Thanks for the replies, below for comments and more info;

- Backfiring thru the carb
- I haven't specifically checked if the carb is leaking down and flooding but it is NOT making oil, and accel pump works immediately after a week - I will check this one further. I had the same problem with the AFB before the TQ went in during the last year.
- Yes I'm sure the rings are a little "loose" it does use oil - more below, I also can't see how this would foul the plugs stationary.
- Interesting on the weak fuel pump, but no issues at WOT and the 5 to 10 mins can include a drive of a couple of miles before it clears so I believe unlikely.

Some other points;

- Engine does use a lot of oil (300 miles a quart) and got worse some time after the rebuilt heads
- All plugs look good after carb fix, although numbers 1 & 2 do look 'richer' but nowhere near fouling
- cylinders 1 to 6 have OK compression - can't get my press-in gauge into 7 & 8
- Bore Inspection gauge shows no dramatic issues like bore gouges from broken rings, but all obviously well used.
- Twin exhausts, both banks put dirty (oil?) smoke on the car after a run but don't smoke visibly once warm.

My thought was that the valve seals could be leaking and fouling the plugs and ports but I'm not sure how this could continue while sitting.

Comments?