Return springs were weakening over time
Funny thing had apparently been happening over some period of time...Probably a couple of years. I noticed last week that when I let off the gas the car seemed to rev just a little higher than usual. When I got home, I checked the carb with the car idling, and noticed that I could close the throttle a little bit more by pushing it shut. I also noticed there was zero tension on the springs when the throttle was closed.
I shortened the 2 return springs by bending the ends and now the throttle closes all the way.
The idle had not been closing all the way and probably hadn't in a long time..... So much so, that I had to turn the idle speed screw over a half turn to get the idle back where it was! The springs were probably marginal to begin with and then weakened over time. Then I'm just obliviously tweaking the idle up to compensate over time.
The morale of the story is don't be like me. Check your throttle return springs once in a while!
72, SB 385, Voodoo roller, 219/227; .515/,530 lift; AFR 180cc Eliminators, RPM Air-Gap, 770 Avenger, 12 bolt 3.42's, Eaton Posi, Pypes 2 1/2" X-pipe system w/Race Pro's
Vehicle weight: 3390 lbs