How does a pilot screw adjustment work?


Quick Answer

Pilot screw adjustments function by changing the pilot jet size, which alters the idle speed and low throttle conditions. The pilot, sometimes called the slow fuel circuit, plays an important role in fuel delivery, especially during idle and at low speeds. Adjusting the pilot screw alters the fuel and air mixture, which may alter engine performance and power output.

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Full Answer

Carburetors employ multiple fuel circuits to control fuel and air mixture and delivery, including the needle jet, which governs mid-range responsibilities, and a main jet associated with high-end fueling. While there is some overlap between the fuel circuits, the pilot jet and screw are primarily involved with adjusting the fuel and air the engine receives while idling or at low throttle. Pilot screws are typically preset in the factory. Only make alterations and adjustments to a pilot screw under necessary circumstances. Pilot screws are sometimes attached to a plug or limiter cap designed to secure their position and limit the ability to make adjustments.

Adjustments typically involve turning the pilot screw by very small increments until the proper idle speed is achieved. Depending on the make and model engine, turning the screw may require a special wrench. Always check the service manual to find specific instructions for adjusting the pilot screw. Never force a pilot screw when making adjustments, as doing so may cause damage.

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