Welcome peter. " that pinging & knocking is Detonation , this will result in a holed piston . timing , or fuel [ high octane] is usualy the problem,, or carbon on top of piston, glows and pre-ignites before piston is at top of stroke, as you can imagine ,if bang is early, its hell on piston, ,
Start with decoke, then check fywheel marks,, as Dt 400 is CDI. it still needs to be timed.. a dial gauge is needed, set to degree ,Before TDC , thats wen pointer lines up with pulser coil , oh spark plug shoulf be B9ES ,,,,