I just got this last fall, and it seemed to run OK, but now it is quite rough. It is by no means a Yamaha or Honda smooth, but I don't really expect it to be. However, it vibrates quite a bit more than I think it should. It has a Mikuni carb, and changing the jet adjustments while it is running does very little until they are cranked all the way in. The engine puts out a bit too much black smoke, too, and fouls plugs pretty quickly. Plugs are never brown when I pull them; always sooty. The fuel is fresh. Any ideas on what I should check first? Timing? Valve lash? Clean carb? Thanks. john