning clockwise should increase tension on Spring

Weak spring and or stuck piston could lead to slow or non activation of power valve