[Kenth]

Originally Posted by srmmmm

I did discover I can close the idle mixture screws all the way and the engine still idles, so the APT must have been "adjusted" and the true possible cause of the remaining issues.

It is not entierly impossible that your engine runs on the off-idle slots when the idle needles are seated, or are they?

Maybe the springs are long enough to stop the needles before they hit bottom and closes off the holes?
Remove the springs and seat the needles, does it still runs?

And, adjusting the APT will do NOTHING for the idle needles position, the idle circuit is a separate stand alone circuit.

FWIW