It'll work..............On your single ig. coil 3 cylinder 2 stroke, hook the new automotive tach sig wire to the boats "-" negative coil wire.(think it's the white wire on your CD unit or at ig sw.) Your 3 cylinder 2 stroke boat coil has 3 pulse/rev (3 cylinder, 2 stroke =
3 pulses/rev).
Once hooked up, be sure to set (as you mentioned) your new 4 stroke automotive tach to "6" cylinder (4 stroke, 6 cylinder also =
3 pulses/rev), you're good to go and seemed to have already knew your answer....:laugh:
FWIW, and as you probably already knew, if that type of motor (single ig. coil) was a 4 cylinder (4 cylinder, 2 stroke =
4 pulses/rev), just set your tach to "8" cylinder (8 cylinder 4 stroke =
4 pulses/rev)............
BTW, analog tach's (front end) such as the one you bought, for the
generality of a pulsed signal derived from inductive sources, manu's/designers will internally de-coulple the input signal through an ac coupling cap (block DC) and/or also limit current typically into 100K ohms or greater to ground. It would be next to impossible to hurt any ignition system/coil, mag's output or alternator's tach output (low voltage sides) connecting up these type tachs, pulse matched or not.