OK Jason, So I'm ready to quit thinking idle circuit problem and go to your earlier thought of vacuum leak.
Your recorded reading of 16 (you said pounds but I think inches/mercury)is not good at all running with the choke or throttle closed---but spraying stuff around potential leak sites showed nothing leaking from outside air.Could the manifold be leaking from the heat box?--where the exhaust manifold and intake manifold are bolted together.There would have to be a crack or perforation inside but I have seen some terrible deterioration in old tractor and stationary engine manifolds.the combination of time and corrosive deposits can make metal go away.
Do you have access to another manifold even a downdraft for a temporary run?
When the valve cover was off did you shake the inlet valves in their guide or visually inspect for a guide terribly loose or missing?
