AOPA Pilot Magazine - April 2018 - 89

positioned in the center of the venturi.
The pressure drop (suction) generated
by the venturi causes fuel to be sprayed
by this nozzle into the airflow, where it
is atomized into a mist of tiny droplets.
The greater the airflow, the greater the
suction, and therefore the greater the
fuel flow discharged by the nozzle. (This
is what makes the carb "proportioning.")
A FEW COMPLICATIONS

This simple scheme works fine at most
power settings. However, when the
throttle is closed and the engine is at idle,
there's so little airflow and so little venturi suction that the main nozzle won't
provide enough fuel to keep the engine
running. Consequently, the carb has a
special "idle circuit" that feeds extra fuel
to a secondary nozzle located just downstream of the throttle butterfly and allows
the idling engine to suck additional fuel
into the airstream to provide smooth
idling. This idle circuit has a needle valve
adjustment that allows the idle mixture to
be fine-tuned.
Another complication occurs when the
throttle is advanced rapidly from idle to
high power (e.g., at takeoff or go-around).
The airflow increases immediately, but it
takes awhile for the increased fuel flow
to reach the cylinders, and that can cause
the engine to hiccup at throttle-up. The
solution is to add a throttle-operated

accelerator pump to the carb to inject a
bolus of extra fuel through the main nozzle to prevent the engine from stumbling.
Some carbs also have a "power enrichment valve" or "economizer circuit" that
provides an extra-rich mixture at wideopen throttle.
Yet another difficulty involves changes
of altitude. The carb's venturi effect produces a fuel flow proportional to airflow
volume, but the engine needs an optimum air-fuel ratio by weight, not volume.
As the aircraft climbs higher and the density of air decreases, the weight of the
induction air decreases even though its
volume remains constant, causing the
mixture to get progressively richer. The
usual solution is to provide a cockpit mixture control that allows the pilot to lean
the mixture as necessary as the aircraft
climbs. (A few fancy carburetors-mostly
on big radial engines-incorporate an
aneroid-operated auto-lean mechanism
to eliminate the need for manual leaning,
but most don't.)
THROTTLE BODY INJECTION

Float-type carburetors have some inherent limitations. One is that they don't work
properly in inverted flight or other negative-G situations. Another is that they're
susceptible to ice build-up in the venturi
and on the throttle butterfly caused by the
sharp temperature drop from evaporation

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A BENDIX-STROMBERG pressure carburetor is mostly found on large radial reciprocating

engines.

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AOPA Pilot Magazine - April 2018

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