Some great replies guys and thanks for all.
The problem originally started at around 200 miles old when the ignition timing retarded itself at this point I assumed that the dizzy had rotated due to being loose. I retimed it and all seemed well for a while.
Then at around 800 miles It happened again slowly getting worse over a 200 mile run to a local kit car event.
Having marked the dizzy I could see that it hadn't moved but the timing had.
Reset it again and all was well but not for long went on a track day around 250 miles later, and the engine began to pink (ping) and got gradually worse over a couple of laps.
Checked the timing trackside and it was all over the place, again the dizzy had not physically moved.
Do to the higher cylinder pressures caused by the timing maladies I broke a piston top land.
Piston now replaced but ignition problem goes on.
I've checked the balancer and carb and gaskets all is OK.
I just pray that if I fit a new HEI all will be sweetness and light and I can enjoy our all too brief British summer.
