1983 240 idles at about 1250 rmp.
I can hear the idle switch open and close (click).
I have swapped the Idle Switch and the Idle Control Valve with
good working units.
I can feel an strong suction on the lower hose to the ICV, both
while at Idle and not at Idle, by turning the throttle.
I have cleaned out the throttle body, was not very dirty.
The idle screw and its nut is set low and the throttle plate is barely making contact.
I have attempted to ground the blue/white wire and adjust the fat plastic
idle screw at the lower area of the throttle, I have uses a hex to
tighten it up but am afraid to damage it if I tighten any more, it is
almost all the way in.
The a/c / heater vents open and close strongly, this leads me to
believe that my problem is not an air leak.
I replaced the engine harness four years ago.

I don't know what to check next? AMM? Oxygen Sensor?

Help,
Joseph in New Mexico