Ok to give an update, it turned out it is a FC TAC.
So Norm has turned up a 100mm x 5mm thick disc on the lathe weighing 290g. It sort of stopped the kick back, but then would still occasionally do it, but even increasing it to 1000 grams fitted on the crank it is still kicking back.
Anyone have an ideas - did the other heavier TAC flywheel slightly change the timing?
