The breaker cam and the distributor rotor are both mounted on the vertical shaft and are rotated at exactly one-half engine speed. Accordingly, since the engine is of the usual four-cycle type requiring two revolutions of the crank shaft for one complete cycle of operation, the distributor rotor and breaker can make one revolution during the completion of each full cycle of the engine.
Distributor Head.—The distributor head contains five high tension terminals. The central terminal receives the current from the secondary winding of the ignition coil and transmits it to the rotor arm by which it is distributed to the four outer terminals. These outer terminals are numbered 1, 2, 3, 4 respectively, corresponding to the firing order of the engine, and are connected to the four spark plugs in accordance with their markings. The distributor rotor in completing one full revolution establishes contact successively between the rotor brush and each one of these four outer distributor terminals, each contact being made at the same moment that the primary circuit is interrupted by the action of the breaker cam. Thus when the spark plug leads are properly connected, the high tension current, as soon as produced in the secondary circuit, is conducted to the spark plug of the proper cylinder just at the moment when the gas in that particular cylinder is ready for firing. If, therefore, the spark plug leads ever have to be removed from the distributor head, they must always be attached again carefully in the correct order.
Automatic Advance Mechanism. ([Fig. 65]).—Combustion does not follow instantaneously upon the occurrence of the spark, however, because a small time interval is always needed for the gas in the cylinder to ignite. Consequently, unless some means are provided for offsetting the lag between spark and combustion, the explosion of the gas could not always be made to take place at exactly the correct moment under varying conditions of engine speed.
Fig. 65. Automatic Spark Advance Mechanism—North East
To compensate for this lag, therefore, there is incorporated in the distributor a centrifugally actuated mechanism, which is capable of automatically advancing or retarding the time of the spark in exact accordance with the rate of speed at which the engine is running.
The operating characteristics of the automatic advance are accurately proportioned to conform throughout the entire speed range with the requirements of the engine; and in order to insure the permanence of this relationship the device is so constructed as to be practically nonadjustable.
Manual Spark Control.—Besides this automatic advance there is also the usual manual control mechanism for changing the time of the spark independently of the centrifugal device. This manual control is for use principally for retarding the spark when starting or idling the engine or for facilitating carburetor adjustments. During normal operation of the engine, the spark lever on the steering wheel quadrant should be advanced as far as permissible without causing the engine to knock, and the actual regulation of the spark position be left entirely to the automatic advance mechanism. The arrangement of the manual control is such, provided the breaker cam is properly set, that when the spark lever is in the position of full retard, and the engine is running very slowly, the spark will occur in each cylinder at 5 engine degrees after the piston has passed the upper dead center of its compression stroke. With the spark lever advanced to the limit of its travel on the quadrant, the spark will occur 15 degrees before the upper dead center position has been reached by the piston on its compression stroke.
Timing the Distributor.—Whenever it becomes necessary to disconnect the distributor shaft from the engine pump shaft the exact relative positions of the two halves of the coupling joining these two shafts, as well as the location of the distributor rotor, should be carefully noted and marked. This is necessary in order to make possible the reëstablishment of the correct relations between the distributor shaft and the pump shaft when original conditions are being restored. Moreover, care must be taken to avoid turning the engine while the distributor is disconnected, because the proper timing relations can only be retained by keeping the position of the pump shaft unchanged during this time.
Should it ever happen, however, that the distributor has been taken off without the proper precautions having been observed, or that the timing arrangement has been disturbed in any other fashion, it will thereupon become necessary to make a complete readjustment of the timing relations of the distributor and the engine. This is to be done always after the distributor has been reconnected to the engine, the first step being to ascertain definitely the relative position of the engine pistons and valves. With this done, the positions of the breaker cam and the distributor rotor are then to be reset as directed below.