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Re: Dwell

To: KENMATTICE@aol.com
Subject: Re: Dwell
From: rpalmer@ames.ucsd.edu (Bob Palmer)
Date: Tue, 12 Aug 97 11:11:04 PDT
Hi Ken,

Yes, it's high time we went out and scared ourselves a little.  Maybe, 
Streets of Willow in Sept, or the big track on October?  As to your 
questions, I really think you should take your own advice and weld your hood 
shut.  Who really cares if your tach runs below 1000 rpm?  I'll bet almost 
everybody wished their problems were this small.  I must confess that my 
tach also misbehaves occasionally below 1000 rpm.  In my case it starts 
reading erratically high.  I also have a large alternator pulley, but never 
thought about it being the problem.  If it is, then turning the headlights 
on should make the problem worse; I'll try it.  What I've been meaning to do 
is put an oscilloscope on the circuit and see what's really going on.  Don't 
know anything about your MOMO unit, but my Smiths cobra type tach requires a 
rather high voltage signal instead of the current signal used by the older 
Jaeger Sunbeam tachs.  To answer Steve Laifman, I use the inductor to 
generate a sizable voltage to trigger the Smiths tach, probably on the order 
of 100V!  I was scrambling to get it working before TUXXII and haven't 
touched it since.  It seems dead accurate and rock solid above 1000 rpm.  
With the previous tach, a 7000 rpm Jaeger, I just wrapped a few more loops 
around the input and it worked pretty well in series with the MSD.  The 
problem with the Jaeger was that it was pretty temperature sensitive.  I 
could have worked on the circuit as per the tech tips, but really wanted an 
8000 rpm tach, so bought the cobra type tach from XKs Unlimited.  Looks 
pretty cool; says 8-cylinder, etc.  However, I find it hard to take my eyes 
off the road when the engine is turning over 7000 rpm.  Guess what I really 
need is one of those eye level indicator (idiot?)  lights that's set to my 
shift point.  But that's against my philosophy of trying to look nonchalant 
while still having the potential to be competitive.  Am I fooling anyone???
Back to your specific problem Ken, I suggest you cozy up to someone who has 
access to an oscilloscope and knows how to use it and get him (or her) to 
help you look at the signal input to your tach and see what changes at low 
rpms that could be causing the drop-out.

Hope this has been a little bit helpful.

Bob Palmer

>I am using the MSD stuff as well but suffer a problem no one else seems to
>have...i.e at idle 800 rpm the tach dies to zero.. then if you slightly
>increase the throttle to say 950 rpm the tach comes right back. It is a MOMA
>unit.. Think it has to do with low voltage as I also have a generator with
>the large hipo pulley... To make it worse, I have a MSD tach adaptor attached
>to the MSD and connect the tach directly to the tach adaptor.  Been thinking
>I should scrap the tach adaptor and run the tach from the coil...any ideas..
>if so say it in English as me and electricty are not acquited.
>Hope to see you at an event soon.
>Ken Mattice

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