1st id like to talk about some things that are idiosyncratic to the f-m mag.
understanding these differences (compared to a battery system) usually helps in keeping your bike starting and running right.
in a battery system the juice for starting comes from a storage battery. a mag has no external electric supply. it needs to make its own electricity.
there is a lot going on in a mag that isnt going on in a battery system, the biggest difference is that your mag reverses magnetic polarity during each rev of the rotor.
in the battery system the flow of current is always in one direction coming from the battery. (this is a generalization as a capacitor flows current in and out during each spark cycle). as far as breaker points are concerned the major juice flow is always in the same direction. that tends to erode the metal from one contact face and deposit it on the other. = pitted points.
if you reverse battery polarity the transfer also reverses = the pit is on the other point face now.
one of the jobs the capacitor does is to 'soften the hit', to keep hi temperature arcing (and the subsequent metal transfer) across point faces as they break to a minimum.
got that?
now to the mag-
because it needs to make its own juice, it needs a coil to turn magnetism into electricity. this happens in the primary side of the mag coil.
as you feed magnetism of a given polarity into the coil, you get current of a given polarity out.
when you reverse (each 1/2 turn of rotor) the magnetism into the coil you also reverse the current flow out of the coil.
that means the flow across the point faces reverses in a mag this keeps pitting to a minimum.
and
it also reduces contact face temperatures as arcing is reduced by less (or no) pits.
low contact temperatures can "foul' points. seen as black (oil based) or white/grey (water based) residue on contacts. once points are dirty pitting is gonna follow. and so is hard starting.
dirty points dont come from capacitors, they are from dirty mags. or low purity contact faces.
when you get capacitor faiure on a mag you will get:
1) no spark (capacitor is shorted)
2) high rpm misfire (broken capacitor mount or an 'open' capacitor with reduced capacity)
if you have a good proven capacitor i seal the wire opening with sealer to moisture proof it. it should last for decades then.
personally i dont use the foil/paper caps i use foil/mylar. the fully epoxy encased units being bullet proof.
these were commercially available in the late 70's. i have no idea where to find them now.
ive been running them,
with no change out, since then.
in closing
your mag is a
a/c permanent magnet altenator that suppies ac spark output with the frequancy of the rotor rpm.you really need to understand ac electricity and apply that knowledge to you mag building and operation. dc knowledge is a poor fit here.