THE SPARK PLUG
The spark plug is a frequent source of trouble, for the insulation may break down, or a carbon deposit may form between the points, either fault permitting the current to pass without jumping the gap. Plugs should be so made that when the parts are screwed together the strain will not come against the insulation. Porcelain insulation is brittle, and may be cracked inside the sleeve, the damage not showing on handling and inspection, but being sufficient to permit the spark to pass inside of the sleeve instead of between the points. Mica insulation is usually formed of a large number of washers squeezed together under great pressure, but the action of the heat and the presence of oil will frequently cause the layers to separate, permitting a short circuit.
The carbon deposit that fouls and short-circuits spark plugs results from a rich mixture, or overlubrication, and this should be prevented by proper adjustments. Under the intense heat the carbon bakes hard, and can be removed only by scraping or by the action of strong ammonia or gasoline. As in scraping the smooth surface of the insulation may be scratched (which would give a rough surface to which a fresh carbon deposit would cling more tightly), the removal of the deposit by ammonia or gasoline, applied with a stiff brush, is recommended.
The points between which the spark passes should be about one thirty-second of an inch apart. After long use, the corrosion due to the heat of the spark will increase this distance, when the points should be bent together. The high compression through which the spark is required to pass presents greater resistance than air at atmospheric pressure, practically in proportion to the pressure, and a current that will produce a spark outside of the cylinder may not have sufficient pressure to give a like result when the engine is running. If the pressure of the compression is four times that of the atmosphere, the current should be able to produce a spark four thirty-seconds, or one eighth, of an inch long outside of the cylinder, and for safety this should be increased four-fold.