stock 318 pinging

Quick way to ball park it for the gas your running on,keep retarding the distributor till the pinging stops at full throttle, when you can barely hear it ping back it off 2 degrees or more .I have also seen heavily carboned engines that pinged due to the excessive carbon raising the cr. Don't know if its still done but I saw mechanics years ago clean carbon deposits by pouring small amounts of water down the carb at a very fast idle.