Good afternoon,
​I recently converted my Slant 6 from a 1BBL over to a 2BBL Super Six setup using a Carter BBD 8371S (date code 1822) that I rebuilt. The engine starts easily, idles smooth at ~650 RPM, and base mechanical timing is set to 10° BTDC with the vacuum line plugged.

​The issue: I went to hook up distributor vacuum advance to the small vacuum nipple pointing straight out toward the passenger side, but it is pulling full manifold vacuum (15–17 in-Hg) at idle. Allegedly this is the proper port for vacuum advance.

​Here is what I've tested so far:
​Backed the curb idle screw out to drop idle down to ~500 RPM (plates nearly choked shut), and the port still pulled steady 15 in-Hg.
​Vacuum drops when the throttle is cracked open/revved, confirming it is acting as direct manifold vacuum.
​Throttle plates were centered and verified to close cleanly against the bores without sticking.
​Idle mixture screws are balanced (~1.5 turns out).

​Given this is an '82 truck casting (8371S), does this specific model route manifold vacuum to that nipple for an ESA/emissions circuit rather than a true ported spark? If so, should I just cap it and run the advance canister on direct manifold vacuum, or is there an internal passage/gasket difference I should check?
​Insight is appreciated.

Show us a photo of exactly which port you're hooking the distributor hose to, please. A photo of your actual carb, not a photo or diagram grabbed from somewhere on the net.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Certainly, here are some photos of it, let me know if there's a specific angle/shot that works better. The port circled in white is the port I've been understood as a vacuum advance port.

That is indeed the intended place for the hose to the distributor. On '75 and later carbs, whether that nipple gets ported or manifold vacuum depends on the transmission type (automatic, manual) and emissions package (passenger car, light-duty truck, heavy-duty truck; Federal, California, Canada) the particular carburetor is for. Here is a report on what happened when I ran a ported-vs-manifold vacuum experiment, and this thread contains ideas on how to emulated ported vacuum with a carb not equipped for it.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.