Well Norm,
You have to keep in mind, that these carbies do work just fine when new. And generally for a considerable time thereafter.
So to my mind, the major source of their problems is going to be the elastomer components, which are the most prone to deteriorate over time. Chief among these are o-rings [on the engine side, including the starter o-rings on PowerTorques] and diaphragms. And often difficult to diagnose, too.
A similar type of problem is with diaphragm replacements in motorcycle CV carbies. The best known one is when using Stromberg diaphragms in Bing carbies on older airhead BMW flat twins. Ya gotta do both at once, or you won't be able to balance them; the Stromberg ones are a bit 'stiffer'...
