A magneto is timed to an engine so that when the spark control is fully retarded, the circuit breaker points are just separating as a piston goes over top center. The engine is cranked until one of the pistons is at top center; the magneto should be in position, but its coupling should be loose, so that the armature can be revolved. The spark control is retarded; that is, it is moved as far as possible in the direction in which the armature turns. The armature is then revolved in the direction in which it will be driven by the engine until it is seen that the contact points are beginning to separate; holding the armature, the coupling is then made fast. It will now be found that the distributor brush is touching one of the contacts; that contact is to be connected with the spark plug of the cylinder that is at top center of the compression stroke. The following distributor contacts are connected to the remaining spark plugs in the order in which their cylinders fire. Should the magneto be suspected of being out of order, the first test is to disconnect a wire from its spark plug, and support the tip ⅛ inch from the metal of the engine while the engine is cranked briskly; if a spark appears, it is evidence that the magneto is operating and that the trouble is elsewhere. If there is no spark, repeat the test with the switch wire disconnected from the magneto. This wire and the switch form a circuit from the metal of the engine to the insulated part of the circuit breaker; when the switch is closed, or in the “off” position, this circuit is completed, and as the magneto current flows over it instead of over the regular sparking circuit, no spark is produced at the plug. It sometimes happens that the switch or wire is defective, and allows the current to take that circuit even when the switch is in the open or “run” position. If this is the case it will be shown by a spark on cranking the engine with the switch wire disconnected at the magneto, and no spark when it is connected. If the switch and wire are all right, examine the circuit breaker to see whether the contact points are clean, and that they touch when the cam allows them to; touch the circuit breaker lever to see that it is free to move and that its spring is not broken. In some tractors the magneto is in such a position that the circuit breaker cannot easily be seen; in such a case hold a small mirror in front of the circuit breaker and examine the reflection. If the circuit breaker is in good condition, examine the distributor to see whether it is dirty, or the brush broken; if these parts are all right, the trouble is of such a character as requires the magneto to be returned for repair.