12 years late, but I have a likely answer! The reason carbs need to boost richness when the throttle is suddenly opened, seems widely misunderstood. With closed throttle and low manifold pressure, most/all the fuel is vapourised. When you open the throttle, the pressure rise causes fuel to condense onto the manifold, briefly starving the engine of fuel. Hence a need to enrich momentarily. The more cylinders a carb supplies, the greater this need. AC's carbs only supply 2 cylinders each, so only a small amount of carb damping is needed. Fuel quantity through an SU jet depends on both pressure and needle position, so although the piston/needle are kept low for a moment, the fuel flow is relatively high due to the pressure above the jet. There is some damping in these early carbs, as air is forced through the tiny hole in the cap.
