Perdue,
Didn't you mean a decrease in RPM with carb ice present?
I'm not so sure about that "need more heat at idle" statement. I understand it is the common belief and it has been a "fact" since forever in aviation.
I have gotten carb ice at takeoff power, at cruise power...but never at idle power (this all with a Cont O-470 in a Cessna 182). I've had a complete loss of power just after takeoff in a Cessna 152 that was probably carb ice -- no other problem being found. (I got back to the runway).
When I put the new engine in the 182 I also installed a 14 channel engine analyzer, one of those channels being used for carb air temp. Something interesting I noted was that the coldest temps were at times of high power. Kind of makes sense, more air being pulled in, higher velocity of air, greater pressure reduction/temp reduction. I also noticed that once the power was brought back far enough the RPM dropped (constant speed prop) the carb air temp would pretty much match ambient air temp.
I changed the way I used carb heat, no more prophylactic heat on with reduced power. While still pulling some power on downwind heat on for a moment to clear any ice that may have formed then heat off once power reduced. Monitor carb air temp and use heat only when ice indicated. I was hauling jumpers at the time and did a few thousand landings operating that way and never had an issue with carb ice.