hello;

what was your timing before?

what is it now?

is your dist vac connected to ported or manifold vac?

are you running stock heat range plugs?

is your eng high perf?

what is your compression?

if your timing is EXACTLY the same at idle it still might be less or more at higher rpms causing more heat in the cyl so it might create pre ignition .

its remotely possible the increased voltage is heating your plug tips up a bit moire and enough to cause pre ignition.

i would retard timing 3 degrees and see if it stops.

if it pings under acceleration try less timing also and/or higher octane as suggested.