The carb has never been rebuilt and kind of oozes a little gas around the base,maybe that should be my next project
This sounds like a worthwhile project.
It just seems like I'm getting less than 15mpg.
As stated above, 15 mpg it not bad. However, besides your mpg being a guess, mpg is dependent upon many factors (types of roads and how fast you accelerate and drive).
I've never noticed hesitation with my setup.
It is ain't broke, don't look for reasons to fix it.
All that being said, your set-up (a non-heated intake manifold) is ((as you very nicely described) not at all like an original set-up with an exhaust manifold physically attached to the intake manifold (with or without a working heat-riser). In the "natural" set-up, heated exhaust gases are deflected to warm the intake for a reason. And, that extra heat is not needed after warm-up BUT there is still a significant amount of heat being transferred to the intake manifold, even after the heat-riser is closed.
