Did you ever figure out the problem? It could be a rotor phasing issue. I had that problem a long time ago with my first stroker motor which is why I started to make reluctors with the different offsets. When I checked my rotor alignment the rotor was pointed somewhat close to the right terminal without the vacuum advance hooked up, but once the vacuum advance kicked in the rotor swung far enough over that the spark could jump to the wrong terminal.
Just for the record, mechanical advance does not change the position of the rotor in relationship to the cap. Vacuum advance does since the vacuum can pulls the entire mechanism.
On my stroker motor there was enough intake manifold vacuum that at low throttle or on decel the can would fully advance and the rotor would move enough to cause mis-fire. I fixed it by splitting the difference on the reluctor. I spaced the rotor so it would be a little past the terminal with zero vacuum and then a little ahead of the terminal under full vacuum. That simple fix solved the problem.
I showed my fix to Eberg and he started selling the reluctors to others. I think he still sells them.
andy, thanks for asking.
i don't know yet if i have solved it, car has been out of commission for the last week or so doing some other work.
what i found was some carbon streaking on the rotor where it contacts the cap at the center. i could also see that only the very edge of one side of the rotor tip showed spark traces, and when i aligned it up with the cap, sure enough it was not aligned with the cap terminal on that side. i used one of the offset reluctors to line it up, and it runs fine, but i haven't had a chance to play with it yet. i want to drill a hole in a cap and make sure that it is in line.
interesting about the vacuum advance though, as mine seems to be misfiring under load, although come to think of it, i'm not sure just how much load the engine is under when doing a burnout. high rpm, but doesn't take that much to spin the wheels.
