Check your muffler bolts. If the bolts are loose you will get a dark brown (usually black) color to the exhaust residue. This is caused by the muffler rubbing against the engine exhaust port. The USED TO provide a gasket that went between the muffler and the engine. I don't know if they still do or not, but in any case, it would not hurt to make a gasket from some gasket material (or in a pinch, some cardboard that come in new shirts) and install it.