Most exhaust emission control carburetor models have an adjustable part throttle feature (Figure 13) used in production to maintain a very close tolerance of fuel mixtures during part throttle operation.  This includes a special power piston and primary main metering rods.  The piston has a pin pressed into its base which protrudes through the float bowl and gasket and contacts an adjustable link in the throttle body.  The metering rods are tapered at the upper metering end so that the fuel flow through the main metering jets is controlled by the depth of the taper in the main metering jet orifice.  During production flow test the adjustable part throttle screw is turned in or out to place the tapered rod at the exact point in the jet orifice to obtain the desired air/fuel mixture ratio to meet exhaust emission requirements.

         NOTICE: The adjustment screw is preset at the factory and capped.  If it becomes necessary to replace the throttle body, the new service throttle body will include an adjustment procedure for adjustment of the A.P.T. screw.

The tapered metering rod, with the adjustable part throttle feature, can be identified by a suffix letter after the part number stamped on the side of the rod.

ADJUSTABLE MAIN WELL BLEED

Some 4MV-4MC exhaust emission control carburetors use an adjustable main well air bleed system or Adjustable Off-Idle (A.O.I) air bleed system, for more accurate fuel control during part throttle operation (Figure 14). The adjustable part throttle or off-idle air bleed system supplements the other main well bleeds.  The adjustable part throttle air bleed screw refines fuel mixtures to meet exhaust emission control requirements.  The screw is adjusted at the factory during carburetor flow test and no attempt should be made to readjust in the field.  A triangular spring clamp forced over the vent tube covers the screw to protect the adjustment from being tampered with and should not be removed.  All service air horns have this screw preset at the factory.

PULL-OVER ENRICHMENT (P.O.E.)

A supplement source of fuel to the main metering system is used on some Quadrajet carburetors to provide added enrichment and improved fuel control during higher engine speeds and carburetor air flows.

A calibrated hole, one in each primary bore, is located either just above or just below the choke valve and feeds fuel from a tube that extends into the fuel chamber.  During high carburetor air flows, low pressure created in the air horn bores pulls fuel from the pull-over enrichment fuel feed holes supplementing fuel flow from the primary main metering system.  The pull-over enrichment system feeds fuel at higher engine speeds to provide the additional fuel needed for good engine performance (Figure 15).

On those carburetor models that have the two calibrated holes located just below the choke valve (See Choke System 4MV), the pull-over enrichment system feeds additional fuel at closed choke for good cold engine starting.  Calibrated air bleeds, located in the air horn, are used with this system.

The air-velocity sensitive pull-over enrichment system allows the use of slightly leaner mixtures during part throttle operation and still provides enough fuel during high speed operation.  This feature gives added refinement to the fuel mixtures for exhaust emission control.

MAIN WELL AIR BLEED TUBES

Some 4MV Quadrajet applications use Main Well Air Bleed Tubes in place of the main well air bleed holes (Figure 16).  The tubes, extending further into each primary bore, help main discharge nozzle response as the main metering system starts to feed fuel.