Trying to tune my 383, but can't figure this out.

It has always run fairly smoothly, good enough anyway, but I wanted to see if I could get it running smoother.

Carb is a Carter AFB Competition Series 9755 S.

I used a vacuum gauge - on the driver side port - to set the adjustment screws to 1.5 turns. That port by the way, has always been capped off.
You can see the short tube with the screw plugging it in the attached pic, on the left side.

The passenger side port is the one that has always been connected to the vac advance. When I hooked the vac gauge up to that port, I only got about 5 psi. Vac gauge connected to the driver side port gives me 20 psi.

Then I set timing, with vac advance disconnected, at 15 degrees advanced.

If I leave the vacuum advance disconnected, rev the RPMs up to 3500, the engine runs smooth and timing advances only to about 30-35 degrees (hard to estimate - I just have an old-fashioned Craftsman timing light, not a dialback).

But if I connect the vac advance, engine runs very badly as I increase the RPMs, and advances to around 45 BTDC.

Thanks in advance.