Hi William and Kingsley
The Rotax engines are fitted with Bing constant vacuum carburettors.
On these, the throttle slide and fuel control needle are not connected to
the throttle cable (like the majority of motorcycle carbs except BMWs), but
suspended under a rubber diaphragm - which is fed from a small venturi into
the choke mouth.
The position of the throttle slide is therefore controlled by the volume and
speed of the air passing over the venturi.
The throttle cable only operates a butterfly valve in behind the throttle
slide. Open the butterfly, more air is allowed to pass and the throttle
slide rises, lifting the tapered fuel control needle and allowing more fuel
in proportion to the volume of air passing through the carb.
The benefit for aviation applications is that as the density of air drops
(when you go up) so the volume of air passing over the venturi will begin to
drop and the slide will drop - automatically lowering the fuel control
needle. It doesn't "lean" the mixture as such - but attempts to keep the
correct stoiometric fuel/air ratio.
By operation, this is altitude compensating, but will also compensate for
anything else that might reduce the air density - like hot or moist air.
Those trained to "lean off" as you fly through cloud really need do no more
than "sit back, relax and enjoy the flight" as we are so often invited to
do!
Apologies for the diversion, I'll let you all get back to squabbling over
engine choice now while I slip back into "lurk" mode ;-)
Nigel
----- Original Message -----
From: "Kingsley Hurst" <hurstkr@growzone.com.au>
Subject: Re: Europa-List: Fwf Prices
<hurstkr@growzone.com.au>
> At 13,000 ft with full throttle, my 912S fuel flow is 14 litres per hour,
> whereas on take-off at full throttle it burns 28 litres per hour. I
> assume
> that proves there is an automatic leaning of the carbs.
Hello William,
With much respect I am wondering if it really does ? At 13,000 ft your MAP
at full throttle is considerably less than at sea level with full throttle.
I suspect if you fly at sea level with the same MAP as you are achieving at
13,000 ft, your fuel flow will still be in the order of 14 LPH or most
likely even less.
If in fact you do have a MAP gauge, I would interested to hear the result of
such a test.
Regards
Kingsley in Oz.
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