Three wires plug into the light box.  [Figure 4-42]  There are  two lights on the front face of the unit, one green and one red,  and a switch to turn the unit on and off. To use the timing  light, the center lead, which is black, marked “ground lead”  is connected to the case of the magneto being tested. The  other leads are connected to the primary leads of the breaker  point assembly of the magnetos being timed. The color of the  lead corresponds to the color of the light on the timing light. With the leads connected in this manner, it can be easily  determined whether the points are open or closed by turning  on the switch and observing the two lights. If the points are  closed, most of the current flows through the breaker points  and not through the transformers, and the lights do not come  on. If the points are open, the current flows through the  transformer and the lights glow. Some models of timing lights  operate in the reverse manner (i.e., the light goes out when  the points open). Each of the two lights is operated separately  by the set of breaker points to which it is connected. This  makes it possible to observe the time, or point in reference to  magneto rotor rotation, that each set of points opens. Most timing lights use batteries that must be replaced after  long use. Attempts to use a timing light with weak batteries  may result in erroneous readings because of low current flow  in the circuits. When replacing or preparing a magneto for installation,  the first concern is with the internal timing of the magneto.  For each magneto model, the manufacturer determines how  many degrees beyond the neutral position a pole of the rotor  magnet should be to obtain the strongest spark at the instant  of breaker point separation. This angular displacement from  the neutral position, known as the E-gap angle, varies with  different magneto models. On one model, a step is cut on  the end of the breaker cam to check internal timing of the  magneto. When a straightedge is laid along this step and it  coincides with the timing marks on the rim of the breaker  housing, the magneto rotor is then in the E-gap position, and  the breaker contact points should just begin to open. Another method for checking E-gap is to align a timing  mark with a pointed chamfered tooth.  [Figure 4-43]  The  breaker points should be just starting to open when these  marks line up. Checking the Internal Timing of a  Magneto 4-25 Figure 4-42.  Timing light.  Figure 4-41.  E50 Magneto Synchronizer. Figure 4-40.  Difference between top center and top dead center.  A B Crankshaft moves about 8° without moving piston Top center Top dead center