Be aware that some of that crud will end up in 2 cylinders, as the other end of that channel of carbon is right at the exhaust bowls of the 2 cylinders. It will probably burn out of there anyway though, so just get as much out of there as you can. And the heat riser is probably no good, that's what makes that stuff pile up in there. The chevies also had the same problem and delayed choke opening etc.