High idle then dies

I fixed my ignition issue but now I cant get a solid idle. it dies if I set it too low but When I set it higher It goes to 1500 RPM right off of start then slowly drops until it dies. The carb is a vacuum secondary 650 cfm holley on a 318 magnum. I have tried adjusting both the idle mixture screws, idle screw, and timing in different combinations to no avail. If its a vacuum leak how do I find it? any other possibilities?