Percolation? Heat soak? Don't drive during summer??

yellow rose said

That I understand. But the green wire doesn’t actually trigger the firing. The switch (points, light or magnet) does that.

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Correct. But the positive side of the coil gets ignition voltage, if a poorly routed wire grounds the negative post of the coil and then disconnects the coil will fire.

But you're right, it wasn't causing the "coil to fire prematurely" it was causing the "coil to be constantly energized wit a poor connection resulting in a constant barrage of sparks".