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Old 04 June 2009, 13:18   #11
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Ok, for reference you've got an 11.25" diameter and 13" pitch prop. (Google conversions are my friend-Inches are so much easier! )
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Old 05 June 2009, 08:18   #12
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quick lesson in alternators....

Alternators work but using a wonderful law of physics that essentially boils down to if you move a bit of wire fast enough through a magnetic field, it will generate current.

Under your flywheel are attatched a number of magnets. Usually 6 or 8, occasionally 4. Coils (usually 2) are fixed under the flywheel and are sited so that the magnets on the flywheel pass very close to the coils, and so the coils are effectively moved through the magnetic fields (the magnetc fields move past the coils in reality) to generate voltage in "spikes" as the magnets pass the coils. Somewhere under the lid of your engine is a thing called a rectifier which essentially takes these 2 sets of spiked voltages, adds them together & flattens out the result to give you a steady (ish) DC voltage to charge your battery.

The Tacho is hooked up to one coil under the flywheel, and so it "sees" a voltage pulsing as that coil is passed by the magnets under the flywheel. As the engine speeds up, more pulses happen in a given period in time. At a given RPM, the number of pulses per minute can be worked out, and by counting them, the electronics inside the tacho moves the needle to that point on the scale.

So, as an example, if you have a 6 pole alternator, each full turn of the flywheel will generate 6 pulses. If your tacho is set to "4", it thinks theres only 4 magnets, so will be expecting 4 pulses per turn, and so if you have a 6 pole alternator, it thinks you are going faster than you actually are, because the tacho just counts pulses per second. Likewise if the tacho is set to 8 , it will see the 6 pulses and think the flywheel has only done 3/4 turn, and will drop the rpm indication accordingly.


Most 4- stroke engines will idle around the 800 - 850 mark, but check your manual - it will tell you the exact number.
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Old 05 June 2009, 14:38   #13
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Thanks 9D280, strange name? but this it would seem may be the solution! The rev counter sits at about 1300 rpm at tickover, a tad too high listening to the engine which is probably at a guess a little more than half that! I will pull the tacho out of the console when I'm next off and see what adjustments it has. My Honda owners manual doesn't seem to quote a tickover but gives a full throttle revs of 5500 to 6000 rpm Thank you!
Nos4R2 any comments about the prop size?
Any one any ideas on the best form of a knotometer....
Steve
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Old 13 June 2009, 07:53   #14
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9D280 do you by any chance have knowledge on the Honda Tachos? I have taken it out of the console it is ZV7-912 and I can find them on Google but nothing about altering them! is the "switch" system likely to be inside the case? I am reluctant to take it apart! without the knowledge that it is altered there.
Thank you!
Steve
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Old 15 June 2009, 06:03   #15
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Sorry, Steve, never had a Honda one in pieces!

I have a Suzi one, which has a screwdriver operated switch accessed through a hole on the back. Teleflex ones are similar. Does the back look like it comes off easily? It may be they decided to seal it properly to reduce condensation or something......

I'll see if I can find a pic that shows something obvious. I'd be very surprised if it didn't, but then if all Honda alternators have the same number of poles, they may not have bothered with the selector. Mind you that theory is instanly killed by the fact you have a Honda engine!

1300 indicated idle sounds like it's in the wrong place. (I had a "1300 rpm" reading when I first put the Suz tacho on the Yam (it also went off the scale at WOT!), one turn of the selector & it read 800 idle / 5400 @ max.)

On the assumption you have no handy access holes for a small screwdriver, do the electrical connections look like they are sealed by a rubber pad? I am thinking that the entire back of the tacho might be a bayonet fit to the front bezel? (I.e front access) What do the instructions say about changing the bulb on the light inside? You may find the adjuster by going in through the front or the lamp fitting at the back?
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Old 16 June 2009, 02:46   #16
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Thanks, major effort on your behalf and much appreciated. I have tried opening the clock but having undone the nuts on the back and loosened the unit inside the front of the bezel is well and truly sealed. Unfortunately I just dropped my camera off the back of my other Honda! never to be seen again or I would send you some pictures. I have tried writing to the company that supplied the outboard but they are strangely very uncommunacative!
There is a local dealer about 35 miles away so I think I'll drop in and have a chat with them. The tacho is a lot more expensive than I thought so no chance of a new one! Thanks again for your help
Steve
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Old 16 June 2009, 04:07   #17
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Steve,

If you have no joy with them and don't mind your dash looking a bit eclectic, in theory any pulse counting tacho should work (Mine has a Suz badge on, and has indicated both Yam & Merc rpms since it was connected to the Suz)

A thought that did occur to me - is your engine a really new fuel injected one? If so the tacho signal might be generated by the engine management box rather than driven straight off the alternator poles, in which case you probably will need the dealer to look at it.
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Old 16 June 2009, 04:42   #18
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Forgot to add - found this on Iboats.

http://forums.iboats.com/showthread.php?t=262285

Implies if they are both branded Honda, you shouldn't have this problem........


Have a trawl round Iboats, the descriptions on the other threads there seem to hint it's a fairly standard setup, (i,.e not digital) even if driven form the ECU. Worst case if you do have to go for an "aftermarket" tacho it looks like connecting it to the alternator shouldn't be too difficult. (apologies for the vagueness there, I've not had the lid off a Honda yet)
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Old 16 June 2009, 05:44   #19
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Quote:
Originally Posted by 9D280 View Post
Implies if they are both branded Honda, you shouldn't have this problem........
the manual for my "fits anything" tacho - would appear to disagree depending on age: http://www.faria-instruments.com/sit...ls/IS0086p.pdf
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Old 22 June 2009, 14:26   #20
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Fitted a few hondas, never had this issue with the tacho, they worked straight off the bat.
My old yam has a pole selector on the back as do the quicksilver etc.
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