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ok so to be clear about your question: you are trying to hook up an electronically driven tach to your 1980 CB750C which uses a cable drive for the stock tach, right?

Couple things you need to know: your cb750C uses a "dual wasted spark" ignition, meaning the coil fires paired cylinders (1-4, 2-3). This is different from a traditional wasted spark which fires all cylinders. You need to know how that tach is setup to run, is it for a 4 cylinder? a wasted spark setup?

Assuming the tach is compatible with what you want to do, here is someone's write up (read Reply #5):

Basically since you have a 4cyl bike with paired coils, you'll need a signal from both so you have to split the black with yellow line to take a feed from both coils.

Since that diagram you posted is for a car tachometer (bikes don't have line lock) I am going to go out on a limb here and say it ain't gonna work for your application at all. Remember most cars use a single coil and a distributor so the tach compensates for the multiple signals for the number of cylinders.
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