From: William Mills <william@wrmills.plus.com>
Date: Tue Sep 20 11:54:28 CDT 2005
Subject: Re: Europa-List: Fwf Prices
That to me would suggest automatic
mixture compensation for altitude, but I am only guessing.
If I can remember, I will certainly take some MAP readings next time.
Best wishes,
William
---> From what I recall reading, the carbs used on the 912 series are what are
known as "Constant Velocity" carbs. Basically, they are motorcycle carbs. In
an old style motorcycle carb, the throttle not a butterfly plate, but a slide
that is controlled by the twist grip. As the slide is raised, controlling the
air admitted to the engine, it also lifts the needle, which admits more fuel.
In a constant velocity carb (aka CV carb), the throttle is now a familiar
throttle
plate, and the slide is now controlled by a diaphragm. This is the round
donut on top of each carb. The diaphragm controls the slide position by way of
manifold vacuum and outside air pressure.
What this did for motorcycles was to keep up the air velocity through the carb
when the twistgrip was whacked open, which improved throttle response as well
as allow for a leaner mixture because the carb didn't have to be set
artificially
rich to compensate for the mixture going lean when the throttle was opened
quickly.
What does this do for the Rotax? Well, at altitude, you will have the throttle
100% open. But due to the low air pressure, there will not be much vacuum to
open the slide. Therefore, the needle will not be lifted up as much and won't
admit as much fuel. Bingo - automatic leaning.
I don't know how well it really works from a fuel mixture standpoint, but there
is no doubt the 912 carbs do lean the mixture at altitude.
What I don't understand is why Rotax is even using carbs to begin with? That
little
engine just BEGS for fuel injection. Rotax needs to make a deal with Bosch
and have a nice little fuel injection setup developed. You could probably
rip it right off a BMW motorcycle.
Maybe by the time we ever get around to buying our engine, Rotax will have a
fuelie
setup.
Chris
A159
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