Most (all?) off the shelf dashboard tachs are made for either 4-stroke battery/coil based ignition (inboards or cars) or pickup from the alternator:

http://faria-instruments.com/site_manua … meters.pdf

And small old outboards are magneto based ignition, and lack alternators.

There’s a circuit out I’ve seen out on the web to adapt a 4-stroke battery ignition tach to use with magneto based outboards, but it required a 12V DC input.

In the 1960’s tachometers were made for magneto based outboards, but they’re hard to find (usually they had a meter and a "transmitter" or a special pickup which is often missing). Complete new old stock tachs are sometimes seen on ePay for $$$ (and you have to get the right one for your brand of outboard); Often what you see is just a meter (movement) from the dashboard, and the transmitter box has been lost.

I picked up an old airguide tachometer and transmitter on ePay, but it was flakey (loose connection), so I painfully extracted the circuitry (it was "potted" in epoxy and aquarium gravel to make the job hard), with the hope of reverse engineering it, and spent a month tinkering, but didn’t succeed. Then I started playing with making a transmitter for old tach panel meters using a tiny "AVR" microcontroller, but making something that people could easily adapt to their meter was daunting…..