Great info. Thank you, Tony.
This runs contrary to the "standard" information. I offer the Jay Pineau / Bob Tindel article in PP's tech forums. See the 7th paragraph - quote from "The Bruce" no less.
Pelican Technical Article: 911 Warm-Up Regulator Adjustability
So I do some more sniffing on Google and find this tasty treat - I bet this is your method.
See video at 1:30 in:
k-jetronic adjusting warm-up valve, system & control pressure on golf mk2 gti - YouTube
Not arguing but I have to believe that bumping the plug has to impact warm C pressure to a degree? Or is it absolutely stand alone for cold control pressure?