One way to get a good rebuilt bike is to have an issue and in frustration start parts changing. When you are done it is usually some small idiotic thing you missed but you end up with a well sorted bike!
Sadly I've ticked all the boxes to no avail. It ain't the coils, the EI, wires, caps nor carbs or anything in the carbs. The valves were removed and checked, they seem fine. Compression is good and very even side to side. The bike can start easily and runs well enough to be used without issue.
BUT the damn plug is black and dry no matter what is changed. The oil gets dirty too quickly.