What happens to engine RPM when carburetor heat is applied?

• A. The RPM rises.
• B. The RPM drops.
• C. The RPM remains the same.
• D. The RPM fluctuates.

Answer: (B)

Applying carburetor heat warms the air entering the carburetor. Warmer air is less dense, so less air mass (and thus less oxygen) reaches the cylinders for each intake stroke. With the same fuel metering, this reduced air mass means less combustible charge and less power, so the engine RPM drops. This drop is the expected response as icing is melted and the air flow changes. Once heat is removed and the air returns to cooler, denser conditions, RPM tends to return toward normal.