What operates an automatic mixture control?

• A. A sealed bellows that contracts or expands with a change in pressure and temperature.
• B. A mechanical linkage adjusted by engine speed.
• C. A venturi air flow sensor.
• D. An electronic sensor module that monitors fuel trim.

Answer: (A)

Automatic mixture control relies on a sealed bellows (a flexible diaphragm) inside the carburetor that expands or contracts in response to changes in intake pressure and temperature. This movement drives the fuel metering mechanism—shifting a needle or lever so the engine automatically richens or leans the mixture as air density and operating conditions vary. This vacuum/pressure-driven device keeps the air–fuel ratio steadier across different speeds, loads, and temperatures without needing a manual or electronic adjustment. Other options rely on rpm-based linkages, air-flow sensing in electronic systems, or electronic fuel-trim modules, which are not how a traditional automatic mixture control operates.