Since it's necessary to drain the float bowl by removing plugs in the bottom of the bowl before tinkering with the main metering jets, maybe you're referring to the idle mixture screws? Other than making the engine idle properly, they have no effect whatsoever on fuel mileage or performance. Adjustable main jets need some pretty sophisticated exhaust analysis equipment (and maybe a dyno) to set them properly, and when more than one barrel is involved, it really gets complicated. Jon's comment on having the proper manifold is spot on.
Jerry
