The combination of fuel and oxygen is the heart of the combustion process that powers all our piston engines in aviation. The two ingredients are mixed in different ways, but the most common, especially on older aircraft, is in a float-type carburetor.

Carburetors work on the Venturi effect, which says that pressure decreases and velocity increases as a fluid is forced to go through a constriction. The carburetor contains five critical parts, the float, the float chamber, the main discharge nozzle, the venturi, and the throttle valve.

Because the air within the venturi is at a lower pressure than the ambient air, it creates a “vacuum” that draws fuel out of the main discharge nozzle. The construction of the nozzle causes the fuel to form into tiny droplets, where it then vaporises in the air. Fuel is sent to the main discharge nozzle from the float chamber, which is primarily just a holding vessel for the fuel. The fuel level in the chamber is controlled by the float, a device that acts not unlike the float in a common household toilet.

The pilot controls this process with the Throttle. The cockpit control is directly linked with the Throttle Valve. As the pilot pushes the Throttle forward, the valve opens, allowing more air to be “sucked” from the engine and through the venturi. Reducing the Throttle does the opposite. The amount of fuel allowed to travel from the float chamber through the main discharge nozzle and into the venturi is controlled by the pilot’s mixture control. Two common systems exist within the carburetor to then meter this flow, either a simple needle valve that acts like a gate, or a pressure differential that restricts the flow of the fuel.

Carburetors are normally calibrated at sea-level pressure, where the correct fuel/air ratio is set with the mixture control at or near the "full rich" position. As altitude increases, the density of the air entering the carburetor decreases. If you don't change the amount of fuel entering the carburetor, your mixture will slowly become too rich. The mixture will become progressively richer as altitude increases, resulting in engine roughness and a reduction of power output. Rich mixtures slow the speed that the fuel/air mixture burns in the cylinder, which means less power output. Running with an overly rich mixture for extended periods can also lead to spark plug fouling, which creates a weaker spark in the combustion chamber, further reducing your power output.

To maintain a proper fuel/air ratio, lean the mixture. Leaning decreases fuel flow, compensating for the less dense high-altitude air.

As you descend from high altitudes, you'll have to progressively enrich the mixture to prevent engine roughness. If you leave your mixture leaned as you descend, eventually, the fuel/air will become too lean causing your engine to run rough and not produce sufficient power. It can also in some cases cause detonation, leading to potential engine damage.

With increasing altitude, the power of a carburettor engine decreases.