You should see a definite drop of at least 150 RPM drop when carb heat is selected on.
That's interesting, our 28 drops about 30 rpm when carb heat is selected. I've checked the linkage to the box, all seems in order. I haven't checked inside the box. General consensus is that 'it just does that.' I have had carb icing in that particular a/c and applying carb heat cleared it.