OK, so I've been musing on the aftermarket diaphragms since last night. Here are my thoughts about how diaphragm stiffness can be a problem for some people (manufacturing consistency?) in our Constant Velocity carburetors.
If the slide is held down (stiffer spring/diaphragm, more weight, etc.), but air demand is increased with an open throttle, then venturi speed will increase and pull more fuel from an equally open main jet in an accelerator pump fashion and probably richen the mixture a bit too.
However, it is not just the mixture that we have to worry about, but the air volume that makes it through the carb. I think this is the root of the problem that some people have had with the stiffer diaphragms.
A low slide will restrict how much air can get into the engine. While the air passing through the venturi can be sped up to some degree, there is a limit to how much this is possible (otherwise we could feed our airboxes through high velocity pinholes and we wouldn't have a variable venturi in our carbs). It becomes a problem of fluid (air) drag. For the sake of simplicity, consider the venturi cross section a perfect circle. As the radius increases the drag (circumference) increases linearly, while the volume is can pass (area) increases exponentially. A slide that is too low can be forced to breathe more via higher velocity, but it breathes less efficiently (and probably is more turbulent too) and there is a limit to how much it will let by since the engine can only pull so hard.
If the diaphragm stiffness (or any of the other variables) goes past the tipping point, the engine will not get enough air through the venturi bottleneck, no matter the mixture.
The automotive engines with the Stomberg carbs are pulling a whole lot more displacement through the carbs, and produce manifold vacuum more evenly (2-4 cylinders per carb), and with a greater manifold air volume to buffer the manifold vacuum level. I think this is why a stiffer diaphragm works fine on those bigger engines, whereas the BMW boxer pulls a small amount of air through a small manifold at 1/2 to 1/4 the number of intake strokes per revolution, and thus has less available low pressure to keep the slide lifted by the time the next intake stroke comes around.
