G643(E) Service Manual
Chapter 5. Engine Management System (EMS)
156
A pressure/force imbalance begins to build across
the air valve diaphragm between the air valve
vacuum (AVV) chamber (above the diaphragm) and
atmospheric pressure below the diaphragm.
Approximately 6 inH2O (14.945 mbar) of negative
pressure is required to overcome the air valve spring
force and push the air valve assembly upward off
the valve seat. Approximately 24 inH2O (59.781
mbar) pulls the valve assembly to the top 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 air valve
diaphragm assembly to move upward a small
amount, creating a small venturi. At high engine
speeds, high AVV causes the air valve diaphragm
assembly to move much farther creating a large
venturi. The variable venturi air/fuel mixer constantly
matches venturi size to engine demand.
Figure 10. Bottom View of Air Valve Assembly
Figure 11. CA100 Mixer Installed with Electronic
Throttle
A main mixture adjustment valve on the fuel inlet of
the CA100 is not used in the MI-07 system, however
an idle mixture adjustment is incorporated into the
mixer (Figure 12). 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 screw with locking threads that is factory
set with a tamper proof cap installed after
adjustment. Accurate adjustment of the idle mixture
can be accomplished by adjusting for a specific fuel
trim valve (FTV) duty cycle with the Service Tool
software or with a voltmeter.
NOTE:
Adjustments should only be performed by
trained service technicians.
Figure 12. Idle Mixture Adjustment Screw
(with tamper proof cap removed)
