The black plastic idle air bypass (base speed adjustment) control is normally held only by the tension of a small spring and the sealing ring and should not be hard to turn. It is in a removable housing and the gasket is available. There may be a buildup of gunk around it and you could remove and soak the housing until it's free to turn. It's also possible that somebody cranked it all the way tight for a racing idle problem at some time. If the engine now idles with the bypass wire (near AMM) grounded or the IAC hose crimped closed, then the adjustment is OK and you don't really need to turn it right now. Save it for next service.