It seems like it's definitely running leaner as the tank level drops. Just for curiousity, run it at the 12,500 with a full tank but then try closing the throttle slowly from full open and see how far it closes before the revs drop a little. Leave the throttle at this setting for the rest of the tank and see what rev increase you get. If there's not much increase then I'd say the carb is too big for the revs you're running. Closing the barrel slightly simulates having a smaller carb that has better fuel draw.