Digital outboard tacho query

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C08

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I have a Yamaha Multifunction Tacho and the settings for the tacho dsplay puzzle me. There is a dip switch for 6 or 12 pole alternators and one for 2 stroke or 4 stroke. As I understand it the rev counter is counting unrectified pulses from the alternator so the number of poles calibration I understand. What puzzles me is the 2 or 4 stroke dip switch. Surely rpm is rpm and the fact that there is a firing stroke for each rotation of the crank for a 2 stroke or one firing stroke per two rotations for the 4 stroke is irrelevant so why the 2/4 stroke dip switch selection? I am obviously missing something here and require some enlightenment.
 
ahabdonal said:
I have a Yamaha Multifunction Tacho and the settings for the tacho dsplay puzzle me. There is a dip switch for 6 or 12 pole alternators and one for 2 stroke or 4 stroke. As I understand it the rev counter is counting unrectified pulses from the alternator so the number of poles calibration I understand. What puzzles me is the 2 or 4 stroke dip switch. Surely rpm is rpm and the fact that there is a firing stroke for each rotation of the crank for a 2 stroke or one firing stroke per two rotations for the 4 stroke is irrelevant so why the 2/4 stroke dip switch selection? I am obviously missing something here and require some enlightenment.
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The real test is whether it makes any difference to the reading. Some people just like to have switches to set and manufacturers indulge them without bothering about the effect on people who think!

Mike.
 
mjcoon said:
The real test is whether it makes any difference to the reading. Some people just like to have switches to set and manufacturers indulge them without bothering about the effect on people who think!

Mike.
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That is an interesting observation Mike as the switches did not seem to make any difference to the tacho reading which used to peak at about 3000rpm rather than the 6000rpm that the motor was rated at. I never did get it to read correctly however I set the dip switches!
 
The only reason I can think of for a 2/4 stroke switch is that the tacho. can take a pulse from either the alternator or the ignition system. That would give an easy way of getting the correct pulses in relation to revolutions. I suppose the sensor could either be on the LT side or some sort of external clip-on for HT side.

The other switch makes sense for a pulse from the alternator instead of using the ignition pulses. However, you also need some method of adjusting for ratio between alternator and engine rpm. Inboard diesels use belt drive and the ratio between pulleys canvary a lot. I wonder if Yamaha outboards use some sort of direct drive where ratio is always the same across all models.
 
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kelmark2590 said:
Although this is not true for all 4-strokes, a lot of 4 stroke motors fire twice per revolution while two stroke motors always fire once per stroke. I assume this is the reason for the switch.

http://www.PartsPak.com
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Don't you mean that 4 stroke motors only need to fire a spark once per 2 revolutions but sometimes do spark every revolution. You are correct in saying that's why you might have a switch. I didn't state it explicitly, just suggested that tacho might use either alternator or ignition and offering that as an answer to the 2/4 stroke switch.
 
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