Testing an ignition coil properly: the gap, the kill wire, and the $8 tester

An inline spark tester ($8) between the plug and the boot shows spark while cranking under compression, which is the only test that counts. No spark: disconnect the kill wire from the coil and test again. Spark now = the kill switch, bail cable or wiring is grounding the coil. Still none = coil. Before replacing it, check the air gap between coil and flywheel (business-card thickness, about 0.010", set by loosening the two bolts with a card between) and clean rust off the flywheel magnets. Coils fail hot more than cold, so a "dies after 10 minutes, restarts when cool" symptom is a coil even if it tests fine in the driveway.