you need not open plugs frequently to clean ... assuming there is no oil deposit on these plugs .. lets count things which make a mixture rich
-- air flow
-- mixture screw adjustment
-- plug gap
-- current in plugs (this includes coils, HT leads, point and plugs)
-- valve and timing
-- fuel quality (some what)
you can follow this list to check the problem area but next time just inspect the silencer and its color will tell you the details . a little more experienced people can even smell unburnt fuel from exhaust gases