Re: [OMC-Boats] Engine RPM and Boat Speed

From: Lee Shuster <lee.k.shuster@...>
Date: Tue, 29 Jul 2008 09:16:33 -0600

So without reviewing all of your data points again, this sounds less
like "slippage" and more like a case of a combination of too little
torque for the combination of altitude, weight, and prop pitch.
Just my, shoot from the hip, two cents worth..


From: omc-boats-bounces@...
[mailto:omc-boats-bounces@...] On Behalf Of Don Mandelas
Sent: Monday, July 28, 2008 8:49 PM
To: Evinrude & Johnson Boats of the 1960's and 70's
Subject: Re: [OMC-Boats] Engine RPM and Boat Speed

On my last note I relayed information incorrectly. I said that the boat
tach is reading lower than the timing light indicates. That is not
The correct information is the boat tach is reading higher than the
timing light indicates.
For example when the boat tach is at 1,100 rpms the timing light reads
1,000 rpms. And, When the boat tach is at 4,000 rpms the timining light
reads 3,000 rpms. 1,000 rpms difference.
This afternoon I rechecked the boat tachometer against the timing light
to confirm these results.
> Date: Mon, 28 Jul 2008 13:42:35 -0500
> From: brodskye@...
> To: omc-boats@...
> Subject: Re: [OMC-Boats] Engine RPM and Boat Speed
> On Sun, 27 Jul 2008, Don Mandelas wrote:
> > This weekend I was able to obtained a timing light and digitally
> > the revalutions per minute of my V-6 155 hp boat engine.
> > I discovered that when the timing light indicated 1,100 RPM's my
> > tachometer was reading 1,000 RPM's. And, when the timing light
> > 4,000 RPM's my boat tachometer was reading 3,000 RPM's. This means
> > tachometer is reading a full 1,000 rpms below what it should be on
> > higher end of the scale and is probably the primary reason why I am
> > slower than normal. I'll replace the tachometer and try the boat
> This sounds a little strange - you say that the tach is reading low,
so the
> engine is actually spinning faster (and making more power) than
> I found the opposite on mine - when I measure the RPM (using a
> inductive pick-up on a spark wire), I see that the tach is reading
> substantially high (1000-1500 rpm). My understanding is that older
> tend to read high due to weakening of the return spring with age.
> Are you sure you had the timing light set properly (some can be
> between various combinations of two-stroke/four-stroke, single/multi
> cylinder, and wasted-spark modes) and it wasn't reading high by a
factor of
> two? I'm not sure how it works with older distributors, whether each
> plug fires once for every revolution or just every second time. I
> it's the latter.
> Tinytachs are a fairly cheap way to have a more accurate RPM reading,
> well as an hour-meter. Unfortunately I don't remember whether the 4C
> the 2C should be used.
> If anyone has figured out a way to adjust the factory OMC facts, I'd
> to fix mine. I haven't figured out how to do that yet.
> Ethan
> --
> Ethan Brodsky
> _______________________________________________
> OMC-Boats mailing list
> OMC-Boats@...


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Received on Tuesday, 29 July 2008

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