While you are turning the idle adjust cable.... can you feel resistance? It is possible to turn the cable out so many turns that it unscrews from the carbs. At that point the cable pops off, the retainer spring drops off and there's no way to increase or decrease the idle.
If that's not the case I'd agree that there's probably a vacuum leak. Under the carbs at the 90Ëš bent hose is a common leak; at both intake manifolds there's hoses that can leak. Remove the hose and cut off 1/4" - 1/2" and reinstall.
There are a hundred other possible vacuum leak areas but these are probably the most common.