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

From: Ethan Brodsky <brodskye@...>
Date: Mon, 28 Jul 2008 13:42:35 -0500 (CDT)

On Sun, 27 Jul 2008, Don Mandelas wrote:
> This weekend I was able to obtained a timing light and digitally measure
> 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 boat
> tachometer was reading 1,000 RPM's. And, when the timing light indicated
> 4,000 RPM's my boat tachometer was reading 3,000 RPM's. This means my
> tachometer is reading a full 1,000 rpms below what it should be on the
> higher end of the scale and is probably the primary reason why I am going
> slower than normal. I'll replace the tachometer and try the boat again.

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 believed.

I found the opposite on mine - when I measure the RPM (using a TinyTach
inductive pick-up on a spark wire), I see that the tach is reading
substantially high (1000-1500 rpm). My understanding is that older tachs
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 switched
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 spark
plug fires once for every revolution or just every second time. I think
it's the latter.

Tinytachs are a fairly cheap way to have a more accurate RPM reading, as
well as an hour-meter. Unfortunately I don't remember whether the 4C or
the 2C should be used.

If anyone has figured out a way to adjust the factory OMC facts, I'd like
to fix mine. I haven't figured out how to do that yet.


<a href=""> Ethan Brodsky </a>
Received on Monday, 28 July 2008

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