not sure about that, back in the day they just stuck the tube in the pipe, no rollers to test it at speed. it was always high co or something like that. those screws always made it pass. then id just open em back up where they were.
being your failing under power the carb maybe f ed up and need a rebuild.
i always liked the Q jet too. nice small primary flaps, ( great for normal driving) then the huge secondary's that just dump fuel in the intake lol and rebuild rather easy too.