"C J Campbell" wrote in message ...

Did you try leaning aggressively? An iced up engine will run richer
and richer as the air portion of the venturi is closed off with ice.
The exhaust will then cool drastically, and surprise (!) there is very
little heat left anywhere to burn out that ice. The problem thus
feeds on itself.

The only way to get on the other side of this with a badly iced engine
is to quickly lean for best power, raising the EGT until enough ice
burns out. then the mixture can be restored to a more normal setting.
Note that this burnout method may cause chunks of ice to go thru the
engine, making the mixture to suddenly go so temporarily lean that the
engine quits again.

I don't know why this isn't taught to students, or even written in the
aviation rags.

A dropping EGT is an effective way to initially detect ice. Our 172M
also ices very slightly quite easily, but i've never had it progress
to the point of serious roughness. I do glance at the EGT many times
when in the air.

As an aside, expect that when an engine at cruise is properly leaned,
suddenly applying carb heat will make it run richer due to the less
dense hot intake air, as raw gas flowing along the walls of the intake
manifold will suddenly vaporize and go thru the engine. Pushing carb
heat off will make it then make it run so temporarily lean that the
engine may stumble momentarily. It takes a few seconds for the
balance of liquid fuel and vaporized fuel to be re-established.