Mercruiser 5.0L rpm, Crownline manual and water in bilge!

[GrahamHR]

It's not got a replacement rev counter, has it ? Marine rev counters for petrol engines usually have a rotary selector on the back, so they can be used with 4, 6 or 8 cylinder engines.

Reading 1600 rpm at idle (double 800 rpm) sounds like it's set for a 4 cylinder engine (or maybe a 6 cylinder if it's idle speed is fast, at a real 1200rpm. Possibly that the selector is between the 6 and 8 cylinder stops; they often have to be pushed in to adjust them)

The signal for petrol rev counters for petrol engines is from the ignition circuit, not from the alternator.

Graham

 

[Spi D]

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