2 stroke poop! A certain amount of this is normal due to the oil/gas mixture you're burning. However, type of 2stroke oil used, amount used, and jetting of the bike can all affect the amount. If it's coming all the way out the back, it's time to repack the muffler. If this is the case, you'll probably notice your bike is a little louder and raspier than before. This also impacts performance negatively and, eventually, the inside of your muffler can start smouldering.
When i ride my dt125x 2005 if i screw it theres a black oily substance coming from exhaust please couldyou tell me what this is and what needs fixing to put it right