Joe Kleifges wrote,
>i have a question... regarding the cd-150's, and changes in altitude.
> will there be a need to re-tune with any great change in the altitude
i
>am driving at. i can, in less than an hour, be driving at 6000 ft, up
>from 1300, and i want to be aware if there is a potential problem.
> while working on an airplane sat. i noticed that the rotax powerplant
>(not a motor or engine!!) used constant depression carbs, and were
>"altitude compensating". i'm wondering if this is inherent to CD type
>carbs, or is it just a feature of the carbs on the rotax. i cant tell
if
>it is a designed feature of the rotax carbs, because i really don't
think
>the owner would like me to pull them apart to find out, and schematics
of
>the mechanism were not readily available.
> i am of the opinion that since the airvalves work relative to air
>pressure, and the ratio of vacuum to outside airpressure will stay the
>same, that they will not have to go through a mixture adjustment. with
>less outside pressure, the manifold vacuum will not "pull" the valve as
>high at altitude, due to the outside pressure not "pushing" the valve
as
>hard as it would down low, with the net result of a "leaner" relative
>mixture that is correct for the lower air density at altitude.
This is more or less correct of CD or variable venturi carburettors.
The fuel mixture is somewhat independent of barometric pressure,
but absolutely not independant ot temperature.
This is one of the main quirks with the Alpine CDs, and why
they seem to need periodic tweaking, the temperature changes
"underhood, and air temp".
On this type carb, the change in ambient air temp can effect the
mixture by 3/10s of a percent per degree C temp change.
BTW, later strombergs had a temperature compensation device
as did the funky 2700 and 7200 ford V.V. carbs.
Jarrid Gross
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