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LP Engine G430 (3.0L)
LP Fuel System (Low Emission Version)
Variable Venturi Air/Fuel Mixer Theory
of Operation
The variable venturi air/fuel mixer is mounted in the
intake air stream between the air cleaner and the
intake manifold. The design of the main section
incorporates a cylindrical bore through the center
(mixer bore) (1) and a gas discharge jet (2).
Incorporated into the Inlet (top end) of the mixer
bore is the air valve (3), Tapered gas metering valve
(4), air valve diaphragm (5) and calibrated air valve
metering spring (6). The gas valve (4) is
permanently attached to the air valve (3) with a face
seal mounted in between the two parts (the air-gas
valve assembly is mounted in the center of and
supported by the diaphragm).
When the engine is not running this face seal
creates a gas tight seal against the gas discharge
jet (2), with the aid (downward force) of the
calibrated air valve spring (6). A throttle valve (7) is
attached to the outlet (bottom end) of the mixer
bore.
As the engine is cranked over it begins to draw in
air. Since the air valve (3) is covering the inlet to the
mixer bore a negative pressure begins to build. This
negative pressure signal is communicated to the air
valve vacuum chamber (8) through four vacuum
ports in the air valve (9). A pressure/force
imbalance begins to build across the air valve
diaphragm (5) between the air valve vacuum
chamber (8) and atmospheric pressure below the
diaphragm (10). The air valve metering spring (6) is
calibrated to generate about negative 4 inches
(101.6mm) of water column at engine start,
negative 6-inches (152.40mm) of water column at
idle and up to negative 14-inches (355.60mm) of
water column at wide open throttle. This is known
as air valve vacuum (AVV). As the negative
pressure in the air valve vacuum chamber (8)
reaches negative 4 (101.6mm) inches of water
column the air valve (3) begins to lift against the
pressure of the calibrated air valve metering spring
(6). The amount of negative pressure generated is
a direct result of throttle position and the amount of
air flowing through the mixer into the engine. At low
engine speeds, low air valve vacuum the air valve
(3) lifts a small amount creating a small venturi. At
high engine speeds, high air valve vacuum the air
valve (3) lifts much higher creating a large venturi.
The variable venturi air/fuel mixer constantly
matches the venturi size to engine demand. As the
air valve (3) rise’s it lifts the tapered gas-metering
valve (4) off of its seat and exposes the gas
discharge jet (2) to the negative pressure signal
generated within the mixer bore (air valve vacuum).
This allows the negative pressure signal to travel to
the secondary vacuum chamber of the pressure
regulator and operate the regulator as previously
described. The tapered shape of the calibrated gas
valve (4), in conjunction with the calibrated
secondary regulator spring, meters out the exact
amount of fuel to match the air flow over the
complete operating range of the engine.
The variable venturi mixer is equipped with a low
speed mixture adjustment, and is factory set.
