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I have an Outrage 18 with twin 1989 Johnson 70s. I have a problem with the speed of the boat so I assumed the props were too small after running it wide open and getting around 6100-6200 RPM, so I emailed Michigan prop company they told me I needed more pitch. OK, so I was pricing out props and never pulled the trigger. I have been running the boat at 4400 RPM and doing anywhere from 19 to 22 knots. I was hoping the boat was more like 24 knots. Then today I decided let me check the tacks with a digital test gauge. WOW I found out that the tacks are 700 RPM too HIGH! So if I'm running the boat at 4400 RPM then I'm actually only running it at 3800 RPM, and WOT at 6200 RPM is only 5500 RPM. So It looks like the boat is right in the ball park where it is supposed to be. Now my question is can I calibrated the tachs so they read correctly? These are original OMC tachs.

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There are settings on the back but not exactly for calibration.

What series of OMC Tach do you have?
http://www.whalercentral.com/articles...icle_id=43
1. Traditional Series
2. Tech Series
3. Other Series?

If you have to buy new Tachs, see this article where I put in the Tech Series with the System Check.
http://www.whalercentral.com/articles...icle_id=72

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Could you describe that process in more detail?

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I have the tech series. We used a digital handheld tester my dads friend gave me that owns a marina. It claps around the top cylinder spark plug wire. Hope that helps.

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I highly doubt both tachs (which are the relatively newer style) would both be inaccurate.
I could see one tach going bad but not both at the same time.

When you turn the keys ON, do both tachs go to zero?
Also, what RPM's do the tachs read at idle?

Secondly, is your charging system working properly?
Do you have voltmeters for each motor?
How many volts is showing when you are at cruising speed?
A faulty charging system can cause the tachs to not read properly.

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Joe makes a good point about voltage. Years ago I owned an OMC sterndrive powered boat that suddenly showed much higher RPM that it should have. The voltage regulator went out and the voltage supplied to the tachometer was around 16 volts and sent the readings sky-high.

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At idle its only 300 rpms off. So far the charging system is working. However sometimes the voltage gauge does read 16 volts when I'm running on both engines. I haven't check the volts with a meter though.yes with the keys on the go to zero.

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1989 Johnson 70s should be running at, or near, 6000 RPM at WOT if propped correctly.

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If you are using OMC/BRP props, you are probably using 15" pitch versions when you should be using 17" pitch versions.

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I think Tom is correct on the pitch.
I had 15 pitch on my 1986 Evinrude twin 70's on my Outrage 18' and here is the info:
http://www.whalercentral.com/articles...icle_id=35

What brand and pitch props are you using now?

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I'm running 14x19 aluminum.

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What is the WOT speed of your boat?

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Rpm I the tacks read 6200, mph don't know I didn't have my gps on. When I had the digital tach hooked up to the spark plug wire it was 5500 rpm so I know the tachs are off now I need to see if I can calibrate them to match the digital tach.

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Again, based on what you report, I think the problem is not with your tachometers. I think you have a too much voltage and the 1989 Johnson 70 used a rectifier, not a voltage regulator. I bet one of your rectifiers is bad and in need of replacement.

You don't say what brand of propeller you are using but 14" x 19" being turned by those motors at 5500 RPM should be pushing the boat to about 37 MPH. That is why it is nice to have GPS data; you can work backward to see the tachometer readings make sense vis-s-vis the speed, gear ratio and propeller.

A rectifier for those motors is a $37 part and can be installed with a screwdriver.

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At 4000 rpms I do 23.5 knots, the props are michigans. I read on other websites its normal to have high voltage at cruising speeds on these engines.

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OK, that sounds pretty normal, 23.5 knots at 4000 RPM is about 9 percent (calculated) propeller slip which is perfectly typical of those 19" Michigan aluminum props.



19 knots at 4400 RPM is 33 percent slip but 22 knots at the same RPM is only 23 percent slip. Either way, far too much.

So, you can understand why I am completely confused by what you are reporting.

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SG, you may wish to read this thread:

http://www.whalercentral.com/forum/vi...pid=111037.

At the end, the poster discusses using a resistor inline to the tach. This prevents it being affected by the high voltages produced at speed by these older, unregulated motors.

Tim

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Excellent idea Tim, though I am still suspicious of BOTH tachometers reading the same, which implies a systemic problem rather than an individual tachometer failure.

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Thanks I am going to look in to the regulators.

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Just to clarify:

- 1989 Johnson 70s don't have voltage regulators. Even if everything is working fine, they are going to put out more than the typical 13.8V when running at speed. I don't remember the actual voltage, but my recollection is that 16 or even 18V is normal.
- the reason this does not fry your battery is that the charging current is very low - 6A if I remember correctly.
- some components such as GPS heads won't run at those higher voltages
- the other poster's Teleflex tach wouldn't either
- If the OMC tachs are also sensitive to higher voltage (I don't know whether they are) then you may want to try a resistor in series with the tach power to see if it helps
- the reason it affects both tachs the same may well be because both motors are running the same speed and putting out the same voltage, which the tachs don't like
- or it could be because both tachs are taking off the same circuit (that was not specified) and one motor really does have a voltage output problem

My point is that you should try to diagnose the problem with a voltmeter before you start throwing parts at it.

Tim