There is a saying in the auto repair community, that a clean plug wire is a happy plug wire. And why not? A thin film of dirt on the outside of the plug wire and around the boots at the ends of the wires can form conductive path to the high voltage when exposed to moisture. The conductive path drains away the current that should be going to the spark plug. This can make your engine miss or run rough, especially in damp weather. So, take care to make sure that your wires are clean, and always wash your hands before returning to work.