Checking the Internal Timing of a Magneto

in Engine Ignition and Electrical Systems

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.


Figure 4-43. Timing marks indicate the number one firing position of a magneto.

Figure 4-43. Timing marks indicate the number one firing position of a magneto.

In a third method, the E-gap is correct when a timing pin is in place and red marks visible through a vent hole in the side of the magneto case are aligned. [Figure 4-44] The contact points should be just opening when the rotor is in the position just described.

Figure 4-44. Checking magneto E-gap.

Figure 4-44. Checking magneto E-gap.

Bench timing the magneto, or setting the E-gap, involves positioning the magneto rotor at the E-gap position and setting the breaker points to open when the timing lines or marks provided for that purpose are perfectly aligned.