does it smell rich?
And you may have an exhaust leak as another possibility.
You got raw fuel which is igniting in the exhaust system. If your bike
runs fine without a miss then i would rule out ignition. It is possible
your timing is off and it is allowing partial burn to happen in the exhaust
and burn the left over HC from the other cylinders. You can have a bad valve
or valve seat on the exhaust side allowing fuel to escape. Worse case because
you need to pull the head off.