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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