Re: Lets Talk Pre-Ignition
What about EGR? I would guess that EEC IV was controlling the EGR valve. Is anything controlling it now, or is it permanently closed? No EGR on an engine that was calibrated with it will quite often ping if it's disconnected. If EGR left disconnected, other changes would have to cover for the lack of EGR effect (EGR "desired" effect is to reduce peak combustion temp in the cylinder, therefore reducing those "harmful" oxides of Nitrogen that are formed by efficient combustion). EGR actually reduces peak combustion temp by diluting mixture with hot exhaust gases (yup, that's right!). Cars of the 70's you could usually disconnect and plug the vac hose to the EGR valve and improve performance. They were just "tacked on" systems then... but into the 80's they became integrated into the whole engine calibration and operation. Seems like I remember something about changes in cam overlap later to accomodate EGR as systems progressed. So all that could be a source of ping.
Distributor vac advance. You mentioned connecting it to the "base" of the carb. Vac advance usually used "ported" vacuum. Ported vacuum is low/none when throttle is closed, increases the more the throttle is opened. It's a vac port below, but close to the venturi. A vac port at the base of a carb is usually manifold vacuum, as it's well below the throttle plates and close to the intake manifold. That vac will be strong when throttle is closed, lessen when throttle opens further. Connecting the vac advance to manifold vac gives FULL vac advance over a lot of the load range. I was able to do just that hook-up on some cars of the early 70's, but they were simple systems, and could take full vac advance without pinging.
Just some thoughts. Don't know if any are your particular problem. UncleDon