77 Johnson 115 Short Shaft - Tach Pulse Setting

[grcsomd]

Folks,

I have (finally) installed a Teleflex tach and am getting some strange results. On a newly rebuilt powerhead, WOT with a 13.25 x 17 prop gives me 30mph (via GPS) and around 2900 RPMs showing on the Tach using the "standard" 6P (pulse) setting on the Tach. The RPMs reported seems somewhat low to me. The motor seems to be running wide open, butterflies horizontal, etc. I am not really sure how many poles or pulses per rev this motor puts out. I can't find the spec in any books I have. I tried using the 3P setting and am getting 5800 RPM at 30 MPH which seems too high but maybe calls for a 19 pitch prop. Is it possiblle this motor has 8 Poles and I should use the 4P setting? This appears like the 4p setting would map out to about 4500 RPMs which seems about right if I am slightly overpropped with a heavy load. In that case I would like to try dropping down to a 15 pitch prop to see if I can get the RPMs up at WOT to more like 5000 RPMs which the book says is standard, and possibly a few MPHs more. I would really like to know the number of poles that this motor has, and what pulse setting is correct for the tach.. and go from there. Lastly.. how to calibrate the tach. The calibration screw seems to give me 250 rpm +- over what its running, but does not adjust at zero with ignition on. How can the correct calibration be done?

Thanks in Advance.


G

 

[Tacklewasher]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

Something isn't right.

The tach should be on 6p, but the rest of your numbers don't make sense.

At 2900 rpm, with a 17p prop, best speed would be 23 mph, and assuming a 10% slip, you should be doing 21 mph.

To do 30mph, again assuming 10% slip, you should be at 4100 rpm.

Ideally you want that motor to run at 5500 to 6000. I've worked hard to get my 81 115 to 5500, including going to a 15p prop.

Here is the link to a prop slip calculator to see where I got my numbers.

http://www.go-fast.com/Prop_Slip_Calculator.htm

Your gear ratio is "2" (as in 2:1)

 

[grcsomd]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

I agree the numbers do not make much sense. There is a +/- 200 - 250 rpm variance in the calibration, but still.. the 6p setting for me seems wrong, even though most people say its right. Could I have some "bad poles" so that I am only getting 4 pulses instead of 6? The math makes it seem like 4P is the right setting for the Tach.. based on pitch and max MPH via gps.

I also have read up some more on how possibly my motor may be too low in the water. It is setup on the top bolt hole notch.. and the cav plate seems to be about 1 inch below the water/hull line. I may try moving that up some based on my research.

On the number of poles/pulses vs what I am seeing in RPMs, MPH, etc.. I am completely stumped still... unless somebody figures out that its really an 8 pole motor. I had read somewhere about a difference in poles possibly between the 20inch short shaft and the 25inch in this model year and HP (shrug) I think I need to start being able to believe the tach before I change anything else...

Thanks again,

G

 

[Dhadley]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

What Telflex tach did you get? And one that has that much (+/- 200-250) rpm variance isn't going to be very useful anyway. The correct setting is position 6 and since you mention it has that it must be for an outboard but it's way off.

BTW - I doubt your motor is a short shaft (15"), bet it's a long shaft (20").

 

[Randyg123]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

You're a 6p

When dealing with tachometer problems, one issue always seem to pop up... the rectifier. Did you verify the rectifier is working properly? Check out the bazillion (yes a bazillion) of posts on here about how to troubleshoot the rectifier. Start with this FAQ posted on the iBoats' site -
http://forums.iboats.com/showthread.php?t=190638

A tach is only as good as its incoming signal

But another issue is that the tach should zero when the key is turned on. If it can't be calibrated to zero correctly then there is a problem with the tach or perhaps the wiring is faulty. But I'd make sure your rectifier is working correctly and not just letting out a partial signal.

 

[grcsomd]

Re: 77 Johnson 115 20inch Shaft - Tach Pulse Setting

Re: 77 Johnson 115 20inch Shaft - Tach Pulse Setting

Thank-you! I will check the rectifier and keep the tach on the 6P setting. The Tach is kind of a universal type, it can work with inboards and outboards and has settings for pulses (ob) or number of cylinders (ib), some of which overlap. I dont' see a way to set it for ob or ib, so it must be able to figure that out on its own.


ps: Sorry about the short shaft/long shaft confusion. At one point dealing with lower units for this motor, I recall it was available in 20 and 25 inch shaft lengths only, which they called short and long... now I know better..

G

 

[grcsomd]

Re: 77 Johnson 115 20Inch Shaft - Tach Pulse Setting

Re: 77 Johnson 115 20Inch Shaft - Tach Pulse Setting

Hello,

I tested the rectifier and sure enough it fails. Not really so surprised actually since it looks like the original equipment. I am getting infinite resistance (open) between the two yellow wires in both directions with the multimeter.

I recall that the stator had two poles that were a little bubbled up last time I had the flywheel off... so I went ahead and replaced both the rectifier and the stator. The tach is on 6p and I will report what my WOT rpms are next time out. I still need to figure out how to zero the tach. With ignition on, the needle goes "near zero" but the calibration without the motor running does nothing to the needle. When running the calibration seems to give a total range of between 200 and 250 rpms. At dead slow idle on the trailer, it will read anywhere from 600 to 850.. so I am not sure where it really is.. I have an old automotive type dwell&&tach meter that I may be able to use to calibrate the new tach.. Is there some way to record the sound of the motor and get the RPMS based on the frequency its putting out?

