My boat tach just quit. I have a 115hp Johnson '77. An OEM replacement
goes for about $130.

There are automotive units commonly available for down around $30, and I'd
love to be able to use one of these el cheapos if it will work.

Most auto tachs have a switch on them for selecting 4/6/8 cyl use. I think
putting the switch on the 8cyl setting (4 cycle, of course) would cause the
scale to read right for my 4 cyl 2 cycle boat engine.

Both tachs seem to be triggered by a 12v pulse from the electrical system.

If I'm right on the above, then it seems I can get away with using the
automotive tach. Thoughts/comments...