I would say if its been sitting a long while the carbon/coke build up looses its adhesion to the inside of the head and muffler etc. It either dries out or it oils up then it all gets blown out the exhaust when you run the motor next.
If the mixture is a bit lean this will make the carbon burn off as well.
I get the same sparks thing with a lot of the older (60's & 70's) 4 strokes i fix up. Even after setting the valve clearances and de-cokeing the head and piston they can still blow sparks. Carbon in the muffler, definately more prone to doing it with a lean mixture.