Thanks again for all the great advice.

grcsomd

 

[grcsomd]

Re: 77 Johnson 115 20Inch Shaft - Tach Pulse Setting

Re: 77 Johnson 115 20Inch Shaft - Tach Pulse Setting

Folks,

Was out on the water this weekend and seemed to be doing well on the 6P setting at idle speeds and up to 1200 rpms. It was too rough out there to really do much more... but was trolling at around 7mph via GPS and running a 13.25" x 17" pitch prop which seems about right. The water finally settled down a bit and was able to make a few WOT runs and was showing 3900 RPM and doing 30mph. While a lot better than before, this still doesn't seem right. Tweaked the calibration screw on the tach and was able to get a variance a dead low idle between 600 and 1400 rpms.. a lot wider range than when it was on the 3P setting. Just left the calibration at about 700 which should be close to the real idle rpms. Just for chuckles tried both the 3P and 4P settings and both of those settings were > 6000 rpms while the motor was idling.. so the new rectifier has definitely changed the environment... 6P seems a lot closer to the right numbers, however don't see how it can be making 30 mph at 3900 rpms.. According to the Prop Calculators out there, its still off by 3.3 mph... (26.7mph is what the calculators show with a 15% slip factor) This is an older (early 70s) heavy (Grady White) 18 foot boat...

Using the formula in reverse it says that the setup should be making 4385 rpms in order to do 30mph.. so the tach appears to be off in the general area of 485 rpms. Can't imagine that my idle speed is not 700 by rather 1185.. but possibly the calibration adjustment is not linear across RPMS..

Even if its able to make 4385 rpms with the 17 inch prop, may be somewhat overpropped for this motor and application so will try stepping down to a 15 pitch prop. If anybody has a good idea on how to properly calibrate this Teleflex universal (gas) type tach for my application.. , would appreciate it. Recall reading somewhere in my research about some motors needing some kind of voltage regulator or resistor in order for this tach to work right... Maybe that would explain the wild rpm swings in the calibration.. Suppose more research is warranted, but its getting closer to being setup right.. Too bad this tach did not come with a manual, but hopefully its available online..

 

[WizardIII]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

hmmm, jumping in here.
I have the Evinrude service manual for the 1977 85hp/115hp that says it has an 8 pole alternator. Wouldn't that be a 4p setting on the tach? I'm about to hook up a tach also, am I missing something?

 

[Randyg123]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

I do not have that model but everything I've ever seen posted on this site would indicate this should be a 6p. Some small Evinrudes/Johnsons can be a 5p or even a 3p, but I've never even seen 4p mentioned in association with a Evinrude/Johnson.

Is the manual an OEM manual or an after-market version?

 

[WizardIII]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

I do not have that model but everything I've ever seen posted on this site would indicate this should be a 6p. Some small Evinrudes/Johnsons can be a 5p or even a 3p, but I've never even seen 4p mentioned in association with a Evinrude/Johnson.

Is the manual an OEM manual or an after-market version?

Hi, yes Evinrude service manual and parts catalog although they are copies from marineengine.com.
I did finally find a thread on here from 2002 asking the same questions about the same motors with 6 amp 8 pole alternators: 'why 6p instead of 4p' settings?, however all the replies from Joe Reeves were deleted.
Anyone remember???
Steve

 

[ezeke]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

Is the setting that you are using on your tachometer "6" or "6p"?

What is the maximum RPM on your tachometer?

Have you tried switching the gray wire on the terminal block to the other yellow wire?

Is your rectifier showing signs of overheating?

 

[WizardIII]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

Is the setting that you are using on your tachometer "6" or "6p"?

What is the maximum RPM on your tachometer?

Have you tried switching the gray wire on the terminal block to the other yellow wire?

Is your rectifier showing signs of overheating?

The first question just might be the key I'm looking for. Please explain the difference. I have a 7000 rpm OMC #174985 tach and says pul/rev with dial settings 2, 3, 4, 6, 5.

I'm working on the transom of a 1960 fiberglass boat and haven't put the '77 85hp 'rude on it yet.......just planning ahead.
Thanks, Steve

 

[ezeke]

Re: 77 Johnson 115 Short Shaft - Tach Pulse Setting

To WizardIII: Your tachometer is OEM and the setting is 6 for your motor.

To grcsomd: Teleflex has excellent support for users and can give you advice based on your specific tachometer:

http://www.teleflexmarine.com/cgi-bin/framesett.cgi?site=home&type=us&link2=1043

Still the correct setting for their outboard tachometers is 6P on your engine. See page 6: http://www.downloadingfiles.co.uk/t...structionmanuals/99320 rev 10 Tachometer,.pdf

Once you have the setting correct, calibration would need to be done using a technical tachometer to verify and adjust yours to the actual RPM.

The tachometers will not work properly if the ground wiring in your electical system is corroded or poorly connected.

 

[grcsomd]

Re: 77 Johnson 115 - Tach Pulse Setting

Re: 77 Johnson 115 - Tach Pulse Setting

Folks,

Good news. I sent the Tach back to Teleflex for recalibration. I believe I am getting good results now. I had to ensure a good ground, replace the rectifier and things are starting to make a lot more sense:

Speed...........RPMs
Idle................700
22 mph .........3200
28 mph..........4000
30 mph.WOT..4250

This is with a 17 inch pitch prop.. and carb butterflies wide open. I think I will try stepping down to a 15 - seems like I could even use a 13 with heavier loads.. The Seloc book shows 5000... but that seems kind of low..

Thanks much -

grcsomd