Bob,
Sorry to disagree with you, but I have a slightly different opinion
regarding the Smiths tach with dual points.
First, the calibration pot is not under the lamp hole. The tach guts
need to be taken out to calibrate it, unless someone has drilled a hole
in the case above the calibration pot. Check out my tech tip at
http://www.classictiger.com/techtips/motach.html to see advice on taking
the tach apart for calibration
Second, the way the tach works is by creating an adjustable width pulse
for every ignition pulse it sees. So, at low RPMs there is a low duty
cycle square wave, and at high RPM there is a high duty cycle square
wave driving the ammeter that is the tachometer. Unfortunately, if the
ignition pulse is shorter than the internal tach pulse, it shortens the
internal tach pulse to match the input pulse, not the desired pulse
width. There must me a finite amount of time before the next pulse
comes in for the circuit to get ready for the next pulse, or the circuit
gets confused.
If a dual point ignition is perfectly set, there is only one ignition
pulse per cylinder firing. In this case, your tach will work fine.
Where the problem comes in is when the dual points create an ignition
signal consisting of two rapid pulses per cylinder firing, then the tach
will lose its mind, with symptoms that can vary from twice the proper
RPM reading to no reading at all to erratic behavior.
The best way to ensure the dual points are set properly is to use a
scope, probably best checked on a dyno at various RPMs.
Mark
------------------------------
Date: Thu, 11 Dec 2003 18:09:57 -0800
From: "Bob Palmer" <rpalmer@ucsd.edu>
Subject: RE: Tach Question
Scott,
There is an adjustment just inside the lamp hole. Turn it until you get
it
about right at mid-range. As Theo says, the dual point just changes the
relative on/off ratio. However, this does have a considerable effect on
how
the electronics in the original tach respond. The doubling you note is
just
coincidental.
Bob Palmer
rpalmer@ucsd.edu
rpalmerbob@adelphia.net
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