What can be the cause of a 72 Chevy Impala stalling when idle?

I've been working on a 1972 Impala for awhile now. I just put on a new carb and reset the distributor cap after performing some other engine work with my father (not sure what all he did to the engine). Either way, it runs now BUT it will not stay on unless we constantly stay on the accelerator. Once the car idles, she stalls. He isn't sure what is wrong with it, and I have even less of a clue. What could be some causes for an old school car like this to stall when idling? I'd like to get her up and running so I can sell it asap.