Starting in the 1975 model year, major casting changes were made to the air horn, float bowl, and throttle body of the Quadrajet carburetor to incorporate a new Adjustable Part Throttle (A.P.T.) feature and, on some applications, the addition of an auxiliary power piston and metering rod assembly. These new models were termed "modified" and designated by the prefix "M" to the mode listing (e.g., M4MC).
ADJUSTABLE PART THROTTLE
The A.P.T. feature on these "modified" models consists of an adjustable metering rod assembly with filler spool (Figure 17-A) or combination aneroid metering rod assembly (for an explanation of aneroid, see Altitude Compensation, below). The adjustable metering rod, with or without aneroid, provides close tolerance control of fuel flow to the main metering system during the part throttle range.
The A.P.T. adjustment is performed at the factory during flow test by turning the threaded metering rod, or aneroid-metering rod assembly, up or down to position the metering rod in a fixed metering jet located at the bottom of the fuel reservoir in the float bowl. This sets the part throttle air/fuel mixture to the desired flow band.
On M4MCA-M4MEA models, a barometric pressure-sensitive aneroid, sometimes called a "bellows", is included as an integral part of the threaded A.P.T. metering rod assembly (Figure 17-B). The aneroid, being sensitive to air pressure change, automatically either expands or contracts with changes of altitude to lower or raise the metering rod in the fixed metering jet to maintain control of part throttle air/fuel ratios.
NOTICE: The position of the A.P.T. metering rod in the fixed jet is extremely critical. Adjustment should NEVER be attempted unless a replacement is required. The threaded A.P.T. metering rod should be adjusted carefully following adjustment procedures (See step 7a, page 38).
As a result of these changes, a new float assembly, plastic filler block (located in the top of the float chamber over the float valve), and new air horn gasket are used with the "modified" carburetor models.
SEPARATED MAIN WELLS
The main metering system of most later "modified" Quadrajet models (Figure 18) operates similarly to that described for 4MV-4MC models, above, except the float bowl casting is revised to provide for separated main wells. The separated main well feature is used to feed each main nozzle for improved fuel flow through the carburetor venturi. On these models, a special aneroid cavity insert is used in the fuel chamber to replace the adjustable metering rod assembly with filler spool, or aneroid-metering rod assembly, formerly used.