If you're running a stock cam, 10-13 inches of vac sounds low to me. most stock or nearly stock motors I've measured will idle at 18-20 inches. The 383 I'm running now has a Comp 262XE cam(218 degrees @.050 duration) and it idles at 18 inches. I always run straight manifold vac. on the advance. I think you MAY have a manifold leak.