Carburetion

Fig. 1 Typical Exploded View Of Hitachi MR08 Carburetor:

The Hitachi MR08 carburetor is a staged two barrel downdraft unit with separate primary and secondary metering systems fed from a common float chamber, Fig. 1. The primary throttle plate is directly actuated by the accelerator linkage and the primary system functions under all operating conditions. The secondary throttle plate is vacuum operated and opens only during high RPM or high load operation.
Fuel enters the carburetor through a conventional float operated needle valve and the float chamber is vented by an ECM controlled solenoid valve. Fuel is drawn into the primary and secondary metering circuits through main metering jets located in the float chamber floor. A piston type accelerator pump provides enrichment compensation for sudden throttle openings and a blade type choke, operated by a coolant temperature sensitive wax element, provides enrichment during cold engine operation. The accelerator pump and choke operate only with the primary system.
Both the primary and secondary systems include conventional main and transition fuel metering circuits which utilize air bleeds and emulsion tubes to improve fuel atomization, and the primary system includes an idle circuit regulated by a sealed adjustable needle. In addition, the primary circuit includes a mixture control solenoid valve and a fuel cut solenoid valve.
The Mixture Control (MC) solenoid is operated by an Electronic Control Module (ECM) and controls opening of the primary system main air bleed. The ECM monitors engine and vehicle operating conditions through a group of sensors, computes the ideal air/fuel mixture ratio for proper engine operation based on information from these sensors, and controls MC solenoid operation in order to maintain the ideal mixture ratio. The ECM cycles the MC solenoid on and off, causing the solenoid plunger to cycle up and down 16 times per second. When the solenoid is energized, the plunger is held down opening the air bleed and air/fuel mixtures are lean. When the solenoid is de-energized, spring tension forces the plunger up, closing the air bleed and providing richer mixtures. Although the number of solenoid plunger cycles remains constant at 16 per second, the ECM is able to control air/fuel mixtures by varying the amount of time the solenoid is ``on'' (dwell period) per cycle.
The fuel cut solenoid is also ECM controlled. This solenoid is used to block fuel mixture flow through the idle circuit in order to prevent engine run-on and backfiring. When the ignition is switched off, or if the ECM senses high manifold vacuum such as during deceleration, the ECM energizes the fuel cut solenoid to block fuel flow through the idle circuit.
The ECM, MC solenoid, fuel cut solenoid and bowl vent solenoid are all considered part of the ``Computer Controlled Emission Control System.'' Refer to the ``Computerized Engine Controls'' section for diagnosis and testing of these components.