I disassembled the distributor (removed the breaker plate) and verified the mechanical advance mechanism still worked (it does). Everything looks to be in good shape, moves freely, and returns back to normal position with the springs. Before reassembling, I moved the anchoring position of one of the springs (to an available hole in the cover plate) to give it better mechanical advantage over the centrifugal advance mechanism in the hope it would delay when the mechanical advance would come in. That seems to have worked; I don't hear pinging anymore and the engine sounds/performs better under acceleration.
I realize it would be better to find a proper set of mechanical advance springs, but I'm having difficultly finding one of those "kits" that would have the springs I need.
