Mike,
You have the 0-80457-S, correct?
The fast idle cam is adjusted by a setscrew behind the choke coil.
I will say the little plastic cam
could be sticking.
And it might not be due to the adjustment at all.
(If it was working before, use carb cleaner with the straw, work it back and forth, then flush it again)
Don't mess with the adjustment, if it used to be okay.
The 4180 that came on your rig had a little metal tab that had to be bent. (Fed emissions 'tamper-proof' mandate)
Holley has the 4160 instruction manual online if you don't have them with you.
Enjoy your trip!
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.