You can rule out direct injection as a cause.Has anyone else experienced or observed this and has a good idea of what may be going on?
On a carbed vehicle you are typically left with things like PCV and EGR ports near the cylinder allowing un-burned hydrocarbons back in. Maybe a loose or leaking vacuum line near the port. Poor quality gas can also be a culprit, but seeing how its only one cylinder you might be able to rule that one out.
Are you seeing the carbon build up on top of the piston as well?