Steve,I assume you have a model B carb, generally the main jet has little effect on part throttle drivability unless the jet is really plugged up.Is your intake manifold heated by the exhaust and does it work properly? A Rochester B on a cold intake manifold is hesitation prone.The power valve is normally in the enriched position,until engine vacuum closes it,the little piston moves freely?The small tight wound spring is present?
Also look down the carb throat, there is a round spray bar running across the diameter. The middle of the bar has one or two small air bleed holes in the top. take a strand of stranded wire, like from 16 guage and push on end into one of the holes. This will richen the part throttle response. See if runs better or worse.Sounds like you have cleaned the carb well, no possibility of crud in that spray bar?
Of course your problem may not be the carb so....
