https://www.mginfo.co.uk/upgrades4mgs/Engines/tuning_the_su_carbs.html
An excellent article that I am following to modify and tune a spare pair of hif4 carbs I have that have the vac port on them rather than on the inlet port – which are quite rare. Read the article as it explains the benefit of these carbs. Earlier hs2 carbs ( on the chrome bumper cars) already have the benefit of the carb mounted vac port for the distributor advance.
MGB tips mods and maintenance 