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Outboard tachometer technical question...

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Outboard tachometer technical question...
Ok, question because just having something that works isn't good enough, I need to understand why...

I have gone through several tachs for my Yamaha F40 (3 cylinder, four stroke) over the years. These are inductive tachs with a wire wrapped around a spark plug wire to pick up the spark. The first couple were Yamaha branded 4 stroke tachs, they didn't last long (sealed internal battery) and were fairly expensive and were somewhat finicky to get a good reading. I had another one that worked well in the interim, but I couldn't find another of the same brand/model to replace it. So, I searched high and low and found a tach that was listed to handle 3 cylinder, four stroke motors. A 3 cylinder, four stroke motor should be a 3 spark per 2 revolution motor per the reading I've done on the internet on several boating forums and per the directions on the tach.

So, the tach does not give the right RPMs set on 3 sparks per 2 revolutions, but it does give the right RPMs on 1 spark per 1 revolution (I verified the RPMs using a laser non contact tachometer on the flywheel). Further the old Yamaha tachs I bought were just 4 stroke tachs, no reference to number of cylinders, so that jives with what I'm seeing with the 1 to 1 setting working.

Can someone who understands this stuff explain it to me in brief?
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Re: Outboard tachometer technical question... In reply to
its voodoo magic, just run with it...

Cool


Carl
Mobile, AL
DHBP Member since 1998

"Life is too short to drink bad beer."
Disclaimer: This post and/or report is not a substantiation of or reflection on the true accuracy of the present surveying methods. It is only a report on or comment concerning local observation and/or results. Your results and observation may vary based on your location, local water conditions, food supply, weather conditions and migratory patterns "
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Re: Outboard tachometer technical question... In reply to
Carl wrote:
its voodoo magic, just run with it...

Cool


I'm OK with that, since I don't really understand it completely anyway, but I was hoping someone had an easy explanation.
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Re: Outboard tachometer technical question... In reply to
I am no mechanic but a each cylinder on a 4 stroke will spark every 2 rotations and on a 2 stroke once every rotation regardless of how many cylinders there are. So if the inductance wire is wrapped around a spark plug wire then it should be either 1:2 or 1:1. If you had the inductance wire on the ignition wire going to a distributor cap then the # cylinders and 2 or 4 stroke would matter because it would be getting a signal for every spark on the motor not just a single cylinder. Why 1:1 on your 4 stroke is working makes no sense to me.
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Re: Outboard tachometer technical question... In reply to
tod osier wrote:
Ok, question because just having something that works isn't good enough, I need to understand why...

Can someone who understands this stuff explain it to me in brief?


Is this some cheap Chinese tach? Got a make and model number?

Four stroke engine as already explained, each separate spark plug will fire once for every two revolutions.

Couple possible reasons come into mind.
(A) user error, you THINK you have it set for 1 spark per 1 revolution but in reality it is set at 1 spark per 2 revolutions
(B) the instructions are written incorrectly, translated incorrectly or in gibberish, leading to the above (A)
(C) the instructions are just pictures/diagrams, leading to the above (A)

See a theme here? Whistle

No you are not going senile, there is no good explanation why the tach is reading correctly, given the information you provide.

Take care, Huntindave McCann Smile SHELL ROCK IA. ,,,,,, "As sailors grow older, the wiser ones move to smaller boats." Thomas Firth Jones, Multihull Voyaging
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Re: Outboard tachometer technical question... In reply to
Consistent with what Dave explained, see my answer!


Carl
Mobile, AL
DHBP Member since 1998

"Life is too short to drink bad beer."
Disclaimer: This post and/or report is not a substantiation of or reflection on the true accuracy of the present surveying methods. It is only a report on or comment concerning local observation and/or results. Your results and observation may vary based on your location, local water conditions, food supply, weather conditions and migratory patterns "
Quote Reply
Re: Outboard tachometer technical question... In reply to
On car engines not outboard some spark every revolution. Meaning it sparks on compression and exhaust. Is it a low rpm reading high? Is this unit any good? I also like to have a tach for are-prop and testing things.
South Jersey
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Re: Outboard tachometer technical question... In reply to
Huntindave McCann wrote:
tod osier wrote:
Ok, question because just having something that works isn't good enough, I need to understand why...


Can someone who understands this stuff explain it to me in brief?



