What initial change in engine performance is expected when carb heat is first applied?

When carburetor heat is first applied, the expected initial change in engine performance is a loss of RPMs and potentially rough engine operation. This occurs because the application of carb heat introduces warmer air into the engine, which reduces the density of the air-fuel mixture entering the combustion chamber. A less dense mixture can lead to a decrease in engine performance, often resulting in a loss of engine power and a rough running condition as the engine compensates for the change in mixture.

Additionally, during this phase, the warmer air may cause vaporization of accumulated ice in the carburetor, which can further disrupt the airflow and fuel flow to the engine until the ice is fully cleared. Thus, the combination of a less efficient air-fuel mixture and the potential for temporary imbalances in the engine's operation leads to the observed decrease in RPMs and can cause the engine to run roughly until it stabilizes with the warmer air being used.