When changing down from 5th gear to 4th to overtake, my engine would splutter until I changed back up to 5th. I've just serviced the car. I see that the old plugs were Champion N9Y. on inspecting them they had all burnt pretty well. no indication of a rich mixture. I have fitted the specified N7Y plugs. Old and new plugs were set to 0.6mm gaps.
I took it for a drive and revved it to about 6000 rpm in all gears. I could not get it to splutter at all.
Just wondering - could the plugs make such a difference. I didn't fiddle with the carbs at all