I am not a veterain six tuner, but that problem often shows up for small block guys with the 20 degree vacuum canister on their HEI when they hook the vacuum advance to raw manifold vacuum and set their idle and ignition timing without realizing they should have set the timing with the vac advance disconnected. The problem is when they release the clutch and the vacuum falls, thier timing is gone and now very retarded under load. I am not sure whether or not this is possible on a six?
