Tach for 2 stroke

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maintenanceguy
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Tach for 2 stroke

Post by maintenanceguy »

But not just any 2 stroke. It's an old mariner with a single ignition coil and two cylinders. The ignition coil fires twice for each stroke of the piston. So, a 2 stroke fires twice as often as a 4 stroke and my 2 stroke fires twice as often as other 2 strokes.

Any tachs that will work for me?
-Ryan

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DaleH
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Tach for 2 stroke

Post by DaleH »

Look up Hardline tachs: https://www.hardlineproducts.com/produc ... ach-meter/ , but I would also definitely email or call them, for the right configuration!

C&K
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Tach for 2 stroke

Post by C&K »

maintenanceguy wrote: 15 Jun 2021, 23:38 But not just any 2 stroke. It's an old mariner with a single ignition coil and two cylinders. The ignition coil fires twice for each stroke of the piston. So, a 2 stroke fires twice as often as a 4 stroke and my 2 stroke fires twice as often as other 2 strokes.

Any tachs that will work for me?
Outboard tachometers don't work with the ignition system. They use the stator's AC sine wave and count the pulses. So an 8 pole stator has four pulses/rev, 12 pole has 6, etc. etc....

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maintenanceguy
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Tach for 2 stroke

Post by maintenanceguy »

C&K wrote: 18 Jun 2021, 20:24
Outboard tachometers don't work with the ignition system. They use the stator's AC sine wave and count the pulses. So an 8 pole stator has four pulses/rev, 12 pole has 6, etc. etc....
Excellent information. Points me in the right direction. Thank you.
-Ryan

C&K
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Tach for 2 stroke

Post by C&K »

You're welcome. The stator on your Mariner is a single phase AC unit and it has two wires going to the rectifier. The other two wires on the rectifier are your DC positive and negative that charge the battery. The rectified DC is 1.414x the AC RMS voltage so the stator puts out 9-10 volts AC RMS. If the stator is 8 pole then it will have 4 positive voltage pulses per rev, and four negative pulses (AC current). In that case a standard automotive tachometer set to 8 cyl will work. An automotive tach has four wires, + and - for power to the tach, back lighting and signal wire. The signal wire goes to either of the AC wires from the stator.

8 pole stators were common in Chrysler, Force, Mariner and some Mercury outboards (less than 50hp). Most others (and higher hp engines) will have 12 pole stator. Count the number of coils on the charging stator. If you have a 12 pole stator a standard automotive tach won't work out of the box because automotive is designed to count 2, 3 or 4 pulses/rev for 4, 6 or 8 cyl four-stroke engines. In that case you need a regular marine tachometer that can be set for 6 pulses/rev. OR - if you are good with electronics you can change the resistor on the voltage divider circuit in an automotive tach to modify it so it will count 6 pulses/rev. There are various tutorials on the internet on how to do this like this one:
https://www.instructables.com/Automobil ... -outboard/

DO NOT attempt to hook a tachometer to the ignition on an outboard unless it is battery powered points ignition (like Chrysler or Force thru 1985 on some 40, 50 and 55hp engines). It will either damage the CD switchbox or it won't run at all because the CD ignition will view the capacitance of the tach circuit as a ground and it won't fire.

If you aren't comfortable modifying an automotive tach for 6 pole stator you can buy a regular marine tach like this one that is adjustable for the number of poles on your stator. But they are more expensive than the typical $40-45 automotive tach's:

old_fart
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Tach for 2 stroke

Post by old_fart »

Not for the faint-of-heart...
http://n952.ooguy.com/tach/#Tach_Image
This is what happens when your retire...

-W