Ignition Help

My best guess is that when you run an MSD ignition with vacuum advance, the rotor to cap indexing is critical because the Msd will fire the spark plug through 20 crank degrees of rotation .When the timing is advanced by the mechanical advance the rotor to cap phasing doesn't change..the reluctor moves with the rotor, with the vacuum advance the pick up coil moves ,this changes the rotor to cap indexing .Probably as much as 15 degrees ( full vacuum advance) + the MSD's 20 crankshaft degrees (at lower RPM's)=35 crank degrees x 1/2=17.5 rotor degrees.Since the 8 plug wires are 45 degrees apart it becomes possible to have the rotor pointing in between to two terminals on the cap when the coil tries to fire the plug (worst case scenerio)Since you didn't mention any mis-fire problems I'll assume you're probably just getting excessive rotor tip to cap terminal clearance under vacuum advance.Ohm's law says...Voltage drops ...Amps go up...up enough to melt Carbon (5000 degrees F) and Since FREEDOM DIED on Jan.20,2009 the only FIREWORKS goin on this 4th of JULY will be under your distributor cap!!!!!