When I pulled the cylinder off of my 2001 yz 250 today, I was shocked to see that the intake ports were completely black. also, the bottom of the piston was completely black. The only thing I can think of is that the rings were so worn that the exhaust was blowing by into the crankcase, and being drawn through the ports back into the cylinder. This is the frist time I have had this bike apart, so maybe I just waited too long? I normally put 2 to 3 pistons in my 125 per year, but I thought I could get away with only one rebuild per year since this is a 250. Anyway, if this is what happened, should the cases be split, and the main bearings be inspected for damage? The last thing I want is for the crank to sieze on the face of a triple. Or was it something completely unrelated to worn rings? Thanks.