You might check these things.
Have the throttle body clean and the plate adjusted as tightly closed as possible.
Check the operation on the throttle switch adjustment as it has to tell the ECU that you want to come off idle and pull away. Look at the general condition and for corrosion in the connector to the switch.
The IAC has to close while setting BASIC idle in order for it to operate in the middle range of air control. This applies if you have a black knob on your throttle body for setting bypass air. The IAC shuts on these 86-year cars by grounding a white wire with the blue stripe located in an open-ended connector above the AMM. Leave the other wire in it alone.
Phil
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