Though I haven't seen any bikes with this mod, it became quite the rage back in late 1975 / early 1976 (or thereabouts) when Cycle magazine printed an article by the late Gordon Jennings showing that cheap (~ $9) K-Mart replacement automotive coils (for GM engines, IIRC) provided MUCH more spark than the typical weak-ass motorcycle coils of the day.
As I recall, Jennings was able to get a high-voltage spark to jump a large gap (maybe 60 thousandths of an inch?), when the stock motorcycle coils (Ducati, I think) gave a very weak spark, and it didn't work well much beyond ~ 20 thousandths of an inch... Keep in mind, I'm working from a piss-poor memory here, but I remember re-reading the Cycle articles (in two parts, IIRC) in the summer of 1995. Of course, I first read those articles back in the day, and I've always credited Jennings with motivating the OEMs to switch to high-voltage ignition systems in the late '70's / early '80's; without his research exposing the pathetic ignition systems of the day, and proving that a simple solution was available, we'd probably STILL be using that Flintstone-era junk he complained about so bitterly...
Keep in mind that most cars use only one coil for the engine; the distributor sends the spark to each plug as needed. Modern motorcycles don't use distributors, so they require one coil per cylinder, or coils that can supply spark to two plugs simultaneously... which is why modern four-cylinder motorcycle engines tend to use coils with two spark leads; that way, two coils can provide the spark for four plugs...
Hope that didn't confuse anyone; it's really pretty basic, if you understand basic automotive ignition systems... This is NOT as simple a swap / mod as it seems; there are motorcycle ignition systems that can be DESTROYED by making this change, so if you're really interested in this subject, I urge you to do the research and be SURE that the changes you make won't ruin your entire electrical system...
