Which adjustment is used to set idle mixture on a float carburetor?

Which adjustment is used to set idle mixture on a float carburetor?

• A. Throttle stop
• B. Idle needle valve
• C. Choke
• D. Primer

Answer: (B)

At idle, the engine relies on a small, separate fuel circuit to mix enough fuel with air when the throttle plates are nearly closed. The adjustment that controls how much fuel this idle circuit delivers is the idle needle valve. By turning it in or out, you change how rich or lean the idle mixture is, which lets the engine run smoothly at idle and stay stable as it heats up or cools down. The throttle stop, by contrast, sets how far the throttle opens at idle and thus controls idle speed, not the fuel proportion. The choke is there to enrich the mixture for starting when cold, and the primer injects extra fuel to aid starting—not to fine-tune the idle air-fuel ratio.

At idle, the engine relies on a small, separate fuel circuit to mix enough fuel with air when the throttle plates are nearly closed. The adjustment that controls how much fuel this idle circuit delivers is the idle needle valve. By turning it in or out, you change how rich or lean the idle mixture is, which lets the engine run smoothly at idle and stay stable as it heats up or cools down.

The throttle stop, by contrast, sets how far the throttle opens at idle and thus controls idle speed, not the fuel proportion. The choke is there to enrich the mixture for starting when cold, and the primer injects extra fuel to aid starting—not to fine-tune the idle air-fuel ratio.