Just wondering if I can put a tachometer on my 1962 Johnson 40 hp motor? if so how is it done? and does anyone know what the WOT rpm should be for this motor?
Buy the tach. Instructions come with it. Automotive tachs will not work. Get the tach right here at iBoats or any store that sells marine accessories (Cabela's, etc). There are four wires. Ground, switched +12V, light, and sense. Sense is pulsed line from the charging system.
The wiring color scheme has been the same since at least the 70's; it might have been different before that, but try this...your control box should have an accessory plug. the plug should have 3 sockets...top one should be wired to a gray wire that gets the alternator output, it's the tachometer wire...bottom left should have purple wire that gets 12V switched (accessory power), connect the gauge lighting terminal to that for illumination (or connect it to the nav light switch)...bottom right should have black wire (engine ground). No accessory plug, look for a gray wire in the wiring harness.
No gray wire...check the wires with a voltmeter with the engine running. Tach wire will be alternator output, accessory power will be battery voltage. Or get your hands on a wiring diagram (seloc or clymer's has it, better yet get a factory manual). Or, try this: www.tinytach.com
Those tachs they sell are for STATOR equipped outboards and you have a MAGNETO. The only tachs that work are old ones from the sixties that are specifically made for Johnson/Evinrude 2 cylinder outboards with magneto ignition. No modern tach currently made will connect up.
An alternative is to use a tach that runs off a spark plug wire pickup like a tinytach from tinytach.com
If you're handy with electronics, we've worked out a couple ways of connecting a 4 cylinder inboard tach to these engines. If you want to know more about it, just ask.
WOT RPM is 4000-5000 RPM, but they seem to like around 4500 the best.
