To help explain [something I once explained long ago, but seems to be gone now?) how the spark jumps the sparkplug, I snatched this (not real clear) picture of the oscilloscope trace of the spark across a plug that is running on my test bench where the Transistor Ignitions get their burn-in testing when they are finished being built.
This test-bench setup uses stock Honda 4.3 ohm coils, switched with a square-wave ON-OFF drive at (usually) 5500 RPM equivalent (about 92 Hz) like points would do, with 0.24uF polyfilm capacitors acting as the condensors, driving the coils at 12.9 volts DC on their Black/White power wires. The sparkplug caps are the NGK 10k ohm type, and the plugs are D8EA NGK plugs. (I use the 10k caps because they last the longest in this rig).
The peak voltage of the spark oscillations is about 8000 volts, dropping about 110 volts on each of the 10 oscillations before the discharge back & forth across the electrodes of the sparkplug wears out (that's the tall, sort of fuzzy lines on the left of the trace) and drops down to the smaller waveforms, which are not jumping the plug gap. These oscillations are going back-and-forth between the coil and condensor as the condensor dampens down the residual power. In purely points ignitions, these latter oscillations are made much smaller by the power used up when the points arc while running, so those smaller oscillations on an actual bike are much smaller in real life.
There isn't just one spark jump: there are several. As the resistance of the plug caps becomes less (like 5k ohm caps), the tall 'wiggles' on the left side of this screen become slightly less tall (lower voltage) and there are fewer oscillations overall. The least number of oscillations occur with 0 ohm sparkplug caps, or with Dyna's 3-ohm coils, where the voltage is higher (clear off this screen) but there are only about 5 of them, a much shorter spark duration. This phenomenon occurs because the current passing between the plug's electrodes is higher with the lesser plug cap resistance, and/or from the Dyna coil's discharge time being very short. Back in the day, some racers tried using 10k ohm caps with the Dyna 3-ohm coils, only to receive short life from the caps when the higher voltage burned up the carbon pellets in the plug caps(!), not an ideal arrangement.
My test bench is set up to run between 4.5k and 6k RPM at the above fixed-parameter test so as to maximize the heat in the Transistor Ignition: it runs hottest at this RPM, so that's my burn-in target for each one. They get 1-6 hours of this, depending on what else I might be doing that day.
Years ago I had a thread where I posted pictures of these traces when developing the Transistor Ignition, and they had more scenarios than just this one-speed, one-resistance setup (and better pictures with the special camera I had back then, now gone).
