At this time you may want to start at "ground zero", check the timing, jet a little richer and take a look at the cyl. head. See if there are any, even very slight imperfections in the combustion chamber, a nick or deep scratch can create a hot spot leading to pre-ignition. Usually any jetting problems will show up on the side of the piston, where as (pre)-ignition, low octane fuel or too high compression will show up on the top of the piston. A switch to 50/50 race fuel/ pump gas might be in order to give you, your piston anyway a measure of safety. I hope this helps and not just confuses the problem.
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