RPM's from outboard?

[J88GTM]

Hello all,

Last few days I've been helping my friend do up a little speed boat, today we're fitting the gauges, so far we have speed, volts and fuel, and we are wondering if theres anyway we can get a RPM gauge going

The outboard he is using is a Mercury 40hp 2 stroke, is it possible to take a feed from the wire comming off the magneto that tells the plug to spark?

I am only asuming such a wire is on the outboard mind, I dont know if this is how the ignition works!?

I've had a qwick look for a kit to do it but not managed to turn much up...

The outboard is fairly old, and the only actual output is a 12v socket on the side I'll point out, I'm guessing the bigger outboards have dedicated take offs on them especially for this job.

Many thanks for looking!

 

[steve66]

You can get rev counters that pick up rpm from plug leads, just a matter of wiring it up to a plug lead then telling the unit how many cyliners you have got. Hope this helps

 

[Spi D]

Or go the universal way:

Just a heads up on universal tacho's: They're cheap, reliable, simple to use and will count rpm on any rotating object.
You put a reflective (or white) tape marker on the rorating item, point the laser and get the reading. No wires, no cables.

I've used it for several tasks including calculation of gear ratio on a Merc Alpha drive

 

[Davy_S]

Google Tiny Tach.

 

[Spi D]

Google Tiny Tach.

Problem being that the built in battery can not be replaced so once flat, you need a new instrument..

TINY-TACH™ unit is potted to prevent damage from moisture and vibration. Lithium battery life 5+ years, not replaceable.

And you still have to mess with wires...

 

[Davy_S]

And you still have to mess with wires

Your solution would be presumably, dab a but of paint on the flywheel and run the outboard motor at full chat without cowl to test the rpm propellor combination? My outboard has no rotating parts that can be viewed easily.
If the OP wants a continuos tachometer for everyday use then a factory type or tiny tach fits the bill.

 

[Rivers & creeks]

Mercury units sometimes come up on ebay but expensive. Tiny Tach works well, easy to fit and lasts at least five years.