The venturi — the duct narrows, the air speeds up to pass through, and faster air has lower pressure, so the throat runs at a pressure below that in the float chamber, which is vented to ambient.
The main jet — that pressure difference pushes fuel from the chamber up through a calibrated orifice into the throat, where the fast airflow tears it into spray. The faster the airflow, the bigger the pressure drop and the more fuel delivered: metering that follows the engine's demand by itself.
The float and needle valve — metering only works if the fuel level in the chamber stays constant, so a float rides the level and its arm holds a needle valve against the fuel inlet: level drops, needle opens, fuel refills; level rises, needle shuts.
The throttle valve — a pivoting butterfly disc downstream of the venturi. It is the pilot's power control: it sets how much air the engine may draw, and the venturi's own metering matches the fuel to it.
Holding the mixture across the power range
The simple carburettor has a built-in error: as airflow increases, its metering over-delivers, so the mixture grows steadily richer as power rises. Real carburettors correct this with two classic devices:
Compensation jet — a second jet fed from a small open well rather than directly from the chamber. Its delivery does not keep pace with the rising pressure drop, so it runs leaner as flow increases; added to the main jet's richer-with-flow delivery, the sum stays close to constant.
Diffuser (air bleed) — air is bled into the fuel passage through holes in a diffuser well. As flow rises and the well's fuel level falls, more holes are uncovered and more air joins the fuel, both leaning the delivery and pre-mixing the fuel into an emulsion that atomises better.
Holding the mixture with altitude
Climbing creates the same problem by another route: the air thins with altitude, but the venturi's metering keeps delivering nearly the same fuel for the same volume of airflow, and the same fuel in less air mass means the mixture richens steadily as the aircraft climbs. The correction is the cockpit mixture control: a lever that reduces the fuel delivered to the jets, either by a needle valve restricting the fuel line or by back-suction, bleeding away part of the pressure difference that drives the fuel. The pilot leans as the aircraft climbs and enriches on the way down; some engines carry an automatic mixture control, a pressure capsule that does the same leaning by itself. How the pilot finds the right setting is the mixture lesson's subject.
Sudden throttle and full power
Two enrichment devices cover moments the steady-state metering cannot. The accelerator pump answers the slammed throttle: air responds to a suddenly opened butterfly instantly, but the heavier fuel lags behind, so for a moment the mixture runs dangerously lean and the engine hesitates or cuts just when power was demanded. The pump is a small plunger linked to the throttle: open the throttle quickly and it squirts a shot of fuel straight into the airstream to bridge the lag.
Power enrichment answers full power: near full throttle an enrichment valve (also called an economiser, for what it permits at lower settings) opens extra fuel beyond the best-power proportion. The surplus fuel does not burn better; it evaporates and cools the charge, holding the cylinder back from the detonation conditions the previous lesson listed at exactly the setting where they threaten most.
Carburettor heat
The carburettor heat system is a valve in the intake that swaps the normal filtered cold air for air warmed in a muff (a jacket around the exhaust pipe). Selected on, it melts ice already formed in the carburettor and prevents new ice while it stays on. It has a price, and the exam wants both halves: hot air is less dense, so the mixture richens and the power output falls (less air mass per induction stroke and a mixture pushed richer than best power). In many installations the hot path is also unfiltered, a reason to use it sparingly on the ground. Carb heat is therefore used deliberately: fully on when ice is suspected or in icing conditions, off when it is not needed.
Priming and the carburettor fire
A cold engine needs fuel it can actually ignite, and cold fuel barely vaporises. The primer pump is a small plunger pump, hand- or electrically operated, that draws fuel from the tank line and sprays it directly into the induction manifold or cylinder intake ports, past the carburettor entirely, so the first ignition strokes meet rich vapour. The other method of priming is pumping the throttle: each stroke works the accelerator pump and squirts fuel into the carburettor. It works, and it is the more dangerous habit, because that fuel is delivered upstream of the cylinders and can pool in the carburettor and air box.
Pooled fuel is the carburettor fire: over-priming or a backfire during start can ignite fuel lying in the intake. The corrective actions run against instinct: keep cranking, because the engine drawing air is the one thing that pulls the flames out of the intake and into the cylinders, where they belong. If the engine starts, it swallows the fire and runs; if it will not, the drill is fuel selector off, mixture to idle cut-off (the mixture lever's fuel-stopping position), throttle open, keep cranking to exhaust the burning fuel, and call the fire services before leaving the aircraft.
Fuel injection instead
A fuel-injection system replaces the carburettor with a metering unit and a spray line to each cylinder, delivering fuel continuously at the intake ports. Its advantages over the carburettor:
No carburettor icing — there is no venturi refrigerating itself and no fuel evaporating in the intake air; only impact ice on the airframe's intake remains a threat.
Precise, even metering — each cylinder receives its own measured fuel, so mixture distribution between cylinders is uniform and leaning can be finer and more confident.
No float chamber — nothing that needs gravity to hold a fuel level, so unusual attitudes and turbulence do not disturb the fuel supply.
Crisper response and economy — fuel arrives at the port, not far upstream, so throttle response is immediate and specific fuel consumption is lower.
The differences in operation are the flip side of the same design: the injected engine is primed with its electric pump and mixture control rather than a primer plunger, hot starts are harder because fuel vaporises in the heat-soaked injector lines, and the intake still needs an ice answer, which the alternate-air section below supplies.
Carburettor icing
The carburettor cools its own air twice over: fuel evaporating in the airstream absorbs latent heat (the heat any liquid soaks up as it turns to vapour), and the venturi's acceleration expands the air and cools it further. Together the drop can reach 25-30°C, so moisture in the intake air can freeze inside the carburettor while the outside air is far above freezing. The ice grows on the venturi walls and, worst of all, around the throttle valve, narrowing the passage the engine breathes through.