Ive noticed on my saito fg19r3 to get a nice idle you have to lean the low speed a fair bit but once you get to mid range its lean..so to have good mid range its stuck with rich idle ..maybe bert designed those holes to lean the idle mix then once up in mid range velocity overcomes those small holes ..just an idea,could fit a fuel nipple to that manifold and fit a needle vave from a os 46la or similar to adjust the air bleed ..would act similar to an airbleed maybe..
im not a professional..its just a thought!!