Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Here's my next attempt.

The black side of the Reed switch contact has current or at least continuity to it. The white side does not but will once the contacts touch thus sending whatever current the black lead carries back through the white wire to wherever it goes. If the red lead went to the opposite side of the Reed switch, bringing the points into contact would simply create a direct short, which I suspect would not be good. Still, the polarity of the red lead must go somewhere useful, or you cannot have any current flow through the system.

Now I must wonder, what polarity from the battery pack is ground, and is that the same as the black lead going to the Reed switch?

I suspect that you should want the polarity of the ground and the live wire to the Reed switch to be opposite polarity so that then the power transmitted to the white lead by the contacts in the Reed switch would then be able to power the spark against the grounded spark plug.

Hope any of that made sense?

One test you may want to make is to take your spark plug out of the head, but hold the threaded part against the hole in the head where it mounts to ground it, then try touching the red wire that is cut off at the Reed switch to the top of the spark plug and see if that gets the plug to spark. If it does, then the black lead to the Reed switch is the same polarity as your ground so that it would seem that the lower arm of the Reed switch needs to have the red wire soldered to it, instead of the black wire ..... maybe.

Perhaps a safer way to test this is to use your voltmeter to check the voltage from the red wire to ground on the engine, and then also test the voltage of the black wire to ground on the engine and see which if any give you a voltage reading.

If neither the black wire nor the red wire provides a voltage reading when the meter is used to ground them against the engine, then I would think that your "coil" or "magic box" is the problem.