"John Theune" wrote in message
news:8obWf.14186$ES3.7989@trnddc02...
I may be perpetuating a OWT but I seem to recall something about carb heat
putting unfiltered air into the carb and that was why it was not always on
in addition to the issue of the hotter air decreasing performance.

I can't speak for all airplanes -- maybe there's one out there that uses a
filter with carb heat -- but yes, generally speaking carb heat means the air
is not filtered.

However, that is primarily a concern while on the ground, where all sorts of
dust and dirt can get blown up and into the air intake. Once you get aloft,
it's unusual for the air to contain enough stuff to cause harm to the
engine. Not impossible, but unusual.

A better argument against constant use of carb heat is that doing so can
take a non-icing situation and turn it into an icing one. I think this
would be very rare, but I don't believe you can rule it out. Of course,
carb heat also reduces performance, since it lowers the density of the air
coming into the engine. Better to wait for signs of carb ice and use the
carb heat as needed, rather than as a general prophylactic.

Pete