UK Triumphs used Stromberg CV carbs, not Zeniths. But yes, a split diaphragm means idle rpm only because there is no direct connection between the throttle butterfly and the actual airvalve. I preferred the SU due to the lack of the diaphragm and still have a car using one of those carbs and three spares in the garage, for other projects.
The SU will be replaced by fuel injection system very soon, I'm working on that project right now.
I've still got a spare Stromberg diaphragm in my cupboard from a Hillman Hunter that I sold in early 1982! Probably a bit crispy by now
One of my favourite words,
diaphragm.
