Slater Aviation · Engine systems

Carburetor lab

Set the weather and work the engine controls. Watch air and fuel move through a float-type carburetor, see what the tachometer does, and find out when carburetor ice forms.

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Part labels

Tachometer

Carb air temp

Fuel-air mixture—

LeanBest powerRich

Venturi blocked by ice0%

Engine

Running normally.

Engine controls

Throttle

Mixture Pull to the left to lean. All the way left is idle cutoff, which stops the fuel.

Carburetor heat

Environment Icing unlikely

Altitude

Outside air temperature

Relative humidity

Try a scenario

Carburetor ice

Fuel vaporizing in the venturi and the drop in pressure can cool the air by 60 to 70 °F. Ice is most likely below 70 °F with humidity above 80%, and it can form on a 100 °F day. In a fixed-pitch airplane the first sign is a drop in rpm.

Carburetor heat

Heat costs power: expect an rpm drop of up to 15%.
With ice: rpm drops, the engine may run rough, then rpm rises as the ice melts.
No ice: rpm drops and stays steady.
Use full heat. Partial heat can warm the carburetor into the icing range.

Mixture

The carburetor is set up for sea-level air. As you climb, the air thins but the fuel doesn't, so the mixture gets richer and the engine runs rough. Lean until the rpm peaks. Pull too far and the engine quits from a lean mixture.

A simplified teaching model of a normally aspirated engine with a fixed-pitch propeller. Numbers are illustrative, not for flight planning. Follow your POH. Reference: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7.