Rusty nut

My 1985 celebrity wagon with 2.8 liter carburated engine with Rochester vari-jet has been running just fine. But this past Saturday it began to not idol with out stalling, Engine hot or cold. Acceleration is excellent. I have not 'fiddled' with the idol screw. Engine has been running just fine, prior to this, I haven't had to lift the hood for months.
There is no back firing.
I see no arcing of the ignition wires when viewed at night.
My inductive timing light indicated the plug is firing when i attached it to each plug wire individually.

What could be the culprit? and how do I check it?