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A little water in the air makes hardly any difference to a
running engine. In fact, there are two beneficial effects to
water droplets.
1) Water cools the intake air as it evaporates, increasing
the detonation margin. Racers use water injection for this reason.
They also add alcohol (methanol or ethanol) to the water to
improve droplet dispersal and prevent freezing.
2) Water entering the combustion chamber removes built up carbon
deposits. An engine running at high power can ingest a steady
stream of water into the carb (or throttle body) and the end
result is a clean combustion chamber.
As to the Cessna recommendation, one more reason to use carb heat
is so that the intake air bypasses the air filter. The fine pores
in the air filter are easily clogged by water (or ice) and could
lead to loss of power as the filter gets soaked.
"the use of carburetor heat in heavy rain is recommended both
to prevent build of of carby ice and to avoid engine stoppage"
I recall reading in some flying magazine article about having
to use carb heat (seems like it was partial carb heat) in a
C172 during heavy rain. If my memory is correct (not necessarily
guaranteed) that was for a specific C172 model and engine
combination. Later models of the C172 did not have this
recommendation. I don't remember ever seeing a similar
recommendation in the Lycoming O-320 or O-360 powered
Pipers or Grummans that I flew. Maybe I should look through
the old POHs that I have in my library.
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