sparks plugs carbonise

Hello, Depending on when the spark plugs were replaced last, carbon build-up could be normal. If this is the first, or even second set of spark plugs, assuming they were changed half-way between the current mileage and new, I would expect some carbon, but not to the extent that the spark plugs are unrecognizable. That being said, there are a few things that can cause carbon buildup. Among them are oil consumption via worn piston rings or valve seals, emissions control system failure causing imbalanced fuel to air ratio, or even incorrect engine temperature. In any case, it is not a bad idea to do a comprehensive inspection of the vehicle, and make necessary repairs to ensure longevity of the engine.