creeping corrosion.
When you setup an ignition system, and all the parts are new, shiney, clean, bright and tight, the resistance issue for the primary is more of a range than a fixed number. Once you hit enough primary resistance so that the coil doesn't get thermally damaged, and that you are not damaging the points, then you can run.. but.. that magic number can be a pretty significant range. it could be 2.85 ohms to 5 ohms.. depending on the coil ( and yeah.. chinese made 12v coils are ALL OVER the place in terms of dc resistance on their primary... it's hard 2 buy 2 from the same manufacturer and get the same reading... I've checked! ).
Now.. what this means is that 2 running systems side by side may have 2+ ohms different in their primary resistance. both make spark and run.
If one has a perfectly tuned carb, and good clean connections.. it's spark could be a little weak but still run.
Add in a year of setting in a barn.. points surface area decreases a bit from micro pitting. and an oxide layer forms, coil that was made in some sweat house had a few more or less winds than the next coil, and some of the wire insulation on a few turns were marginal, so you have some shorted turns after a couple years of heat cycles... bat cables are only 70% battery has aged, nominal coltage is down .02 vdc. wire connection on top of coil is getting some corrosion on the brass contacts.. the pigtail setting in the points concave screw has lost some copper wash and it's surface contact resistance has gone up .5%, the air gap at the distribuitor cap posts and the rotor has widened 2$ from arc jumping.... etc, etc. Add all that up and you have a machine that was running with too much resistance .. but 2 years later.. enough 'stacked' tolerances' added up and suddenly that magic 'range' was exceeded and it went from running to non running. Removing that resistor that wasn't needed thru it back into that magic range.
