The model

The fuel-passage area tool computes geometric area, not flow rate. It is a comparative estimate to support tuning decisions, not validated jetting advice. Tune at your own risk.

What is calculated

The free fuel-passage area (mm²) across throttle, from:

• the needle profile in the needle jet bore (the annular area between needle and bore);
• the main jet, modeled as a restriction in series with the annular passage. The effective area tends toward the main jet but never reaches a hard ceiling.
• the pilot jet, modeled as a constant floor present across the whole curve;
• the venturi, which sets the needle stroke and therefore how far the curve goes. It does not change the shape of the shared section.

The curve starts at idle, with the needle anchored at the start of its taper, and rises with throttle.

What is NOT calculated

• Flow rate (vazão). Turning area into flow needs the discharge coefficient, fuel density and the pressure drop, which are out of scope here.
• Transient and dynamic effects, the air-side cutaway of the slide, and temperature.

Known approximations

• Pilot jet area is taken proportional to the jet number; the calibration constant is pending bench data.
• Main jet is approximated by its nominal diameter.
• Venturi is approximated by the needle stroke.
• Other carburetor brands (e.g. Keihin) are modeled from their own catalog data; cross-brand equivalence is approximate.

Sources

• Catalog data (Mikuni needles, needle jets, jets) from jetsrus.com.