Why do you think you need lower vacuum than you already have on the vacuum port? What is wrong with the way it runs, now?
I've read that on the spark port, (not intake vacuum) if you are pulling vacuum on the gauge, than you're not completely in the idle circuit on the center carb. As I close the throttle blades on the center with the idle adjust screw, the vacuum on the spark port continues to decrease.
Once I have the engine idling where I want it with the spark port vacuum reading at zero, then I'll know I'm idling on the carbs idle circuit.
