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Re: DIY O2 (Lambda) gas analyser

To: <Menno.Meijer@ict.nl>, <mgs@autox.team.net>
Subject: Re: DIY O2 (Lambda) gas analyser
From: "Rick Lindsay" <ROLindsay@Emeraldgrc.com>
Date: Thu, 11 Apr 2002 10:34:54 -0500
Menno writes,

> Dear List,
> 
> As a novice MGB mechanic I started reading all I could
> find about SU tuning. (Although "real" experts will call
> what I do "idle setting" instead of "tuning")

   The biggest mistake most amateur <and many professional>
mechanics make is tuning BEFORE testing.  This is a soap-box
concept for me so please humor me.  It is in this light that I
like your idea of testing.  Here are a few thoughts.

> After reading several articles it seems to me that the
> "Art" of tuning SU's (or in fact any carburettor) is
> easier to learn by using an exhaust gas analyser. (Or
> can be used to confirm a correct setting (14.7:1))

Let me make a few points:

+ Setting the mixture on an SU is most influential at
   idle.  That is why so many people call it "idle setting."
   That is not to say that the position of the jet does not
   influence the mixture at larger throttle openings.  The
   profile of the needle is dynamic and sets the correct
   running mixture.  That is what you need to change if
   after setting the idle mixture, you car does not perform
   well at your altitude.  The idle setting is a static 
   adjustment adding to <or subtracting from> that profile.

+ 14.7:1 is the chemically right air-fuel mixture for correct 
   combustion.  That is, it is the ratio of air to fuel that
   neither leaves unburned fuel nor excess oxygen in the
   exhaust -- in a perfect engine.  However, 14.7:1 is
   NOT the best air-fuel mixture for maximum power.
   That happens at about 12:1 -- and produces a dirtier
   exhaust.  It is the cost of extra power.  Please do
   not read this to mean, "If a little richer mixture makes 
   a little more power, a LOT richer must make a LOT 
   more power."  Such is not the case.  12:1 or so is 
   about the best compromise.  Modern cars run at 
   14.7:1 for optimized fuel economy and clean
   exhaust.  On many cars, there is a switch that
   richens the mixture at WOT -- effectively taking
   the lambda circuit out of the equation.

+ I tune with a vacuum gauge attached  to the intake 
   manifold (not the vacuum advance port on the carb
   as that is a modulated vacuum).  This is an idle-only
   setting but as discussed above, that setting influences
   the whole curve.  Maximum vacuum occurs when the
   engine is running at its highest volumetric and combustion
   efficiency.  That is also what you are tuning to achieve.
   It then follows that the goal is the set the mixture, keeping
   the idle speed fixed, that produces the highest manifold 
   vacuum.  Cheap gauge to buy, excellent technique to 
   implement.

   As an aside, the vacuum gauge can also spot a plethora 
of other engine problems like sticking valves, broken rings 
and ill-sealing valve seats.  Give it a try.  Its a cheap test.
   I hope my rants in some way, help.

Best,

rick

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