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.

I had a similar problem two years ago after fitting new stay up floats, the left cylinder was always slightly richer than the right.

In the end after reading some advice I took both float bowls off and mounted them in a vice with a remote supply tank and using a ruler made sure both fuel levels were exactly the same level, this cured the problem.

Graham - He has a single Mikuni.

Assumption is that the side with the sooty plug is the issue, could it be case of the 'normal' looking side running weak for reason or another. With the bike on the centre stand and left overnight, remove the inlet manifold and inspect the valve stems and valve guide noses in the ports, a guide and or seal could be passing a small amount of oil.  

As someone pointed out in previous comment that what I had said had already been explained. Not so, as in a previous post, swapping the HT leads could move the issue to the other side (this proves the ignition side or not as the case maybe) but before doing this it would usually be noted by misfiring or a different warm up rate if you feel the silencers from cold prior to swapping the HT leads, however, with a single carb and well maintained ignition system the silencers I would expect to warm at similar rate irrespective of one plug getting sooty as the single carb should deliver a very similar amount of air fuel ratio to each cylinder. With good compression and both cylinders readings being very close this suggests that the bore is sealing well at compression. Now combine the warm up rate of the silencers and both cylinders on good compression suggests that each cylinder is pulling a similar air fuel ratio mixture volume through the single carb - this leaves two things to look at  - oil getting past the guides and / or valve stems, or air leak on the non sooty side, it could be the rings in the bore, but in this case I would expect to see a difference of more than 10% in the compression readings. A compression test which is just an indicator only gives a value from when inlet and exhaust valve are both shut until TDC, what the rings scoop up off the cylinder wall on the way up the bore until the valves are both shut has no real input to the result, though the oil could aid in getting a higher figure.
Exhausts could have different through put from each other, but with a well set up carb the air fuel ratio is the same, it's just the volume of it that could differ and it would need to be considerably different in volume to make a difference, you would notice lack of performance or poor running first if this was the case. 
This bike is fitted with a single carb, once ignition is proven not be the issue, it leaves very little else to consider - oil leak to cylinder or a crack in the head around the inlet port of the non sooty side. 
  

Had it been running rich previously?  

Possibility of an air leak at the manifold/head joint? 

I once spent months trying to fix a 'sooty plug' problem by checking, rechecking and replacing the carb and ignition system and got nowhere. I had convinced myself it was running rich somehow.(Commando with single Amal) The problem was glazed bores so a rebore and running it in properly fixed it. The clue was the amount of black oil in the silencers.