It could be an "age thing". I have noticed that when fully warmed up, I tend to slow down, and I am not quite as old as your trucks (don't miss it much
).
Seriously, it would be useful to know which Carter and which Rochester you have. The service replacement Carter and some of the Rochesters have a fast idle circuit, such that when the choke is activated, even slightly, the curb idle is increased.
If you have either of these, and were engaging the choke, the idle speed would decrease when the choke is completely disengaged.
Jon.
