The FMJ series was very popular and widely used. They were offered in 1,2,4 and 6 cylinder, base mount, flange mount and vertical configurations. This bulletin is for the FMJ4A and FMJ4B series. It is a modern serviceable magneto.Here is some information to share by kind permission from Fairbanks-Morse to keep yours alive and well.
Dirty points are the usual cause of failure. If it is a no sparker, start here. Use fine abrasive paper or a stone to get the rough blemishes off then polist to a mirror finish by working the contacts over a flat surface with 400 or 600 grit paper. If the contacts are smooth, they will stay cleaner much longer than if left rough. Point files are much too coarse, remove too much material and leave a rough surface. If the old points still have meat on the contact surfaces, by all means use them. Put a dab of black Molybdenum axle grease the size of a match head on the cam. Condensers do not give much trouble. Always test for leakage first. A leaky condenser will overstate capacity in a capacitance test. if a condenser is leaky, replace it.
Coils of the day, especially the varnished tape wound variety suffer from the usual woes of sticky goo that finds its way onto the magnetic rotor. Modern replacements are readily available. If you need to remove the coil, be very carefull of the two setscrews on top holding it in. Too much screwdriver torque and one side of the slotted head will break off and we have a problem . If the setscrews do not come quietly, heat the top of the housing with a propane torch, carefull to aim the flame away from the inside parts. The nonferrous housings will expand more than the steel setscrew and also release a thread keeper placed at the factory.
If the distributor end cap screws are stuck, add some BTU's with the propane torch. Point the distributor enc cap downward while heating to keep from overheating it and the screws nearly always let go.
These are good words below. Before assaulting the magneto, first check the spark. It could be a fouled plug, improper resistor wires or resistor plugs, and/or all of the below mentioned.
This s very important to be sure that when the points open (firing position) the rotor is pointed at a spark plug terminal post. The small steel gear usually is beveled and/or painted red and should be pointed with the witness mark straight up. If the markings on the fiber distributor gear are gone, point the rotor at the raised ridge (Timing Boss) inside the distributor cap/coilcover and slide things it all together. You may need to turn the rotor slightly to engage the gear teeth. If so, turn the rotor slightly the opposite direction it normally rotates. For a clockwise magneto as most are, turn the rotor slightly counterclockwise. The converse is true. When replacing the distributor cap/rotor cover, make sure that the brass rod is making contact with the springy tab (High tension terminal) on the coil when the screws are tightened.