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, as the sum of two parallel contributions:

• the main circuit: the annular passage between needle and needle jet in series with the main jet. The effective area tends toward the main jet but never reaches a hard ceiling;
• the pilot circuit, added in parallel as a constant contribution present across the whole curve.

The venturi 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 main circuit's contribution at the idle position summed with the pilot — and rises with throttle as the needle lifts and the annular area opens.

The catalog covers Mikuni and Keihin (FCR 28-41mm, PWK/PJ 34-39mm). Drop-in clones such as NIBBI, KOSO and OKO share the original geometry, so the same catalog applies to them.

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 Keihin area is derived from its nominal diameter (jets numbered in 1/100 mm). Mikuni's is taken proportional to the jet number as a placeholder; the calibration constant is pending bench data.
• Main jet is approximated by its nominal diameter (modern round-head Mikuni and Keihin alike).
• 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 and Keihin needles and needle jets) from jetsrus.com.
• Keihin FCR/PWK jet-needle geometry from the official Keihin PDFs (keihin-na.com).