Dan
Application of carb heat causes a change in rpm's because it changes
the fuel/air ratio.....NOT because there is carb ice. If carb ice is
present, the heated air will hopefully melt the ice and it will then
cause the engine top stutter a little as it goes thru the combustion
process.
If you have carb ice conditions, you can run with full heat on to avoid
the ice formation. However, on the ground, this will by-pass any
filters and you can suck dust into your carb with the dirty air and a
potential for serious engine problems.
In severe low temps, sometimes it requires full carb heat just to get a
close to useable air density for decent fuel/air ratios.
Another trick to stick in your pilots bag of magic is to use the primer
knob if your engine gets starved for carb ice and stops. The primer
system on most carbureted engines bypasses the carb and goes directly
to the intake manifold. Your fuel schematic should show you that. I've
used it a couple of times to keep the engine runing in severe
conditions around the country. The engine will run for a few seconds
then die again so you keep stroking the primer knob to keep it running
until hopefully the heat will come up enough to become effective again.
The addition of a carb heat gauge is worthwhile for most aircraft
(normal aspirated). A periodic carb heat check in the air doesn't hurt
either.
Discuss the situation with your local mechanic and he can explain it
better. If he can't, find a more experienced mechanic!
Cheers and safe flying