Yes, it is carbon build up. I have an old 8hp tiller that does this the first couple times of use in the spring. I am a generator tech and yesterday I was running a 1500kw unit. It had about a 1/2" build up of unburnt fuel and carbon. I did a 2hr load bank test on it to clean out the wet stacking. For the first hour it was like black snow. It covered everything!
