Re: how to test if a coil is bad
The way these magneto systems work is extremely simple:<br />1) Magnet in flywheel magnetizes the iron core of the ignition coil, and sets up a flow of low voltage electricity through the primary of the coil, via the closed breaker points. This takes time.<br /><br />2) Breaker points open suddenly, interrupting the primary electricity flow. This collapses the magnetic field in the core. The collapse induces a current in the secondary winding of the ignition coil. Since the secondary winding has so many turns, the voltage is very high. This current flows through the spark plug (sparks).<br /><br />3) The condensor (which is an obsolete name for a capacitor) allows the breakers to open suddenly by absorbing the energy that would otherwise cause some arcing at the breakers' contacts. This also keeps the contacts from burning.<br /><br />The above is a bit of a simplification. There's a nifty feild reversal done on those three legged ignition coils. Oh, and shorting the primaries of the coils to each other kills spark in each of them. This is how the kill switch works.<br /><br />The normal failure mode for these coils is that cracks appear in the insulation around the coil, and it arcs over. Since spark voltage goes up with flywheel speed on these magnetos, sometimes a spark that goes to the plug at cranking rpms arcs over internally at higher rpms.<br />Another failure I've seen is where moisture creeps into the coil through a crack in the insulation, which effectively shorts them out internally. In my cheapskate past I tried to dry one of those coils out, aiming to reseal it. It did not work. <br /><br />You'll note that neither of these problems can be diagnosed with a multimeter. You would need a hi-pot tester to find the leak.<br /><br />It seems to me that the pink/red insulation coils are the worst for this. Although the green insulation coils can be bad too. If they're original coils, they're almost surely bad. Since new ones are pretty cheap (~$30 each OEM, ~$15 each aftermarket), they're worth replacing even if you simply suspect them. Never just replace one - the other is not far behind.<br /><br />Make sure neither of the (low voltage) wires coming from the armature plate are shorted to ground anywhere. If one was, it would kill the spark on the associated cylinder only.<br /><br />I hope this helps!
