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[QUOTE="dirt bike dave, post: 1290851, member: 18302"]
I think you have a form of preignition that is different from the conventional knocking under heavy load and low rpm.  Check out the description of 'wild ping' in the research paper linked below.

I would examine the head, piston top, plug and top of the cylinder bore for any pitting or loss of material.   As for cures, maybe a thicker base gasket or head gasket would help, as might a colder plug and bigger main jet.   Personally, I would pull the cylinder off already to inspect for any damage before riding again.  Check for bearing play, too, as more compression combined with a brutal high rpm pre-ignition would be very hard on your big end and wrist pin bearings.

It is possible the reduced cooling efficiency caused by a leaky head gasket is causing the problem.  Also, what oil and ratio are you using?  You could try adding more oil, to improve cooling and lubrication.

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