Refer to figure.
When applying the carburettor heating, hot air is supplied to the engine. Hot, less dense air will reduce the engine’s power output by roughly 15% and will create a richer mixture.

Because the risk of detonation is much higher at high power settings, carburettor heating shouldn’t be selected at power settings greater than 80%. It should be used for a sufficient period of time to restore engine’s original RPM, only when icing conditions are observed along with an unexpected drop in RPM. Bear in mind that when applying the carb’ heat the RPM will initially drop and then increase again provided the accumulated ice has been successfully removed.

Due to the decrease of engine power, carburettor heating is normally not used during take-off and climb power settings. Especially important, when taking off from short fields, you want all the power your engine has to offer, as carb heat robs your engine of some power. If you have carb ice, applying carb heat will cause your engine to make even less power (hot air is less dense than cold air).