Chevrolet Repair: idle problem, Vacuum leak?, intake manifold gasket, vacuum gauge

Question
Hi, I have a 1984 el camino with a 305 v8.one today i was driving it and suddely the rpm decreased from 750 in gear to about 500. The car has been running rough for the pass two months and i belive its a vacuum leak.I ve changed the intake manifold gasket and i have replaced the carb gaskets,the two that are in the carb itself
and it still runs rough!When ever i put my hand over the choke throat, only when engine is warm, the rpm increases and the car sounds like the way it was before the exhaust sounds better too. I have checked all the main hoses and they were fine. Please give me another idea of were a vacuum leak can occur or if my problem is a vacuum leak.

Answer
Could be the vacuum advance in the distributor.  Check the vacuum line going to the distributor.  Also , if you choke the car out, but putting your hand over the carb, and the engine idle gets better, you definitely have a vacuum leak.  If it's that bad you should be able to hear it.  If it's not a vacuum leak, per say, it could be the idle air mixture, for the carb.  If you have a vacuum gauge, put it to one of the ports and see if you have enough vacuum, in the green on the gauge.  I will look into this further for more info, and hopefully come up with something
Jimmy