G420E/G424E Tier LP Engine System Operational Overview 21
When the engine is cranked over it begins to draw in
air, creating a negative pressure signal. This negative
pressure signal is transmitted through a port in the
check valve plate to the AVV chamber. A
pressure/force imbalance begins to build across the
air valve piston between the AVV chamber (below the
piston) and atmospheric pressure above the piston.
Approximately 6” W.C. (Water Column) of negative
pressure is required to overcome the air valve spring
force and push the air valve assembly (piston)
downward off the valve seat. Approximately 24” W.C.
pushes the valve assembly to the bottom of its travel
in the full open position.
The amount of negative pressure generated is a
direct result of throttle position and the amount of air
flowing through the mixer to the engine. At low engine
speeds, low AVV causes the piston air valve to move
downward a small amount, creating a small venturi.
At high engine speeds, high AVV causes the air valve
piston to move much farther creating a large venturi.
The variable venturi air/fuel mixer constantly matches
venturi size to engine demand. To prevent engine
reversion pulses, commonly encountered in small
displacement engines, from having an effect on the
piston AVV chamber, a check valve is incorporated on
the check valve plate control port to the AVV chamber.
The check valve is held open with gravity and
remains open with any negative pressure signal from
the engine. If a reverse pressure pulse, caused by
engine reversion, travels up the intake manifold
toward the mixer it will close the check valve for the
duration of the pulse, preventing the pulse from
entering the AVV chamber.
A main mixture adjustment valve on the fuel inlet of
the N-CA55-500TR is not used in the MI-04 system,
however an idle mixture adjustment is incorporated
into the mixer (Figure 19).The idle mixture adjustment
is an air bypass port, adjusting the screw all the way
in, blocks off the port and enriches the idle mixture.
Backing out the idle adjustment screw opens the port
and leans the idle mixture. The idle mixture screw is a
nylon type screw that is factory set with a tamper
resistant cap installed after adjustment. Accurate
adjustment of the idle mixture can only be
accomplished by adjusting for a specific fuel trim
valve (FTV) duty cycle with the service tool software,
and should be only be adjusted by trained service
technicians.
Idle Mixture
Adjustment
Screw
Electronic
Throttle
Air
Intake
N-CA55-500TR
Figure 19
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