My Spit Mk 3, standard engine 1293cc no mods. Has run on since its rebuild 800 miles ago. I've just changed the head gasket (to cure an oil leak) and was suprised to see how much carbon had built up in that time. Carbs set up by ColourTune and plugs a good colour. Plugs are Denso W16EPR-U, as supplied by either Canley or Rimmer (forget which), fuel - generally Sainsbury standard Unleaded
Changing plugs may help run on, but surely won't affect carbon build up.
Peter