Message text written by McFadyean
>Nigel Charles (?) also mentioned carb. temperatures he had measured.
I would argue that the close proximity of the rear cylinder exhaust header
pipes (on the Classic, not the XS) provide a considerable amount of
radiated heat to the body of the carb.<
Whilst the aluminium used in the construction of the carbs is a good
conductor of heat or cold I believe there is likely to be quite a
temperature gradient across the carb. Many of us have fitted tray type
heatshields below the carb to protect the float chamber against the heat
of the exhaust manifold. Even so the ambient cowl temperature tends to warm
the carb anyway. In contrast temperatures within the carb in the vicinity
of the throttle butterfly often go below zero during colder weather. My
sensor protrudes into the carb at just this point so is a reasonable
indicator as to the temperature at the point where ice might form. Like
most other Rotax operators I have not experienced icing but I monitor the
engine more closely once I get a warning that the carb temp is below zero.
As an aside it is possible to cause carb icing using carb heat. Carb icing
is most likely to form in the temperature band zero down to -5degC. If
during cold weather operation, with a carb temp at say -10degC, carb heat
is used it will bring the carb temp up to the danger zone. With this in
mind, if carb heat is to be used, it is important that it can always lift
the carb temp above zero. I think the large source of heat provided by the
coolant in the Skydrive unit should be able to achieve this but this is a
relevant point for aircooled engines. In case anyone thinks they are not
likely to operate with carb temps down to -10degC the drop in pressure and
loss of heat through fuel vapourisation reduces the carb temp well below
the OAT(in early October I saw the carb temp read -5degC with an OAT of
+5degC).
Nigel Charles
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