[Kenth]

A lean mixture does not ignite as easy as a rich mixture and will not make the engine ping, just loose power.
A too rich mixture will make the engine ping.
Cylinder peak pressure should occur at 14 degrees ATDC, and that´s when the combustion should be finished.
That´s why there are different ignition advance curves depending on combustion chamber shape, compression, fuel used,amount of fuel used, rpm´s and so on.
If you have a too rich mixture it will burn off faster than necessary (before 14 degrees ATDC)and creating ping.
Excessive oil consumtion will also create ping.