patchoy

my car is daewoo espero model 1995.why is it that my spark plug turns out to have a black color and does not maintain to have its natural grayish color.air intake is closed, hose is in its proper condition and idling pick up is ok. what do you think is the better solution to keep my spark plug in its best condition? engine is newly overhauled, electric relay is giving its best usually bluish in color.any one could give comments or suggestion regarding this matter.thanks.