I have a 360 with near stock cam and MSD 6a box. Engine has good vacuum at idle. I have been having some ignition issues with it. First it would not spark one day when trying to leave. Each stage of my problems I tested the MSD box and it seemed to do what MSD said it should. It apparently turned out to be the reluctor wheel was corroded causing the pickup coil not to break the signal, it also had some plastic parts fall out of the distributor while I had it removed so I ordered a distributor to replace it with. Cleaned the reluctor wheel up and ran fine for a while. This vehicle is kind of neglected so I kept driving it with off and on hard starting. Finally one day it would not start. Tinkered with it and finally got it to start and made it home. Finally (about a year after I ordered it)I installed the new distributor along with a new Accel coil, wires and new plugs. Cranked the engine and set the initial timing to about 8 deg. revved up the engine with the vacuum advance off and sounded good. Hooked up the vacuum advance to the ported vacuum and gave it some throttle and it spit and sputtered and backfired. Timing seems to be all over the place. I don't have a good timing light so I used a paint marker on the balancer to get a relative idea of where the timing was going. I painted marks that should have been close to 30 deg and with the vacuum advance it would go to a point that I couldn't see them anymore. Take the vacuum advance off and it seems to run fine. Hook the vacuum advance to full manifold vacuum and spits and sputters. After this long paragraph I am now starting to wonder is it something else that is causing the surging and backfiring or do I still have some sort of ignition problem?