what was the appearance of the carbon? satiny and hard or kind of soft and flat black like candle carbon. Carbon coating can happen very quickly (to a point) even in the best tuned engines and is normal. The additives that the petroleum companies put in the mixtures when blending summer and winter fuels can cause carbon build up (normal type-minimal thickness kind of hard and dry and a little satin shine). What did the plugs look like? You mention tearing the engine apart twice, did you rebuild the first time? New rings, Valve job?