Pontiac began using the Rochester Quadrajet carburetor on certain V-8 applications in ’67, and it was the Division’s four-barrel of choice from ’68 onward. With literally hundreds of thousands of Pontiac-spec castings produced over the years, you’d expect that finding a suitable replacement for your Pontiac build would be an easy endeavor. That’s simply not the case.
The last Pontiac-spec Quadrajets were built during the early ’80s. Originally installed on the 301 in those years, factory metering isn’t sufficient for larger engines, but once modified correctly, they can function quite suitably for myriad applications. Those same modifications, which we outlined in “Calibration Celebration” (HPP, Feb. ’07; also available on HPP’s website), can be applied to any Quadrajet regardless of its original application (Pontiac or not), making essentially any Quadrajet casting a suitable candidate for your project.
You’re likely to find an assortment of Quadrajets at any swap meet, but just because it isn’t an original Pontiac casting doesn’t mean it won’t work for you. Like Pontiac, the Quadrajets used by other GM Divisions had certain characteristics to fit particular engine designs, and that can present you with some installation challenges. Additionally, there are some differences in the various castings over the years.
When selecting a casting to rebuild and modify for your project, here are a 12 important characteristics to consider. Good luck with the hunt!
Note: Some of the photos in our story show individual components of a disassembled carburetor. This is merely for photographic clarity.
1 Fuel-Inlet Investigation
A Quadrajet’s fuel inlet is in one of two positions, and the original application is generally the deciding factor. Those intended for Chevrolet, Cadillac, Buick (’67 only), and GMC applications typically contain a fuel inlet that’s at a 90-degree angle with the threaded opening pointing toward the vehicle’s passenger side (top). Buick (’68 and newer), Olds, and Pontiac castings have a fuel inlet pointing toward the front of the car (bottom). Neither type offers any performance advantage. If originality or ease of install is important to you, then a front-inlet casting may require less effort.
2 Vent-Hole Variations
The float bowl must be vented to outside atmosphere for proper carburetor function. Beginning in ’71, Pontiac incorporated a large vent tube at the front of the air horn on its Quadrajets for even greater effect. That characteristic can be used to quickly identify any Pontiac-only unit when walking through a swap meet. The vent tube was rotated slightly when the new M4MV Quadrajet was introduced in ’75. Pontiac air cleaners are specific to the two types, but the air cleaner base opening can be elongated accordingly in a few minutes with a grinder. If you’re limiting your search to a ’71-and-later Pontiac Quadrajet for your build, just look for the large vent tube.
3 APT Alteration
Adjustable Part Throttle (APT) is an excellent concept that Rochester incorporated into its M4MV Quadrajet introduced in ’75. The system used that model year was rather complex. A revised system was introduced in ’76, and it was much more effective and easier to use—most every GM Division was fitted with it.
APT allows full adjustability over primary metering rod depth in the jets, placing the tapered portion of the rod in the desired position to achieve optimal fueling for maximum part throttle performance. The APT is adjusted using a modified bolt or small pair of needle-nosed pliers, and it’s accessible through the air horn.
So which Qudrajets have the most desirable APT system? The answer is those with this aluminum plug located in the center of the casting. If you plan to make future APT adjustments, the aluminum plug can be driven out easily, and the casting tapped for an appropriately sized threaded pipe plug.