Why does the engine RPM drop when carburetor heat is applied?

When carburetor heat is applied, the result is often a drop in engine RPM, primarily due to the use of hot air from the engine compartment being mixed with the fuel. Hot air is less dense than cold air, which means that when you introduce hot air into the carburetor, there is less oxygen available in the fuel-air mixture. This decrease in the density of the air reduces the amount of fuel that can effectively combust in the engine cylinders.

While it is important to ensure that the engine operates effectively in various conditions, when running on hot air, the mixture can become richer, as there is less oxygen available to fully combust the fuel. In other words, although you may have the same volume of fuel, the reduced density of the air translates to a higher amount of fuel relative to the oxygen available. Consequently, this alteration can hinder engine performance, causing the RPM to drop.

In this context, it's vital to understand that the engine's performance relies on the correct mixture of fuel and air for efficient combustion, and the introduction of carburetor heat disrupts this balance, leading to the observed reduction in RPM.