tach issues

[allen456]

I recently replaced the tack on my 81' 175 johnny and for some reason it only shows 4200 rpm at WOT.  The old one would work occasionally and show 5100rpm.  On the back of the new one it ( and the instructions) show to put the arrow on 1/2 of the number of alternator poles that your outboard has.  there is also an option for # of cylinders for I/Os and inboards.  The old tach points to a 6, but i am not sure if that is for cylinders or half of the # of poles.  Any help would be greatly appreciated.  thanks.

[slvrlng]

Does the back kind of look like this?

[allen456]

almost the exact same.

[Dukester]

Same question about the setting as my new Tach looks the same on back.  I have a Yamaha four stroke 115.  Thanks for any help!

[johnsjmc]

Outboard engines activate the tach with a signal originating in the alternator. It doesn,t matter how many cylinders. both the V4 and V6 Johnnyrudes use the same number of poles. The setting is 6 . The shop manual for the yamaha is needed to find outit's setting. It may use a more or less setting depending on the pole number in it's stator.

[allen456]

Thanks for the help.  I figured that it need to be set on 6p.  Now that opens another door with the set.  If i am running at 4200 to 4400 rpm, have prop issues or i am getting a bad reading.  prop is a 15 1/4 x19

[RebelYell]

Outboard engines activate the tach with a signal originating in the alternator. It doesn,t matter how many cylinders. both the V4 and V6 Johnnyrudes use the same number of poles. The setting is 6 . The shop manual for the yamaha is needed to find outit's setting. It may use a more or less setting depending on the pole number in it's stator.

Outboard engines don't have an "alternator". Instead, they have a stationary stator(windings) and a trigger...

Magnets in the flywheel move to create the electromagnetic field necessary to fire the coils and charge the batteries. An alternator is the opposite; the magnets are stationary and the windings move to create the electromagnetic field.

Bad tachs are usually associated with:
1) Bad tach ground.
2) Voltage regulator going bad.
3) Stator going bad.
4) Bad tach.

If you are getting 14 volts charge at the battery and all cylinders fire OK then you can eliminate #2 and #3.

I would check the ground at the tach and the buss bar first. Then I would make sure it was set properly.

If you have good ground continuity and the engine fires ok and batteries charge ok then I would toss the tach and get a new ( or lightly used ) one.

Hope this helps.

 :joker:

[hwsiii]

Allen, if you will give me your gear ratio, RPM and WOT speed I an tell you what your prop slip is and possibly from that we can figure out if it is working right.


H

[johnsjmc]

the charging system is indeed an alternator. whether the stator is stationary or fixed or with permanent magnets or using exiter coils it is still producing alternating current. hence the need for a rectifier to convert the output to DC for the battery. It is called an alternator in the Jonnyrude shop manual all through chapter 7 electrical system. as well.

[bdtsr]

Allen, what are the compression numbers on the motor? A 19 inch pitch is on the high side for that motor and that boat but should still be turning up around 4600 to 4800 rpm. Check to make sure your timing and carbs are synched properly. A 15 to 17 inch pitch would be more realistic. Your lower unit should be a 1.86 to 1 ratio.