Thanks. When visualizing spark to time an engine, my default is to pull the lead off the plug, and arrange it so when the box fires, the lead arcs to the engine head. I set it up with maybe 1/4” or less gap, and that’s always given an arc.
It is quite simple to wind some thin (bare) copper wire around the spark plug base when it is out of the engine and then to earth it to the engine frame. Beats blowing up the ignition box for sure AND it leaves you with both hands free to make adjustments if needed. You should get a healthy crack from the spark plug so connected, so you know when it is firing.
As an aside here which won't apply to most of you, when my box died and I was making a new Jan Ridders type system, I had to cut the two wires down inside the base which came from the encapsulated reed switch down by the flywheel (the early blue painted model had a Hall Effect sensor down there, but the later models have a reed switch). it was very easy to time the spark by connecting a multi meter on a resistance setting to the two sensor wires. As you rotate the flywheel back and forth the resistance suddenly goes from infinite to zero. If the multimeter has a continuity buzzer, the timing job becomes even easier since the tone sounds just as the engine would fire if was running. Anyway, this made it very easy to time too.
TDC seems a good place for the spark. These engines are so slow running that no amount of advance on TDC should be needed.
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