[SAJ]

I think your plugs are a bit sooty, probably because you are running too rich.
Or, possibly you plugs are too cold running to burn off deposits.? You do not say what brand and grade they are.
I am running Champion W16Y in two higher compression engines. Both run just slightly tan colour on insulators, grayish on earth electrode and black on bodies.
This is with GAV valves about 1/3 rd turn open. At 1/4 turn they burn almost white insulators. This did cause exhaust valves to almost burn at sustained high speeds many years ago, and since running at a very light tan colour I have had no more trouble for 30000 miles or so.
Where your GAV needs to be set depends on the flow rates of your particular jets.
To assess running mixture you need a mile or so of sustained running and then immediately cut the motor, clutch pedal down and cruise to a stop in neutral. Remove and exami e the plugs.
Otherwise, if you idle first for more than just a few seconds you will be reading idle mixture, not running mixture.
Recently we pulled the plugs of several members cars after a running through the mountains and they looked just a bit richer than mine, but still with a light tan colour on the insulators. Some were Motorcraft plugs, TT100 I think, and some were Autolite brand.
From the above I think too lean and white risks burnt valves, but the Model A should run with tan coloured insulators and clean earth electrodes. The ends of the bodies do seem to run black, but crisp dry carbon not fluffy or oily.
SAJ in NZ