The black stuff is talking to you. Over and over it's saying it isn't broken in yet. That experienced giant scale guy needs to do a better job of listening. Look for a tiny leak around the base of the carb and the insualtor block, then spray some soapy water in the same area and look again. Worst case is you remove the insulator block and fine sand it flat if there is a leak. Get those put of the way and we'll work another area if the sputtering problem doesn't go away. The black stuff will go away when it completes the break in.