Glad to hear that the ignition timing made a change. If it were me and this was helping to fix the problem, then go with it. Put more advance in and don't worry about the vacuum advance. Listen for the knocking and keep advancing to see if it eliminates the your popping. As far as the 47 degree figure, that would be with total timing INCLUDING vacuum advance and you would only see that at light cruise. I would shoot for the high 30's with your light, with the vacuum advance REMOVED, and high enough rpm for the all of your mechanical advance to be in. I will also agree that a slightly out of tune shouldn't cause your problem, but what if your recurved dizzy has a problem that amplifies an out of tune issue. Anyway, advance away and keep an hear out for knock.