Quoted From college man:
try tapping your IAC valve with the handle of a screw driver to see if the idle changes. if the idle does change then
replace the IACC8-)
There’s no idle air control valve on this beast. That truck probably has a Quadrajet carb on it. Those things are beastly when tuned right, but hard to get tuned right. You may try backing the idle metering screws out half a turn on each side and seeing if that helps at all. If not, a rebuild may be in order. I’m going to refer this to my buddy Yarddog who is our resident carb guy. Also, does that carb have the metering solenoid built into it? You will know because there will be a connector on top of the carb. It may be bad. Does the truck run fine otherwise? If so, you may just be dealing with the idle circuit of the carb. You may also want to try blasting it with some carb cleaner for kicks, in case there is some gunk keeping the throttle plates stuck open, which would be a vacuum leak in itself. These plates are at the bottom of the carb, under the venturis, not the butterflies on top of the carb. I would check this along with checking for other vacuum leaks as suggested.