O God, here I go again with my simple diagnostics that everyone seems not to like.
Back the distributor off (clockwise) about a quarter inch and try it.
If it's better or gone, then you know exactly what it was and why it was doing it.
I'd check where the initial timing is before doing anything, especially if your idle quality is good. Then find the total number that works, using above method, and tailor mechanical advance to hit your total number.
You might want to back it off just a bit more than where you no longer hear pinging. There could be inaudible pinging, better safe then sorry.
Verify where the total timing number is now. Please don't pick a RPM number and stop. Rev the engine up until it stops advancing. Some of the MP distributors go as high as 4K before they quit advancing. It may be really high, if it is you may want to start fresh by figuring out a total number that works, then get into what the engine like best for initial.
