I have two schematics for this project. This one is the more complicated but more robust version, and the one on the next page is a simplification by Bill Yacey.

The circuit works by rectifying and combining the two primary magneto lines. Typically these are both routed up to the dash to the keyswitch where they're used to stop the engine. Since together they'll produce two pulses with each revolution of the engine, they can directly drive an automotive or inboard tachometer setup for a 4 cylinder, 4 stroke engine. This eliminates a complicated and sensitive pulse multiplication stage. It also has a further advantage as the typical inboard tachometer is only scaled to 6k rpms, which is more useful for older outboards.
The three primary design goals of the black box were construction simplicity, insensitivity to noise and battery voltage, and reasonably low power consumption.