The metering rod spring hanger (the long arm the rods go on) has a sngle piston in the center. The screw head you see is the top of a long rod that goes downt o the bottom of the baseplate. The lever in the baseplate works like apower valve t force the rods out of the jet, richening the full hrottle mixture to the max the jet and rod will flow whetehr there is vacuum or not. At all other times and throttle openings, the rod hanger is controlled by engine vacuum operating against the step up spring, which is under the piston on the rod hanger. If you engine is not stock, my impression is the factory spring is incorrect and pushing the rods to the rich end of the spectrum all the time. You can adjust that center screw all you want, if the spring isn't properly matched to the combo, you'll be either rich or lean all the time except full throttle.