SETTING
Fig. 12.—Timing Diagram, Two-Spark Magneto.
In setting up a system of ignition with a magneto of this type, the firing order of the engine must be observed, and then one of the pistons brought to the point in the compression stroke that it will occupy when the spark is at the greatest advance. As has been explained in the case of engines equipped for the make and break system, this point will usually be from one-half to three-quarters of an inch before top dead center, the greater proportion of designs placing it at one-half an inch. The driving gear of a Bosch magneto is not keyed to the armature shaft, but shaft and gear are tapered, so that when the nut is drawn up tight there is no possibility of slipping. After the magneto is secured in position its gear may be meshed with the gear that is to drive it, but without drawing up the nut. The armature may then be turned by hand until it is in the vertical position, where it will be retained by the lines of force (Fig. 12), and the gear nut drawn up tight. If this position of the armature cannot be obtained by the sense of touch, it may be exposed to sight by the removal of the bridge that carries the safety spark gap, and of the dust cover that protects the space between the upper sides of the pole pieces.
The loosening of the three-armed frame on the front of the magneto will permit the removal of the covers of the interruptor and timer, and it will be seen that the moving part of the distributor is beginning to make contact with one of the stationary pieces; this should be connected to the spark plug of the cylinder that is in the firing position. The other contacts of the distributor should then be connected to the remaining spark plugs in the order of firing.
Ignition is cut out by short-circuiting the primary winding, and a binding post will be found on the front of the magneto under the three-pronged frame to which is connected the wire that leads to a switch making a ground connection when closed.