Tighten or loosen the screws in order to adjust the air/fuel ratio or achieve the highest vacuum reading.

• A. Throttle Linkage
• B. Idle Mixture
• C. Main Jets
• D. Fuel Pump

Answer: (B)

At idle, the air/fuel ratio is set by the idle mixture screw on a carburetor. By turning this screw in or out, you adjust how much fuel is supplied at idle, which directly changes the idle vacuum reading. The goal is to reach the position where the engine produces the highest vacuum, which indicates the smoothest, most efficient idle. Turning the screw in generally reduces fuel (leaner), while turning it out increases fuel (richer). The other components don’t meter fuel at idle: throttle linkage mainly controls how open the throttle plates are, main jets affect fuel at higher RPM, and the fuel pump supplies fuel but doesn’t set the idle mixture.