I agree with swapping the carbs. But before going into that elaborate job, have you compared pressure in the inlet manifolds? Either with twin manometers or with a long plastic tube half filled with water?
The difference in colour on the spark plugs seems to point to differences in mixture, though it could alse be due to some issue with valve timing.
Which might be harder yet to diagnose than swapping the carburettors, I'm afraid.
Last edited by Jan Olieslagers; 11th Aug 2016 at 14:42.