Originally Posted by
jmmoric
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?
Are you sure it wasn't already hot and you moved it to cold (easily done in a brain fart moment). Assuming the carb heat is wired the correct way if there is ice the action of checking carb heat should cause a drop , some rough running as the water runs through the engine, then when set to cold a rise against the original RPM. I've noticed a lot of people don't check the before and after rpm to see if there was ice. Also when testing it you should test it for 10 seconds to allow the ice to melt away. A lot of people just flick it on and off. Since carb ice is so rare ( i only notice it once or twice a year) the check gets shortened. Now does seem to be the season for it.
Does any of that help explain what you saw?