If the compression is not the same in all of the cylinders, a closer adjustment of the igniters may be made that will improve the operation of the engine, the tappet of the high compression cylinder being adjusted to operate the movable electrode slightly before the others, so that the larger volume of mixture will have more time in which to burn. The switches through which the igniters are connected to the bus-bar may now be brought into use, and when three of them are open the engine should run on the single cylinder of which the circuit is complete. Testing the cylinders in this way, running one at a time, gives an opportunity for comparing their action, and for noting those in which the power is weak. If the weakness is due to ignition trouble, it may be corrected by a further adjustment of the tappet.
TROUBLES
Failure of ignition due to magneto trouble has already been described.
Any short circuit of the system will prevent ignition, for the current will not flow across the igniters if another path is open to it. Short circuiting may be due to the chafing of the insulation of the wires, to a frayed end of a stranded cable making contact with the metal of the engine, to the carbonization or breaking of the insulation around a stationary electrode, or to the sticking of a movable electrode in contact with a stationary. Operating the igniters by hand will show the presence of this last named condition, and inspection of the insulation and terminals the presence of any defects. Loss of power is frequently due to the leakage of compression around the plate carrying the igniter, around the stationary electrode, or through the bearing in which the movable electrode rocks. The side of the bearing toward the combustion space is built like a valve, and when worn may be ground to a seat.
If the engine runs well at low speed, but misses at high speed, the fault may be located in faulty insulation of one of the stationary electrodes, which holds the low-pressure current developed by the magneto at low speed, but fails to retain the higher pressure developed when the speed is increased. The faulty igniter may be located by running the engine one cylinder at a time.
The heat developed by the spark is very great, and provisions must be made to preserve the contact points from undue corrosion. These points are usually made of platinum, or of an alloy of platinum and iridium, and must be kept clean and smooth. The spring that draws the movable electrode against the stationary must not be too strong or it will bring the contacts together with sufficient force to batter them out of shape. This spring should be of sufficient strength to bring them together promptly, but without undue force. The tappet spring should be of considerable strength, in order that the separation of the electrodes may be quick and positive.
After long running the contacts will become worn to such an extent that the spark occurs too late in the stroke to permit the engine to develop its full power. This may be corrected by readjustment of the tappets, so that ignition takes place earlier. A wearing of the outside arm of the movable electrode, or of the tappet head, will have the same effect.
Another cause of failure will be the weakness of the spring of the movable electrode, which is so weakened that it will not do its work at high speed.
L. T. MAGNETO WITH SECONDARY COIL
The advantages of a magneto as a current producer for the low-tension system pointed the way to its adoption for the jump spark system also, but it was at once recognized that it would be impossible to lead the current direct to the primary winding, as the current from a battery is used. The reason for this is that while the battery delivers a current of constant value, the current obtained from a magneto is fluctuating, its intensity depending on the positions of the armature as it revolves, and the speed at which it is driven. A current is induced in the secondary winding of the coil as the magnetic field set up by the core changes its strength, and as has been explained, the induced current is strongest when the greatest change in the strength of the field occurs in the shortest possible time. The effect of the flow of the magneto current through the primary winding of the coil would be to magnetize and demagnetize the core slowly, as the magneto current increased to its maximum and died away to the minimum, and these gradual changes in the magnetic field of the core would induce currents in the secondary that would be too feeble to produce ignition of the charge. When a battery is used with a coil, the operation of the vibrator produces rapid magnetizations and demagnetizations of the core, but as the field set up by the core dies away more rapidly than it is established, the greatest current is induced by the breaking of the circuit. In the best known method of applying a magneto to the operation of a secondary coil, the magnetization of the core is caused to occur with exceeding rapidity, and the current induced in the secondary winding as this occurs is sufficient for ignition.