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The MJ series appeared somewhere in the 1930's and essentially brought in a modern age design with fewer moving parts, more reliable and less mainteinence. They were and are very robust and reliable ignition equipment. They employed a stationary coil and stronger magnets. They typically outlast the engines and will probably last forever with proper service and storage. They just plain work even when not in good repair.
The MJ series as the MJA, MJB, MJC, MJK and to some extent the MV... shared many service and repair procedures as well as coupler configurations. The couplers used by American Bosch were robust and reliable despite years of neglect and abuse. There is a synopsis to follow of the various couplers used and information on setting the lag angle with the base mounted devices.
There is also a brief discussion of edge gap that seems to baffle many souls and cause much grief. Recall that edge gap describes the optimal time to open the points (breaking the electrical) circuit after breaking the magnetic path between the magnetic rotor and stationary pole shoes. MJ series magnetos have adjustable breaker plates to allow for optimizing edge gap once the points clearance is set for maximum output. For devices with variable timing, as a rule the optimal edge gap is set with the timing at full advance. Improper edge gap will cause a wimpy spark.