Pulled a mechanical msd out of my 69 recently. Decided to go with the petronix hei with vacuum adv. (Had recently put one in my 68 without any issue's). Started right up with the new dist but on test drive had a pre ignition from hell. Tried retarding several times and many degrees without any change. Manually tested the advance... Never had this issue before. Low miles on a thirty over, nine and a half to one build. Only the best top shelf gas. Does fussing with the advance curve sound like the place to start? Maybe a lighter spring? I know you can adjust a mechanical dist with lighter and heavy springs but is that the case with vac adv dist's as well? Never had to adjust curve before. Maybe it doesn't like hei's and is just spoiled? Any advise or comments are very much appreciated.