Idle Mixture Screw Types

The two possible types of idle mixture (aka pilot) screw are fuel screws and air screws. If the screw is downstream of a passage that is fed only air, then it is an air screw. If the screw is downstream of a passage that has fuel connected to it (via the pilot jet), then it is a fuel screw. Fuel screws are usually located downstream of the slide, while air screws are usually located upstream of the slide. A fuel screw always has a washer and an o-ring under the spring. An air screw sometimes has a washer and an o-ring under the spring, sometimes has an o-ring groove in the screw, sometimes has no o-ring at all, and in the latter two cases doesn't use a washer. A fuel screw has a small tip with a step below, whereas an air screw has a blunter tip without a step. An air screw sometimes has an axial hole drilled through, whereas a fuel screw never has an axial hole. A fuel screw is oriented vertically or horizontally, whereas an air screw will be oriented horizontally or at an angle. If the carburetor is a CV type, the idle mixture screw is usually a fuel screw. The mixture is richened when a fuel screw is opened (turned counterclockwise), and leaned when it is closed (turned clockwise), whereas the mixture is richened when an air screw is closed (turned clockwise), and leaned when it is opened (turned counterclockwise).

Idle mixture screw shape

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