A couple things might be causing this. First, you probably aren't getting the engine warmed up properly before riding, and not riding long and hard enough to reach full operating tempertures. You also may be jetted too rich. If your jetting is stock and you are any where near sealevel, that could be a problem. That stuff is known as spooge and is mostly unburned fuel due to poor evaporation within the combustion chamber. You may have a stuck float or float valve that is dumping extra fuel into the motor. You may be riding with the choke on. Warm up the bike properly, ride it with some umph, then see what it looks like. You may still spooge due to a pipe load already in the exhaust so you may want to flush that out first.