Piper AZTEC D PA-23-250 Owner's Handbook Manual page 18

THE PIPER AZTEC "D"
SECTION II
airflow measuring signals into an air metering force. Fuel inlet
pressure is applied to one side of the fuel diaphragm and the
pressure of the fuel, after it passes through the fuel control
(metered fuel pressure), is applied to the other side of the dia-
phragm. This creates a force across the diaphragm which is
referred to as fuel metering force. Relatively low airflow signals
develop high fuel metering forces by virtue of the diaphragm
areas selected. The requirement for low airflow signals makes
possible the use of a relatively large venturi which keeps engine
induction system air losses to a minimum. During idle operation,
when air intake is too small to create pressure differential re-
quired for operatioo of the diaphragm, a constant head idle spring
is used to operate the diaphragm and supply the required fuel
for idle.
The fuel control system, which is also incorporated in the
Servo Regulator, consists of an inlet fuel screen, a rotary idle
valve and a rotary mixture control valve. The idle valve is ad-
justable to obtain good idling characteristics without affecting
metering at higher power settings. The mixture control valve
gives fuel rich mixture on one stop and a progressively leaner
mixture as it is moved toward the idle cut-off stop. The setting
incorporated in the fuel control system is worked out to meet
the engine requirement for all power settings without compro-
mise. The full rich stop defines sea level requirements, and
the mixture control provides altitude leaning.
The Flow Divider, which is mounted on top of the engine,
is provided as a fuel distributor point. Six individual lines are
connected to the Flow Divider, then routed to the cylinders. The
Flow Divider contains a spring loaded positive shut-off valve
and is ported to accurately divide fuel flow to the nozzle lines.
Located in each cylinder are the airbleed nozzles. The
cootinuous flow airbleed nozzles incorporate provisions to elim-
inate the adverse effect
of
low manifold pressure at idle. Through
this, lines can be maintained full of fuel to provide good dis-
tribution and acceleration characteristics. Actual fuel metering
is provided by the Servo Regulator, not the nozzles, which
permits leaner operatioo for economy and longer engine life due
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