Quadrajet History and Design

The large-bore Pontiac carburetors have a carburetor-mounted thermostatic coil assembly on the main body. Buick’s model received a manifold-mounted thermostatic choke coil. Both of these Quadrajets are considered the best of the 4M-series for high-performance use. Since the Super Duty carburetors used the hot-air-style choke, the choke pull-off was moved to a separate bracket bolted to the front of the carburetor. The 4M casting was redesigned for 1975 and is referred to in Delco literature as the “Modified Quadrajet” and designated the M4M series. Most retained the larger 1 7/32 inch primary bore like the Buick and Super Duty Pontiac 4M, though some castings bound for low-performance, small-displacement applications retained the 1 3/32 inch bore.

Regardless, the new modified castings also included several design changes. The most notable was a much smaller float design. This increased fuel-bowl capacity while maintaining sufficient pressure on the needle/seat assembly to control fuel level. Other changes included an adjustable part throttle (APT) system located in the right front corner of the main body.

Auxilliary Power Piston in Quadrajets

Some 1975-76 models used an auxiliary power piston with a single metering rod
and pressed-in jet. This system worked in conjunction with the APT system and
the main jets and metering rods

A brass cap in the air horn was located directly above an adjustment screw and was used to fine-tune part-throttle air/fuel mixture ratios.
In addition to APT, many 1975 model-year Quadrajets received an additional power piston and single jet/rod assembly. The intention was to provide closer control over the air/fuel ratios while operating on the primary circuit. However, this was not as effective as initially desired and was removed from most carburetors by the 1976 model year. The carburetors using the auxiliary power piston assembly are somewhat difficult to set up for performance use. Although they can be modified with some additional effort, it is usually better to simply obtain a later-style carb.

The 1976 castings are the first of the more desirable M4M Quadrajets. They contain many features that make them an excellent starting point for high-performance use. With an external appearance identical to the 1975 castings, they too use the small float design. The APT system, however, was relocated next to the primary power piston in the void left from the auxiliary power piston that was used in 1975. A black plastic cup filled the cavity left from the 75-type APT.

Quadrajets and APT

This later APT system replaced the single pressed-in jet and metering rod used in the 1975 and some 1976 models. The new system had a provision to raise and lower the power piston.

The power piston was equipped with a small pin that
contacted the APT adjustment screw, which could be adjusted externally. This new-type APT system allowed for full control over part-throttle air/fuel ratios by simply repositioning the tapered portion of primary rod in the primary jet. Because of this flexibility, these carburetors are much easier to tune than earlier units.The Quadrajet continued for the rest of the 1970s without any significant changes. In 1979, however, the plastic limiting caps on the idle mixture screws were eliminated. Hardened steel caps pressed into the base plate took their place. This made any additional tuning impossible and prevented any adjustment that could affect tailpipe emissions. Several methods of removing these plugs have evolved over the years, but the most effective method was printed in AC Delco service literature. Centerpunch and a sharp hacksaw blade are used to score and cut the front edge of the base plate. The main drawback is the rough cutting marks left behind on those that have had the steel plugs removed. Once removed, however, these base plates are as good as any other.

The start of the 1980 model year was virtually a carryover from the previous year. One noticeable change was the choke system. The screws that typically fastened the thermostatic choke coil to its housing were replaced by rivets. This was another attempt for better control of emissions by removing the chance of an incorrect choke valve adjustment……