vaccuum leak?

Hello , We have had the same issue with our 66 Dart /6. After running all the checks i.e. leaks,timing,valves and alot of head scratching we dicovered it was the idle circuit in carb and accel pump adjustment. Not to say this is fixall but it is a tough problem to diagnois and can lead in many different directions. We rebuilt carb , put new filter and the Dart runs Fantastic!. Great idle and no more big drop in RPM when put into gear. Hope this helps and good luck.