As to BMW Bing carbs, there are NO 28 mm carbs, never were on classic fifties or sixties types, Earles or plunger rear end, non concentric floats. The touring types had 24mm max. and the sports types like R 50 S , R 68 and R 69 +S had 26 mm . The maximum Bing got out of the small cases was 27.5 mm. Otherwise you´d need a bigger case and bigger slides so there will be some good fit and no air leaking past the slide. There is a bigger type of 28, 30 and 32 mm Bings, non concentric, like in my photo below on a 460cc Horex, but this series was never sold on production bikes.
The /5 series had a new type of Bings, concentric with rubber membrane slide and 32mm on the 750 and simple Bowden operated slide on the 500 and 600 types, 26mm I think.
Vic
R 69 S with Kayser alu cylinders 734 cc:
32mm big Bing on Horex 460cc: