I was going to add, that you should check out all of your cooling system for problems. Sounds like detonation to me and can be caused by any number of things:poor quality fuel, overheating from being lean or cooling system issues, impropper squish clearance, too much timing advance, fuel delivery problems (plugged petcock, cap not venting, impropper float adjustment, bowls running dry ect.) too much compression for octane, air leaks ect. Just a few things to consider. You need to diagnose why it burned down or it will do it again.