Will says:
>I always thought that if I opened up a vacumn hose attached to the carb that
>the air would rush in and the car would run faster at idle. In the process
>of trouble shooting my mixture problem I did this and it ran slower. If I
>plugged up the hole it made the engine run faster. I am thinking that the
>mixture is lean and that this causes the engine to run quicker when less
>O2 is present because the air/fule mixture is close to being correct? Right?
Hmmmm, not so fast, Will. First, I don't think your question can really
be answered without knowing more about your car and the particular
vacuum hose you removed.
For example, let's say you removed a vacuum hose leading to the
distributor. It might provide advance or retard, and its source might be
located just upstream or just downstream of the throttle. If the source
was just upstream of the throttle, it probably wouldn't change the
mixture strength significantly, but loosing vaccuum might significantly
change the timing of the car, which might make it run faster or slower.
If the source of the vacuum was just downstream of the throttle,
removing the hose might affect the mixture more significantly, but
without knowing more about your car, it's hard to say weather its affect
on timing would contribute to or tend to negate the effects of the
mixture change.
And the same type of interrelationship might be true of other vacuum
hoses, too. For example, most of the vacuum hoses I remember on my
old '79 Honda Civic wagon were upstream of the throttle, and removing
any one of them never seemed to affect mixture dramatically (although
when I started diagnosing mixture problems with that car, I discovered
that having them *all* rotted away made a big difference). On the other
hand, removing the vacuum hose that ran from the base of the carburetor
to the windshield-wiper motor on my '49 Cheverolet half-ton pickup truck
would have made a significant difference in mixture strength, because it
was a big hose and it was downstream of the throttle.
Now, if your car is a '72 TR6, like mine, then there are only 3 vacuum
hoses to choose from; remove the vacuum advance, and the change in
timing throws off any carburetor diagnoses you're trying to make; remove
the giant vacuum line that runs from the manifold to the brake booster,
and you'd lean the mixture way out; remove the hose that runs from the
valve cover to the air filter, and mixture doesn't change, but your air
filter stays cleaner and you get more oil spots on the garage floor.
But all of this aside, I think the heart of your question is, "If I lean
out the 'correct' mixture, does my car run faster or slower." Well,
it's still a hard question to answer. There are at least 2 typical
"correct" mixtures. The richest gives you optimum power, but incomplete
combustion. This mixture is at the apex of the power-to-mixture curve,
and at this point in the curve you'll get the most dramatic change in
power for any given change in mixture. If you lean out that mixture,
you get less power. The next "correct" mixture gives you optimum
burning---no leftover carbon, hydrogen, or oxygen after combustion.
It doesn't give you optimum power, but water vapor and clean carbon
dioxide (in a perfect world) come out of the tail pipe, and you have
this warm, green feeling about letting your intern-mechanic-daughters
run around the garage! Lean out this mixture, and the car runs slower;
but the power-to-mixture curve isn't quite as steep here, so changes in
power aren't quite as abrupt.
But then reality steps in. Today's modern, computer controlled fuel
injection systems do a pretty darn good job of maintaining a (you name
the one you're aiming at) correct mixture during all phases of engine
performance: from flat out acceleration, to engine braking, to cruising,
to idling. But your typical carburetor in your typical LBC (especially
if that LBC is more than, say, 10 years old) tends to do OK at steady
state cruising and much less well at any other point. And how often do
you steady-state-cruise in your LBC? I do it as infrequently as
possible in mine!
Constant Depression (CD) carburetors, such as Zenith-Stromberg and SUs,
do a really good job at steady state, and they aren't bad during
transition, as long as transition isn't too fast. And if your
mechanical/vacuum advance/retard are in good shape, the whole system
together does a good job during even rapid transition. But good mixture
at idle can be a particularly tough target to hit, especially if there's
slop anywhere in your engine or ignition---the load tends to vary too
rapidly for the carburetors to cope with at such a low rate of air-flow.
Various circuits applied to the carburetor are designed to deal with the
specific problems of idle mixture, but these circuits have a bad
reputation, both in the literature and in practical application.
Therefore, Will, your carburetor might be doing a great job, except at
idle, and adjustments you make to your car's performance at idle might
not apply satisfactorily to its overall performance. For example, if
the car's running rich at idle, leaning it out would increase
power---but it might make the car run worse overall.
But take whatever I say with a grain of salt. All of this that I'm
telling you as if I knew what I was talking about is really just a
regurgitation of what I think I remember reading---my practical
experience is pretty limited, and I'd be delighted to have any of my
misconceptions corrected, if anyone out there has read up on it more
recently than I have or can spot practical errors in my theory!
Kevin
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