>>>*Preignition at idle usually means preignition at speed also. In full throttle situations that can mean the next step which is detonation, that is often found by hearing the loud noise and big cloud of smoke.

One point is not correct, the valve timing does not change 3? for every .010" machined off the head/block, it is closer to just 1?.

I would suggest moving the cam timing to a straight up timing setting as very early event valve motion can lead to detonation. Besides, early event affects lower RPM, and later moves the power peak up the RPM scale. Logically your combo is intended to run at upper RPM's.

Still, preignition can be cured by retarding the ignition timing, and I would say always unless the cam was radically out of time.....Kind of a curious situation, can you describe the problem or perhaps tell me how the diagnosis was reached?..The reason I ask is I would question that if everything else is close to normal.....*EB