Piston engine and carburettor heater?
I had the most odd experience the other day, while running the checklist before flying.
When activating the carburettor heater, the engine RPM did not drop, on the contrary it rose ever so slightly, almost unnoticable on the RPM meter, but was able to hear the increase in RPM.
I've never tried that before, do anyone have any clue to why that happens?