Thansk for the info guys, what a good forum. I changed to an 8 grade plug and it is a better colour, (dark brown as opposed to black) and the back firing was an interesting one. The valve shims were done recently and this must have thrown the carbs out of balance - quick check with a vac gauge and one was way out. Rebalance and she runs lots better and has lost most of the low speed snatchiness
