It has been my observation over the years that problems attributed to aftermarket diaphragms seem to vanish when OEM ones are fitted AND the more careful rebuild to fit them cures the problem that caused the poor running in the first place.
Yes the non-genuine diaphragms tend to be thicker than genuine - so what?
Vacuum is a powerful thing and the only difference a slightly less supple diaphragm will do is slow the carburettor's reaction to a change in vacuum by a small fraction of a second.
Read the above sentence again - it is important.
Even if there is a difference in the suppleness of a pair of diaphragms they will still ultimately rise of fall the same amount, only the time to do so will vary, by an even smaller fraction of a second than discussed above.
Why do I say they will still rise or fall the same amount? Flex a diaphragm in your fingers and feel how little resistance it has. Then compare that to the power available both by vacuum and in the spring. The diaphragm component is as irrelevant as the structural strength of a chicken egg about to be run over by a road roller.
Now in one sense I may not know what I am talking about, you see since about 1980 when I replaced my first Bing diaphragms (40mm Bings to which I fitted Stromberg diaphragms) I have NEVER fitted a genuine diaphragm.
Yet strangely my BMWs have always run correctly.
Worldwide are only one potential problem. The carbon that builds up on a slide can cause hesitancy in slide movement. I have spent a lot of time with carb cleaner and polish ensuring that slides move with as little resistance as possible.
In tuning carbs I balance at 3000rpm and leave sufficient slack in the cables so that idle can be set up entirely separately.
If there is a big vacuum discrepancy off idle I spend the time to find out why, without immediately blaming diaphragms.
On my R65 one carb is slower to react to a snapped open throttle than the other, albeit that they equalize in a fraction of a second
I can see it on the carb sticks, but I can't feel it when riding - so I ignore the hell out of it :-)
The cause by the way is that one slide is ever so slightly outfit round (measured with a micrometre) and it leaks vacuum.
For what it is worth, other potential causes include inlet trumpet leaks, uneven valve sealing, incorrectly ground cam lobes (more prevalent than you think - they were churned out by machine and assembled by unskilled guest workers after all).
There are lots of other things to worry about than poor bloody diaphragms
