Gentlemen;
The fuel mileage he's getting (12 city 23 hwy) is 'normal' that carb etc.
No matter what the manifold or CFM numbers are, there "out of the picture" at idle as the only air allowed past the closed throttle plate is determin by the adjustment (speed) screw. The only fuel allowed in is determin by the mixture screws.
The fact that it idles fine for a "few minutes" tells me the adjustment is okay and would indicate a internal fuel leak or a modification as mentioned.
As far as the intermediate (accelerator circuit) is concerned, that 'set up' is just overkill for that size engine. Remember; GM put a small 1 bbl carb there.
I would suggest a faster/leaner idle to help things out. The other is part of owning a Hot Rod.
Good luck.
