Like I said, there are only 2 jet sizes used on Autolite carbs, including the 2 bbls, so I highly doubt that is the problem. The 2 jets aren't that different in orifice sizes, so I can't see it making that big of a difference in mileage. Unless of course, since you said that the carb has been messed with, somebody already drilled and tapped them, then threw in a jet that was way off. You can check by trying to thread in a standard jet. If it threads in, it's been messed with. If they won't thread in, it has factory jets in it. Often times people forget to put in the check ball for the secondaries, and if they are opening to quickly, that can kill fuel mileage.
If you really want to 'tune' the carb, you'll have to drill and tap them for standard thread sizes. If it's a stock engine, that typically won't be needed as the 2 factory jets will usually get you where you need to be. If for performance use, it's a must.