Bill Eastman writes:
> However, manufacturers, having some sense, usually
> design the fuel curve such that this operating range runs on the rich side
> when the cruise mixture is correct.
Oh, but that's a dangerous assumption!
> SU's are wonderful devices but they are not the world's best fuel mixers.
> One of the reasons that our LBC's run such terrible mileage
"Terrible mileage"? Are any of us getting "terrible mileage"? I get
between 25 and 28 mpg (US) from a 1/3 city - 2/3 freeway driving style.
I don't consider this "terrible".
> is that they do
> not lean out enough at cruise. They respond to the velocity of the air
> across the jet bridge such that a given velocity of air gets a given amount
> of fuel.
This is erroneous. Were the jet needle absent, this would be true. But
the truth of the matter is that at low engine speeds, the piston drops
in order to keep the velocity constant. When the piston drops, the
needle drops, stuffing a wider portion of itself into the jet, which
lets less fuel flow. So the mixture curve is entirely (well, the piston
spring has an effect here) determined by the needle.
Don't like your mixture curve? Get a different needle. Simple.
> This in not what you really want. An engine accelerating at low
> RPM and an engine cruising at high rpm may have the same air velocity but
> they would optimally use very different mixtures. On Acceleration, you
> want to be around 110 to 130 percent fuel which is the "best power" range
> if I remember correctly. At cruise, you would want to lean down below 100
> percent to the "best economy" fuel air ratio.
I can only assume that you mean "110 to 130 percent of stoichiometry, or
13.4:1 to 11.3:1". Actually, I remember reading that the stoichiometric
ratio of 14.7:1 IS the "best economy" mode, with something around 11:1
being "best power" mode. I know, I know, References...
> Engine tolerate rich
> mixtures better than lean mixtures so you tend to tune on the rich side in
> general so that you never fall below the lean limit.
But you want to be very close.
> Personally, I tune my carburators the same way that I set my timing- by ear
> and experimentation.
I have been giving serious consideration to tapping my exhaust pipe for
an oxygen sensor, and running the wires up to a portable voltmeter in the
cabin. Such a temporary system would allow me to accurately assess
mixture in ALL sorts of conditions. An O2 sensor is perhaps $40, and
the only other real work necessary would be welding a nut to the pipe.
> This weekend my SO and I went for a ride. This is significant since she
> considers my cars to be more trouble than they are worth. In the 15 years
> that we have been together, she has ridden in one of "my" cars less then
> ten time total. She never rode in the Corv**te (an interesting story) and
> took three years to do two rides in the Spitf**e. She has already ridden
> in the A twice now and I believe she enjoyed it! Further proof that MG's
> have a magic of their own.
... or perhaps that she's breaking down.
--
Todd Mullins
Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
'74 MGB Tourer on the rich side
That's me at the corner.
That's me at the stoplight,
Losing my transmission...
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