If there is no evidence of
oil on the electrode or in the side pipe, and only on the porcelain, then my first guess would be a valve cover gasket leak. Check the bolts that hold the valve cover on to make sure that they are tight. If they are tight, remove the valve cover and take a look at the gasket. If the gasket is overly compacted or torn, replace it. The valve covers need to be snug, not overly tightened. Sometimes people tighten them too much and it destroys the gasket.