Is this some cheap Chinese tach? Got a make and model number?

Four stroke engine as already explained, each separate spark plug will fire once for every two revolutions.

Couple possible reasons come into mind.
(A) user error, you THINK you have it set for 1 spark per 1 revolution but in reality it is set at 1 spark per 2 revolutions
(B) the instructions are written incorrectly, translated incorrectly or in gibberish, leading to the above (A)
(C) the instructions are just pictures/diagrams, leading to the above (A)

See a theme here? Whistle

No you are not going senile, there is no good explanation why the tach is reading correctly, given the information you provide.



I'm at work (gasp) so I can't double check that is is 1 per 1 or 1 per 2 , but I will later. The display readout and directions are very clear on what the pattern is. Of course it is Chinese.

link:
https://www.amazon.com/gp/product/B07DZB57H5/
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Re: Outboard tachometer technical question... In reply to
Tod,

Here is a thought; (I know,, wow w00t )

Clicked on the link and found this note;

Given that the tach is connected to the spark plug wire for one cylinder, as far as the tach is concerned it recognizes a one cylinder engine. Now the only variable is; does the plug for that cylinder fire once per two revs or once per one rev.

Previously, I would have said all 4 stroke engines fire once per two revs (per cylinder). However the above note, brings that "old school" knowledge into question.

I can not confirm nor deny the firing pattern for your Yamaha engine. That said, I firmly believe the it may well be possible this engine is designed to fire once for every revolution.
(A) Once at the top of the power stroke
(B) Once at the top of the exhaust stroke

Obviously the spark firing at the top of the exhaust stroke does not add anything to the hp output of the engine. Yet I see no reason, firing at the top of the exhaust stoke, would be detrimental to the engine. (All depends on the exact timing of the spark in relation to the exact timing of the intake valve opening.)

Firing at the top of the exhaust stroke MAY actually ignite some of the unburnt gases being exhausted from the engine, reducing pollutants expelled into the water.

Note; I use "top of the stroke" for discussion only. Typically most engines will fire slightly before top dead center.

Take care, Huntindave McCann Smile SHELL ROCK IA. ,,,,,, "As sailors grow older, the wiser ones move to smaller boats." Thomas Firth Jones, Multihull Voyaging
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Re: Outboard tachometer technical question... In reply to
OK, I have some more information. I confirmed the tach is set at 1P 1R. I found a tach that I had bought and never installed because I had determined that it would not work given the information that I had that the motor should be a 3P 2R motor. The unused tach has 1P 1R, 1P 2R and 2P 1R options. I made up a short wiring harness and the 1P 1R option on that tach gave the correct RPMs.

I appreciate all the thoughts and suggestions from everyone. I think I'm OK with where I stand :). I know the tach is providing the right RPMS and that future tachs should be 1P 1R.
Quote Reply
Re: Outboard tachometer technical question... In reply to
Huntindave McCann wrote:
Tod,

Here is a thought; (I know,, wow w00t )

Clicked on the link and found this note;

Given that the tach is connected to the spark plug wire for one cylinder, as far as the tach is concerned it recognizes a one cylinder engine. Now the only variable is; does the plug for that cylinder fire once per two revs or once per one rev.

Previously, I would have said all 4 stroke engines fire once per two revs (per cylinder). However the above note, brings that "old school" knowledge into question.

I can not confirm nor deny the firing pattern for your Yamaha engine. That said, I firmly believe the it may well be possible this engine is designed to fire once for every revolution.
(A) Once at the top of the power stroke
(B) Once at the top of the exhaust stroke

Obviously the spark firing at the top of the exhaust stroke does not add anything to the hp output of the engine. Yet I see no reason, firing at the top of the exhaust stoke, would be detrimental to the engine. (All depends on the exact timing of the spark in relation to the exact timing of the intake valve opening.)

Firing at the top of the exhaust stroke MAY actually ignite some of the unburnt gases being exhausted from the engine, reducing pollutants expelled into the water.

Note; I use "top of the stroke" for discussion only. Typically most engines will fire slightly before top dead center.


What you describe is a "wasted spark" ignition system and that looks like what I have. I've reread the threads that I originally read when researching and I don't see anyone mention it, but I have found casual references to wasted spark ignition on yamaha four stroke outboards. Motor is running like a champ and my work on it for the year is HOPEFULLY done, so I should be happy.