WHY low vacuum on freshly rebuilt motor?!

Yes to the adjustable rockers. The carb was taken off and put on a 396 Chevy. Vacum 15 inches ran like a sewing machine. I have ran the motor with vacvum advance. Also put one of those orange revinator box'e with no advance hook up. They insist they run better with no advance . I will try cranking up the initial and get it as high as I can on the starter. What seems to be weierd is the motor idles fairly smoooth cold and when it warms up with the choke open it starts to run very rough. Have been through coils /dist/ ignition boxes ballast/ wires / differnt carbs etc. Still the same . Its probably the cam timing . Although the guy that assembled the motor runs mopar at the track for 30 m+ years. This i\s why I took the motor initially to him.