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Old 22-01-2008, 01:03   #1
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measuring revs

I have replaced the Tacometer (rev counter) on my Nanni 62hp engine,due to a scratched face. The new instrument is reading significantly lower than the previous taco. I suspect the previous taco was reading incorrectly (too high), but the new one is also incorrect (too low).
How do I measure engine speed to calibrate the new instrument ?
Thanks John
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Old 22-01-2008, 05:31   #2
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You need an optical tachometer. You basically attach a sticker to the flywheel and use a stobe to measure how fast the engine is turning over. It's the same with a car. You cgte or borrow one and most auto parts stores. The tach usually has an adjustment perhaps both high and low. You fiddle with the settings until it's pretty close in the middle of the range you actually operate at.
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Old 22-01-2008, 05:40   #3
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You can get them on E-bay for about 1/2 the price. I bought one and it works great. Just search for optical tach. If you were on this side of the world, I would loan you mine.
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Old 22-01-2008, 05:47   #4
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Thanks for the reply.
I assumed a car optical tacometer got its flashing rate from the cars ignition and therefore would't work on a boat diesel, but I understand from your answer they have an internal clock to set the flash rate.
Thanks John
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Old 22-01-2008, 05:57   #5
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Think you might be confusing a timing light with a tacho. The timing light is fired by the ignition to illuminate the timing marks on the flywheel while the tacho light illuminates the sticker or paintmark on the flywheel and counts the flashes to give rpm. so the tacho light will work on a diesel.
Just a thought............. the 62 hp Nanni is 5 cylinder I think. Have you put a tacho on for a 4 cylinder engine? or if you've put the right one on now did it have the wrong one before?
Chris.
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Old 22-01-2008, 06:31   #6
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Dear chris
Thanks for the thought. The yacht has a 2 cyl nanni 12v generator and a 5 cyl main engine. The previos owner had an extensive linventory of spares including a complete engine panel. I took the taco from this it has the same part number , but may be switchable betweer 2,4 5 and 6 cylinders.
Time to pull the panel off again.
Cheers john
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Old 22-01-2008, 22:40   #7
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You won't like the answers...

http://www.foxriversystems.com/products.php?cat=23

Design Technologies. Hand-Held economical tachometers. Wireless tachometers.

In the second one look at tech tach TT-700

Basically you need an optical or mechanical tach. Or a tach that can pick up it's signal from the injection system.

Mechanical tach - If the engine has a mechanical port you will add the pickup, the wiring and the tach head. Usually picks up a one per rev from a gear tooth on the flywheel or something.

Optical Tach - basically you attach a marker dot to the flywheel. The head unit is a strobe as Pbalis describes but you "tune" the strobe to freeze the dot in the strobe light. The coresponding frequency converts to RPM.

Pulse tach - Basically the injection pulses disturb a magnetic field in the handheld tach. The frequency of the interuptions converts to RPM considering a 4-stroke cycle.

Probably the tach on your boat is sensed off the alternator. In this case the frequency of the DC power wave is measured against the known pulley ratio and RPM is determined. Changing pulley ratios, slipping belts, incorrect frequency (6 pole vs. 7 pole alternator) can all impact the accuracy read at the tach head.

Have you changed pulley ratios as well or something else along with the tach head swap?
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Old 23-01-2008, 02:10   #8
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Teleflex has instructions for various types of Tachometer at:
Teleflex Marine Teleflex Marine -

You can also contact their Tech Support (Gauges & Instrumentation) at:
jfedersel@tflx.com <jfedersel@tflx.com>
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Old 23-01-2008, 05:35   #9
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Thanks for all the replies.
I am currently in Scily and finding equipment difficult to get and post is slow and unreliable, but will try hunt out an optical tachometer.
I have adjusted it to read 850 RPM at idle so hopfully it should be approximatly correct.
Is 850 about the correct idle speed for a 5 cyl 62hp engine?
Thanks John
see my blog at wwwamzerzo.blogspot.com
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Old 23-01-2008, 07:46   #10
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I've got an optical tacho and in the early summer we're passing by on our way to Greece if you can wait that long. lol
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Old 24-01-2008, 01:42   #11
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Dear Capt hook
Thanks for the offer. Unfortunately I plan to leave Italy before then , but may stay in Greece on my way to Croatia. So we may bump into each other.
Thanks John
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Old 24-01-2008, 01:57   #12
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Don't miss Corfu or Paxos, that being said all the islands in the Ionian sea are worth a visit, or were 30 + years ago, either before or after you head for Croatia. Try the levkas canal as well.
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