Re: Vacuum Reading / carb adjustments
If you are using the tranny vac side that, at least on mine, is the smaller 1/8" size vs the other side of the "T" fitting feeding the brake booster. If your tranny vac side is as large a port as brake boster use reducer fitting into the vac gauge
Typical low vac would be a leak somewhere but you noted you don't have a vac leak. I use a can of spray starter fluid to see if idle increases around manifold/carb
FWIW my rebuilt 327 has about 19". Camed motors would have less. Had same vacuum with the Rodchester 2 brl as it does now with the Edelbrock 600 I put on
IDK if the Quad, for whatever reason, inherently knocks vacuum down to 5-10 (not seeing how it would without a big vac leak) your vacuum seems quite low to me
What is your timing at idle?