Ignition circuit on an old motorcycle

In January of this year I bought an old motorcycle, a 1946 Harley Davidson WL. This is a civilian model of a motorcycle that HD made several hundred thousand of during WW II. The engine has two cylinders displacing 45 cu. in., in what they call a "side-valve" configuration, what I've always called a flathead.

Being that this forum is concerned with electronics and such, I thought that some readers might be interested in how the ignition circuit works on this old beast.

On the right side of the engine is a component that looks something like a distributor, but unlike most car distributors, there are no spark plugs attached. The unit has a set of points and a capacitor inside, just like older car distributors, but unlike them, there are no spark plug wires attached to it. It's also referred to as a timer, or circuit breaker, rather than a distributor.

The ignition circuit is pretty simple. The primary circuit consists of the the battery, coil, and timer. The secondary circuit consists of the coil and the two spark plugs. When the engine is running (or when you're starting it), the timer rotor turns, and its two lobes cause the points to open and close at specified times related to crank and valve position. When the points close, current flows through the primary circuit, which induces a current in the secondary windings of the coil, and across the gaps of both spark plugs. When the points close, the electric field in the coil collapses.

By firing both spark plugs whenever the points close, they didn't need to distribute the spark to an individual plug, and this is why there aren't any plug wires coming out of the timer. The disadvantage of creating a wasted spark for a cylinder that isn't ready to fire is a weaker spark on the one that needs it, but hey, that's the way the engine was designed.

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