84 Chevy K10 idling high

Sounds like a vacuum leak. When you open the throttle blade, the condition disappears because the engine is ingesting the correct fuel/air mix for the position of the throttle blades and it overcomes the amount of air the leak is letting in. Check top end mating surfaces. You had the carb off, check the gasket underneath, stuff like that. Intake manifold gaskets leak all the time especially when they get old. Check your vacuum hose connections and check the ends for cracks, make sure they're routed to the right place etc. Spray carb cleaner/solvent around all these places while the engine is running (cautiously!), see if the idle changes. If/when it does, you've found a leak source. Might be more than one so be thorough.