Why does applying carburetor heat lead to a decrease in performance?

Applying carburetor heat leads to a decrease in performance primarily because hotter air is less dense than cooler air. When carburetor heat is applied, it warms the incoming air, which can result in a decrease in the air density relative to the cooler air that the engine would otherwise be receiving. The reduction in air density means fewer air molecules are available for combustion, leading to a decrease in engine performance and overall power output.

In aircraft engines, particularly those that utilize carburetors, optimal performance is achieved using cooler, denser air that can mix with fuel more effectively, providing a better fuel-air ratio for combustion. Thus, when carburetor heat is used, while it helps prevent ice formation in certain conditions, it simultaneously reduces the engine's potential power due to the introduction of warmer, less dense air.