Here are some mumblings about servicing some of the impulse couplers used by America Bosch. They were a welcome addition to the early direct coupled magnetos of the day with "Armstrong"(hand crank) starting. The impulse couplers retarded the spark at cranking speeds and also gave the armature a quick flip for a hot spark. They were well made and held up well. Over time dirt and debris can accumulate in the slot in the drive cup causing the impulse not to engage, springs can break, and rust and the usual crud can inundate the mechanism. Getting the coupler off warrants special attention. Always remember that once the coupler is removed from the shaft, remove the small half moon Woodruff key or it will broach a slot in the front of the magneto housing if the armature is removed. We use a pair of diagonal cutters or a small nail puller to grab it and tip it out. With a stubborn key one may use a small punch.
Here is some factory service information covering the most common couplers to keep yours alive and well.
There are two threaded holes in the impulse hub with 1/4-28 threads. To pull the hub, thread grade 8 bolts tight into the holes and usually the hub pops off. If not, tighten the bolts careful not to over tighten and strip the threads and give the end of the armature shaft a smack with a brass drift or non marring hammer. . Remember to remove the Woodruff key before removing the armature.
When disassembling, wash all moving parts of dirt and gunk and re lube with a light oil. Polish any rust on moving parts with fine emery cloth so the pawls can move freely. Be careful not to let grease get on the pawls as it may prevent them from falling into the slot and engaging. As a rule, when remagnetizing a magneto, do so with the impulse coupler removed. Not as big a problem with these as others but if parts of the coupler get magnetized the pawls will not engage.
Always wash out the slot in the bottom of the arrestor plate like Figure 3. If debris and rust accumulate there, the pawls will not fall down and engage. Watch for wear on the edge of the arrestor plate. Excessive wear can cause the pawls to slip over the edge of the slot and not engage. If this happens, replace it but save the arrestor plate as if it is a right hand plate, the wear will be on the left and no wear on the right side of the slot. it can be used as a replacement on a left hand mag and vice versa. Arrestor plates for base mounted and flange mounted mags are not interchangeable as a rule. More on these later.