Which part is adjusted to control the engine's idle speed?

The idle speed screw is the component that is adjusted to control the engine's idle speed. By turning this screw, you can change the amount of air and fuel mixture that enters the engine at idle. A higher idle speed can be achieved by increasing the opening in the throttle plate through this adjustment, allowing more air to flow into the engine. Conversely, decreasing the idle speed involves reducing the air-fuel mixture.

While the idle circuit refers to the part of the carburetor or fuel system responsible for delivering fuel at idle conditions, and the throttle plate controls airflow, it is the idle speed screw that directly impacts the engine’s idle RPM. The plug is unrelated to idle speed adjustments, as it serves a different function in the ignition system. Understanding the role and function of these components helps clarify why the idle speed screw is specifically the correct choice for adjusting idle speed.