low vacuum problem?

[PeteF]

UPDATE: low vacuum problem solved. My trusted local mechanic ran a smoke leak test and found a bad vac advance diaphragm. He then disconnected the vac advance and retuned the engine to run on centrifugal advance only (until I can replace the vac canister). He said he increased the torque on the intake manifold bolts slightly, and raised the carb float levels slightly, and set the initial timing at 12 degrees. Then adjusted the idle mixture screws. The engine now idles smoothly at 15 inches of vacuum and runs great. I am amazed and baffled, because I could never get more than 13 in-Hg vacuum, and I tried increasing the timing with the advance disconnected, and got no improvement in vacuum. I don't yet understand how increasing the float levels could have helped the vacuum, but that's the only thing he did beyond what I had already tried. He told me that he raced Boss 302s in the 70's, so he knows how to tune Fords. He also recommended running the vac advance to the ported/timed vac port above the throttle plates. I have always believed that manifold vacuum was the best source for pre-emission engines. He said for best performance you want advance to come with rpm, and to make sure the total timing does not exceed about 38 - 40 degrees. (If you Google this subject, you get mixed reviews of ported vs manifold vacuum advice). I may just leave it centrifugal only, and give up some mileage efficiency. I had a dual point no vacuum distributor on my '66 Shelby years ago, and it ran fine. I'll see what kind of mileage I get, and decide from there. Thanks for all the help and comments. Maybe this experience will help someone else someday.