Hi Volvo Saint,
when I was trying to get a handle on the carbs on my '71 164 with B30A motor and thus twin Strombergs like yours I read in my Service manual (specific to my model) that the extra butterflies (secondary throttles) were meant to control the fuel mixture flow such that the intakes from the two carbs were shared/equalised and warmed by the manifold at low load. It's for emission control. Where you live you could possibly just remove those butterflies.
ie, The manifold has TWO cross connecting channels between the carb inputs. At low load the butterflies in the manifold (secondary throttles) are closed, which directs the air-fuel mix from BOTH carbs across the bridging channel to BOTH intakes. The point being to get the mixture a bit warmer, and uniform. See image attached.
When you accelerate, the secondary throttles open and the suction in each half of the manifold is such that it takes the mix directly into it's "own" three pots, and the sharing process stops.
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