On my personal bikes I have 2 pk80 engines, both have over 6000kms on each of them, 1 the clip holding the the piston pin went and it snaped a ring, the second just broke. No failure rate involved! Things happen. I do not sell PK's as I do not find them as quick as the ones I sell. Anyway I thought this was to help amy's problem? It for sure is not back firing as that is a pretty easy sound to determine, as is the sound and the way he discribes his problem, check your rings, best case it's not them, worst case, i'm right. Wouldn't that suck?
Thanks