a spark plug , wherein a central electrode is electrically insulated from anuter electrically - conductive casing by an insulator sealed in the casing , separating the central electrode from the casing and having a taper - nose portion which is designed to be exposed to unburnt carbon in combustion gases of a combustion chamber , is modified to facilitate carbon removal from the taper - nose portion . discrete isolated particles or isolated aggregates of particles of vanadium oxide are adhered over the exposed surface of the taper - nose portion to improve the self - cleaning ability of the spark plug without decreasing the degree of electrical insulation provided by the insulator . the particles of vanadium oxide are formed from a suspension which is applied to the surface of the taper - nose portion of the spark plug . the resulting coating is then dried in situ . further heating results in having vanadium oxide particles , in insular form , firmly adhered to the surface of the taper - nose portion .