It could be several things. I'd look down the throat of the carb, actuate the throttle, and make sure the accelerator pump shot is consistent, strong, and starts the instant you move the throttle. I'd do an ignition tune up (cap, rotor, plugs, and wires) if you haven't done so already and make sure you have a good spark at the end of that. Go through and check for vacuum leaks. You can do a visual inspection for lines that are cracked, disconnected, etc. If your base gasket wasn't replaced after the rebuild, you need to get a can of starting fluid and spray around the base to check for vacuum leaks. If the idle speed changes while you're performing this test, you know you have a vacuum leak. It should be a relatively slight but still easily noticeable change if there is one. Also check to make sure the EGR valve moves freely - remove the valve and clean the pintle. Check your timing also. You can try 10* initial, and just leave the distributor at stock curve unless something's not moving right in there or the springs have lost their tension, and then you'll need to do a recurve. It would probably be a good idea to get your total timing also to make sure that the advance mechanism in its entirety is sound. Finally, are you positive that the rebuild was done correctly? Not saying anything at all against your uncle, but Quadrajets can have minds of their own when something goes wrong.
