Idle Jet (38/120) for SI Carbs for Scooters & Mopeds

Idle Jet for SI Carburetors in Scooters and Mopeds

The 38/120 Idle Jet is a part used in SI carburetors on a range of scooters and mopeds. It controls the fuel and air ratio across the lower third of the throttle travel. Getting the jetting right matters for smooth acceleration, since an idle jet that is the wrong size can cause performance problems such as a flat spot when accelerating. It is one of the core scooter carburetor and jet components that can be tuned to improve performance.

Function and Why It Matters

The idle jet threads into the carburetor beside the main jet. The two work together to balance fuel and air at different throttle openings. Jet size is written as two numbers: the first refers to the fuel bore and the second to the air bore. As an example, a 38/120 passes less air than a 38/140, which makes the mixture richer.

Available Idle Jet Sizes

Idle jets are offered in 38/120, 42/160, 45/120, 45/140, 48/160, 50/120 and 55/160. Certain older models use "plugged" jets that have no air hole. When choosing a size, keep in mind that a smaller air bore produces a richer mixture. Tuning the idle jet usually takes some trial and error, and jetting guides offer further detail.

Specifications

• Position: Beside the main jet
• Size: 38/120
• Type: Idle jet
• Fits: SI carburetors on scooters and mopeds (specific model not listed by the source)

Fitment

How to choose: Use the first number to match your fuel bore and the second to match your air bore. Sizes available are 38/120, 42/160, 45/120, 45/140, 48/160, 50/120 and 55/160.

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