Sooty Plug

If it has  twin carbs ,then one is richer , could also be oil soot from an inlet  valve seal, could be one plug is not firing as well as the other , faulty lead ,cap, etc.  I still have to be convinced that one plug firing backwards is good for the life of that plug!.

Same, dry sooty plug, compression test exceptable, thought head but when taken  off ended up with new pistons/ rings/ honed.
.  Plus valves. 
 

I always understood that oil soot would be blue and of course if you are burning oil on one side you should see it from the exhaust. According to what is written above the machine has two coils in which case they should both be firing the same way. If it had a twin output coil set up then there is the possibility that one plug would 'burn differently' but this is the case with all twin magnetos, one plug fires with positive to earth (of the plug) while plug no 2 fires with negative to earth, hence one spark is is aided by thermionic emission the other plug is not, but we don't get sooty plugs one side!
Back to the plot, as you have Pazon ignition (wasted spark) you can swap the HT leads so that left coil fires right plug, if it is a spark issue then the issue will move sides. The other thought is that is there a bias on the carb-perhaps making the apparent correct plug weak? Or an air leak?
OR could it be a difference in exhaust pressure? Swap silencers will sort this one.

Alan raises an interesting point, carb bias is a possibility , you could experiment with a slightlly tapered manifold spacer to direct the airflow more towards the weaker/ cleaner side. Also switch plugs from side to side as one may inexplicably run hotter than the other.