Roger,
Pinging, in the mechanical vernacular is the sound made by the fuel/air mixture pre-igniting, or igniting too soon BTDC. If you are actually hearing a ping, it is most likely a timing related problem.
I think pre-ignition can be caused by carbon deposits in the cylinder/piston area, or a sharp worn valve surface (hot spots) but on a probability scale, these would be right down at the bottom.
Tune it up and fiddle with it..it will probably run great after that.
Stuart
