BSA Bantam - Homemade 12V Ignition System

When I bought my Bantam Trail Bronc it was, lets say, a bit rough:

50 bucks, but the tetanus was free!

At that point, maintaining originality wasn't my concern. Just getting it running again would be a big enough hassle, so I left it mostly as is, and gave it a full engine rebuild and some new tires.

I won't get into the details on the engine build today, because that could take up several posts. I could write for pages about that job, so instead, I'm gonna focus on the modernized electrical system that I installed.

The original system was a low output flywheel ignition coil. I replaced all the bits with aftermarket parts, and was able to get the thing running:

BSA Bantam First Start

And then, after that? Nothing. The bike never started again. It sat in my garage for a year while I rechecked, modified, and adjusted every possible part. I never suspected the coil because, well, it was brand new, and the plug sparked whenever I held it against the case. Then, 3 weeks ago, I tried the same process yet again, holding the plug against the case and kicking it over. Amazingly, at that exact moment, the coil which had apparently been internally shorting all along completely died and produced no more spark. Eureka!

It turns out that if the output is too low, a plug that sparks freely when held in open air may not necessarily spark inside the engine, where the gasoline wets it down. Before rebuilding, I wanted to precisely set the spark timing, but do away with the need for a timing disc. I modified a used spark plug with an adjustable screw and a soft aluminum cap and then inserted into the plug hole.

I carefully turned the crank until the piston hit the plug, and drew a reference line on the flywheel aimed at tab on the case. I then repeated this in the other direction, and punched a reference line between the two pencil lines. This marks top dead center.

 Next, I marked the location of the proper firing position, about 16.5 degrees before TDC, and marked that, as well as labeling it.

Next up was the electrical bits. I stripped out the coil and magneto core, and reused the stock points and condenser with new wiring.

I used these to trigger an aftermarket 12V Yamaha ignition coil that I mounted next to an original Bantam battery tray. I wired it all up and used a miniature hobby 12V battery to power the whole system.

Now it will reliably start on the second or third kick. I will have to take some video as soon as possible.