This question makes me feel pretty dumb because I think I should know the answer but....nope!
All I do know for sure is that 4 stroke engines fire the cylinder every other revolution. So, the tach's "scale" for a 4 stroke engine would have to be ×2 per pulse as compared to a 2 stroke which fires every revolution.
I'm surely missing something here but.it seems to me that it shouldn't matter how many cylinders because an ignition pulse on a plug wire occurs once every two revolutions and whether it's one cylinder or twelve, rpm is the same for all cylinders on the crankshaft.
It shouldn't even matter to an inductive pickup which cylinder it's attached to because the pulse intensity for each spark plug wire should be near the same.
While I realize each of the three coils on that outboard have different part numbers, I believe it's because of the slight difference in plug wire lengths.
One thought is that the tach you purchased is meant to be used with the 2 cylinder and 4 cylinder engines that have what's known as "waste spark" ignition systems that fire 2 cylinders with 1 coil.
For example, the older Honda 8hp 2 cylinder engine uses a single coil with 2 plug wires attached. When one cylinder is firing, the other cylinder is also receiving spark at the same, but wrong time.0⁹
The older 90hp Honda uses 2 coils to fire all four cylinders. Again, each coil has two plug wires attached and as the coil fires one cylinder of the engine, the other cylinder that coil is attached to also gets a "wasted spark" from the coil because that cylinder isn't yet ready to fire the mixture.
The BF25D uses one coil per cylinder so the tach "scale" for counting revs is different from those other 2, older engines.
Sorry for all the words and no clear answer but you caught me thinking out loud.
