tachometer help: pulse alternator pole settings

[1 FMF]

have a teleflex tachometer, for a boat, vector series I think.
Typical setup to it, a +12v and ground connection, plus the sender wire which goes to the ignition.
The one I have has the selector on it for the following:

2P/4C
3P/6C
4P/8C
5P
6P

where p = # alternator poles or pulses, I don't know.
and C stands for # cylinders.

The instruction sheet i got with it has a chart showing make/model of outboards and what to set the selector to.
But, it has 2 columns. One for "alternator poles" and another column for "switch position (pulses)".

On the back of the tach, the letters around the dial are just: 2P/4C , 3P/6C , 4P/8C , 5P , 6P.

Now, say for yamaha outboards it says 6 for alternator poles and 3P for switch position (pulses),
or for another model 12 alt poles and 6P.

For kicks I hooked it up to my lawnmower and left the selector on 2P/4C. It seemed to give an accurate reading- 1200 rpm at idle and 2800 rpm full throttle.

can someone explain how the pulse signal coming from the coil is interpretted by the tach based on the selector setting, in a practical sense?

I changed the selector from 4C to 6C and to 8C on the lawnmower. The rpms were half for 8C than for 4C. The lawnmower is a single cylinder engine (Honda HRX mower), yet the tach reading seemed correct so I don't get the alternator pole/pulse and cylinder setting scheme

 

[TallPaul]

I would call the manufacturer for tech help. That's what I did for the tach I bought for my motorhome and they (Autometer) looked it up and told me exactly which wire to hook it to.

 

[1 FMF]

I wanted to use the tach on my new outboard, a 2006 yamaha 25hp.
Been trying to get the answer from my yamaha dealer but no luck yet
Played with it today, the selector switch at 3P or 4P yielded the most believable numbers.
I figured out I was reading the table from the instruction sheet wrong. For yamaha outboards it's saying the alternator will have 6 poles which means you put the switch on 3 Pulse (3P). But my outboard doesn't have an alternator, so I don't know if the 3P setting is correct. The tach seemed to read best on 4P. For the 3P setting it read kinda high rpm (1000-1100) at slowest idle which should be not more than 800 rpm. It's a brand new motor, < 10 hours on it.

 

[TallPaul]

Well, if they say run 3P and it doesn't seem right, give them a call. Maybe they account for your engine not having an alternator in their chart. How many culinders is your engine? Possible idle really is 1000?