The usual method employed in cleaning spark plugs is to use a small brush and gasoline and polish the end of the plug. As most plugs are constructed so that it is impossible to thoroughly clean the porcelains in that way, it is advisable to take them apart and brush every particle of carbon from the porcelain. If the carbon is baked on hard use a sharp knife to remove it. Never use emery cloth to clean porcelains as that cuts off the glaze and then they will absorb enough carbon to destroy their insulating qualities.