The vacuum line raises the fuel pressure up and down with the manifold pressure. It's there to keep a constant pressure difference on both sides of the injector. You take it off so that you have a constant to set the fuel pressure at.
Kurt
So even to adjust the fuel pressure regulator i should remove the vacuum line?
I adjusted the regulator with the engine running and without removing/pinching the vaccuum line. Pressure dropped fine.
Is this not advisable?