With a car that old, there could be many issues but more than likely it will be easy to find and fix due to the relative simplicity of the engine. Examine the vacuum lines and insure none are split and all are properly connected. Next would be a check on the ignition system - cleaning and re-gapping or replacing plugs, inspecting or replacing plug wires, dist cap, rotor, and ignition points. If that checks out, timing setting would be the next check. After that, you will need to look into the fuel delivery through the carb. even through you said it was recently rebuilt, something could still be the cause of the rough idle such as the fuel jets. Sorry it is not a quick process but on that old Falcon, everything should be pretty easy to access and correct. Good luck.
1 people found this helpful.