Carburetor Jet Sizing

A carburetor meters fuel based on air VOLUME flowing through it, but at altitude the air is less dense, meaning less actual oxygen mass per volume. A jet sized for sea level would deliver too much fuel relative to the thinner air (running rich), so a smaller jet is recommended to cut fuel delivery back in proportion to the reduced air density.

All three reduce air density in the same direction -- hot air is less dense than cool air, and humid air is slightly less dense than dry air (water vapor is lighter than the nitrogen and oxygen it displaces). Since all three corrections push the same way, they combine additively into a single total correction rather than working against each other.

Density Altitude estimates the altitude at which the engine's actual air density would be found under STANDARD conditions -- it combines your real elevation with an adjustment for how much warmer than standard temperature the air actually is, since hot air at low elevation can have the same low density as cool air much higher up. A higher density altitude means the engine is effectively "breathing" thinner air than the raw elevation alone would suggest.

They're widely used rule-of-thumb approximations from motorcycle and small-engine tuning practice (roughly 1% leaner per 1,000 ft of altitude, roughly 1% per 10F above the 59F reference), not a precise thermodynamic calculation. They give a reasonable starting point for rejetting, but final tuning should be confirmed with plug chops or dyno testing on your specific engine and carburetor.