Carburetor Jet Correction for Altitude and Temperature

A carburetor is a fixed-geometry device: it meters fuel in proportion to the airflow through the venturi and it was calibrated for one air density. Take it up a mountain and the air thins while the fuel metering does not, so the mixture goes rich -- and rich costs power on top of the power already lost to the thin air, fouls plugs, and on a two-stroke can load up badly enough to stall. The correction follows the density ratio, which is the pressure ratio times the inverse absolute-temperature ratio, and temperature matters as much as pressure while pulling the other way: cold air is dense, so a cold morning at altitude leans out less than a hot afternoon at the same elevation. The part that gets people is WHICH quantity the ratio applies to. Fuel flow through a jet goes as its area times the square root of the venturi depression, and that depression itself falls with air density -- so jet AREA scales as the SQUARE ROOT of the density ratio, and jet DIAMETER as its FOURTH root. An engine jetted at sea level on a standard day and taken to 8,000 ft sees a 0.743 pressure ratio, a 1.058 temperature ratio, and a 0.786 density ratio: it is breathing 21 percent less air, a jet stamped by flow number goes from 160 to about 142, and a jet measured by diameter goes from 0.040 in to 0.0377 in, a change of less than four thousandths, which is why diameter-measured jets look deceptively insensitive and get under-corrected. Power falls roughly with the density ratio, about 21 percent here, matching the familiar field rule of about 3 percent per thousand feet. Note the temperature term's size: at the same 8,000 ft on a 90 F afternoon the density ratio falls to 0.700 and the engine wants a smaller jet still, a full step from the cold-morning answer at the same elevation. A first-order correction for a fixed-jet carburetor's MAIN circuit. Idle, pilot, and needle circuits have their own calibration and their own altitude behavior, and a needle position or clip change is often needed alongside a main jet. It assumes the engine is otherwise correctly jetted at the baseline, which is frequently not true. Altitude-compensating carburetors, forced induction, and any form of closed-loop electronic control invalidate the whole approach. Two-stroke engines are additionally sensitive because the jetting also carries the lubrication, and running one lean at altitude is how a piston seizes. Exhaust gas temperature, plug reading, and a dynamometer are how jetting is actually confirmed. The engine manufacturer's altitude kit and specifications govern.

density ratio = (P / P0) x (T0 / T) on absolute pressure and absolute temperature; jet area ratio = sqrt(density ratio); jet diameter ratio = density ratio ^ 0.25; corrected flow number = original x the area ratio; power ratio is approximately the density ratio.

The air-density ratio from the absolute pressure ratio times the inverse absolute-temperature ratio, and the carburetor jet-scaling relation in which fuel flow follows jet area times the square root of the venturi depression while required fuel follows air density -- so jet area scales as the square root of the density ratio and jet diameter as its fourth root, by name. A first-order correction for the MAIN circuit only. The engine manufacturer's altitude kit and specifications govern.

Ratio arithmetic on the user's own pressure, temperature, and jet size; no manufacturer jetting chart is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: baseline_pressure_inhg, baseline_temp_f, actual_pressure_inhg, actual_temp_f, jet_flow_number, jet_diameter_in, pressure_ratio, temperature_ratio, density_ratio, area_ratio, diameter_ratio, corrected_jet_number, corrected_jet_diameter_in, power_ratio

• Area is the square root fuel flow goes as area x sqrt(depression) and the depression falls with density, so area scales as sqrt(density ratio) and diameter as its fourth rootcarburetor metering practice
• Main circuit only idle, pilot, and needle circuits have their own calibration; a needle or clip change is often needed alongsidecarburetor tuning practice
• Invalidated by compensation altitude-compensating carburetors, forced induction, and closed-loop control break the whole approachengine manufacturer data