RE: Black goo
I think of carbon as a hard baked on sort of blackish brown product. What I had was goo. Something like molasses in antartica. It washed off with little effort with the brake kleen. The carbon I've dealt with in the past was not this junk. This goo could easily move around, including to the plug area, which is at the bottom on my all my singles. I think it was gathering there during storage and there was an obvious v-shape across the top of the piston pointed at the plug. This coating had become maybe 0.5mm thick. This was still a fluid that to me should have burned, not been left behind as this goo.
When this goo made it out of the exhaust, it left definite little black spots on the model that were not so easy to clean off. Not the normal brown oily stuff like you get with regular 2-stroke oils. Also of note, when I was running this mix, there was never any smoke that could be easily seen. On my other mixes I do see at least some smoke under some circumstances... particularly after choke on start up.
Some carbon build up on the other hand can increase compression and add some power to an engine (a tiny argument for some carbon). I expect that and look forward to when that starts to build up. This goo... well, I guess it did add some compression, but engines that don't run right or start and run...