Fuel Metering Systems
Carburetion
The float carburetor meters fuel with a venturi and main metering jet; the throttle valve controls airflow, and the mixture control changes the fuel/air ratio. Know the float chamber and needle valve, idle circuit, accelerator pump, economizer or power valve, and mixture control.
Carburetor ice is the classic question. Ice forms as temperature drops in the venturi and fuel evaporates; the danger zone is roughly 20 to 70 degrees F with high relative humidity, and it can occur at cruise power on a clear day. Carburetor heat richens the mixture because heated air is less dense.
Fuel injection
Continental and Bendix/TCM systems use a fuel/air control unit (servo) that meters fuel in proportion to airflow, then a manifold valve distributes it to injector nozzles at each cylinder. Fuel injection gives better distribution and avoids carburetor icing but is susceptible to vapor lock and is harder to start when hot.
Traps
Leaning to peak EGT for economy is a target, but many engines must run rich of peak (often about 100 degrees F) at high power to protect valves and cylinders. Leaning past a certain point reduces power, and idle mixture is set separately from cruise mixture.
Rule of thumb
For maximum power, set mixture about 100 degrees F rich of peak EGT; for economy, lean toward peak within the manufacturer's limits while watching CHT.