When the insulation of the coil or condenser breaks down, as may result from overwork or long use, it should be returned to the maker, for the construction of a coil is exceedingly delicate, and repairs on it require expert workmanship. It is not advisable to take the coil apart in any way, beyond the vibrator parts, for inexperienced handling may injure it beyond repair.
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.
THE IGNITER
The stationary point of the igniter is carried on the end of a screw that passes through the cylinder head or wall, and by means of this screw its position in relation to the moving point may be altered as desired. The distance between the points when they are separated should be about one sixteenth of an inch; more than this will result in the formation of a longer spark between them, and as the passage of the spark through the high resistance of the compressed charge will produce great heat, the igniter points will be burned and corroded more than need be. The adjustment may be made by loosening the lock nut on the stationary point, and running the screw down until the stationary point is in contact with the movable when the latter is acted on by the cam. The lock nut should be run down until it bears lightly against the cylinder and the point then unscrewed, the lock nut being carried with it, until the latter is about one sixteenth of an inch away from the cylinder. This gives a corresponding distance between the igniter points, and running the lock nut down firmly will secure it.
The tappet spring should be of considerable strength in order to snap the movable point from the stationary when the tappet ceases to act. Too great strength will bring the movable point against the stationary with such force that the platinum plate that it carries will be battered out of place. The principal difficulty that is encountered is the corrosion of these points, the flow of the current being decreased or stopped by the insulating film that covers them. The blow with which the two come together tends to knock this off and to keep the surfaces flat, but when they become badly worn and pitted they must be filed smooth.
The bearing in the cylinder wall in which the movable point rocks is made after the manner of a valve; the opening must be tight in order to prevent the leakage of compression, but sufficiently free to move as the cam acts on the tappet. This joint cannot be lubricated with oil because of the heat, and the two parts must therefore be kept as smooth as possible. To attain this result they are ground together as engine valves are ground.