biffo

Hello i hope someone can help me with this: i went to buy a TRX 850, cosmetically the bike was in a nice clean condition, but something bothered me, at the join between the head and the cylinder there was a thin line of discolouration all the way around the joint, but only on the left cylinder, i could not work out what this was i.e. oil, grease, or a disintegrating gasket. So i revved the bike and at mid range there was a dark colouration from the exhaust not a lot, just a thin viel of dark grey. from the exhaust. can anyone shed some light as to what this may be.