Modern fuel typically does NOT leave deposits, carbon build up is generally from oil reaching the combustion chamber. This can usually happen 3 ways, through the intake, if crankcase is vented to airbox, check airbox for oil. Through worn valve guide seals, or through worn piston rings or piston/cylinder damage.
If the wrong spark plug was used, is it longer or shorter than the right one ? These bikes run tight cylinder head clearances, so if to long, there would be impact damage to piston & the side spark electrode on the plugs would be damaged. If neither then the plugs are not your problem.
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