> We had a Posa carb. and every time it came to start the plane by hand
> propping it would be hard. The adjustment on the carb is so hard. You
end
> up to rich or to lean.
>
> We replaced the Posa 40 mm. carb with a AMF 50 mm. carb. from a 1600
Harley
> Davidson MotorCycle. We run a 1700 cc. Monnet converted VW. The plane
> starts easy and runs good.
For what it is worth, this was my experience with the POSA carburettor-
Engine - 1835cc VW, prop start with gravity feed, low wing aircraft, fuel in
fuselage behind the firewall.
Carburettor - under the engine, with mixture control, ex HAPI.
I had to reshape the taper on the needle with a file and micrometer to get
a workable result across the engine speed range.
Eventually I was able to fly but the thing was a brute to start sometimes
it was easy (very rare).
If, for any reason it stopped, you had to be super quick to turn off the
fuel or you had a flooded situation and a real challenge to get it started
again. Its favourite place to stop was when turning into wind at the end of
the runway; usually when a non POSA trained friend was flying. This habit
was traced to centrifugal effects because it only happened when turning
sharply right. I learned to avoid it turn gently.
A big factor was the difficulty with maintaining correct mixture in cruise
because fuel flow is related to the head of fuel in the tank. "Head" being
the vertical distance from the surface of the liquid to the carburettor. As
the level in the tank dropped, flow reduced. The result was mixture was
hardly ever correct. I blamed this factor for troublesome build up of
deposits on the valves. When in doubt I run rich.
An important relevant law of physics that I have never seen mentioned on the
lists is -
the flow of liquids due to gravity is proportional
to the square of the head.
It is not a simple case of fuel level half as high means half the flow.
Considering the above it may be that a high wing with fuel in the wing could
be a better choice for the POSA.
Given the above messing about, I quickly ended my love affair with the
simplicity of the POSA. However, I am still a believer in simplicity as
applied to most things.
I have been given to understand that the POSA was intended for alcohol
fuelled motor cycles where it is common practice to use really excessive
fuel to aid in cooling not reasonable excess as in aviation engines but in
the really big heaps that you can be used with alcohol over very short
periods of time. In that case some variation of flow can be tolerated.
Alan T
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