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. N-CA55-500TR Air Intake Idle Mixture Adjustment Screw Electronic Throttle Figure 19 G420E/G424E Tier LP Engine System Operational Overview 21