Adjustable main jets are available for Holley 94 carburators (The 3 bolt 2bbl flathead kind). They are adaptable to a 1bbl manifold. You can order them from
http://www.vintagespeed.com/ I have a set, but I have not tried them yet. They are an adjustable needle valve that pokes into the main jets. The adjustment screws stick right out the side of the carburator so you can futz with them without even taking off the air cleaner.
