My 340 getting 6 mpg

I see the ignorance of Newbomb Turk rises to the top in post #107 like cream rising to the top.
Larger main air bleeds will lean the mixture, not richen it. Same dope blames the carb he has on the dyno because he does not have the jetting parts on hand to jet it. It's the carb's fault!
As I said earlier in this thread, I believe the carb being used has some fundamental internal problem & a different carb should be tried if possible.
Below on air bleed function is from the Holley book, by Holley Engineer Mike Urich

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