If not, I have two Q-jets: a 7040262 (I looked this up on a GM Web site and it said Pontiac 350 - 428 carb) and a 7042240. (This was off a '72-'74 Buick per the same site.)
The guy that I bought it from many years ago said that it came off of an SD-455 drag car. How could I tell if this is an 800-cfm carb? Would either of these carbs be suitable and what size jets and metering rods would I need for the R/A-IV 400 that I have? I would really appreciate your help on this.Christopher M. HollandVia Internet
Rocky Rotella Responds:
Having used both Holley and Quadrajet carburetors on Pontiacs, I can tell you that both are excellent when properly tuned for that application. However, I have found that the Quadrajet's spread-bore design seems to make it the choice for street applications where maximum throttle response and fuel economy are a concern. When properly modified, it can perform as well as a Holley, too.
If you're looking for bolt-on performance, I don't think an off-the-shelf unit available from a local parts store or large mail-order supplier would be a step in the right direction. Each original Quadrajet was precisely calibrated for its application and, when attempting to install a casting onto a non-original high-performance setup, the result isn't always successful, which lends to the negative reputation.
With a few simple modifications that expand on the Quadrajet's versatility, any casting can be made to work in virtually any application, and a custom-calibrated unit like that from Cliff Ruggles of Cliff's High Performance (www.cliffsqjet.com) is likely your best choice. If you like to tinker though, forego that option and consider modifying one of the castings you have available. Cliff's new book, How to Rebuild and Modify Rochester Quadrajet Carburetors, will guide you through that process.
Being a new subscriber, you might not have seen the two Quadrajet-specific articles we prepared that were printed in HPP several issues ago-"Quadrajet Quotient" (Dec. '06) and "Calibration Celebration" (Feb. '07). In those stories, we learned how to differentiate between what are referred to as 750- and 800-cfm castings and then used Cliff's book to modify one for a specific application. It's delicate work, and small changes can make a significant difference, but I think you, too, will find the results well worth the effort.
Of the two carburetors you have, No. 7042240 ('72 Buick 455) has a greater airflow capacity than the No. 7040262 ('70 Pontiac) and as such, may provide a slight performance boost above 3,500 rpm. If originality isn't a concern, I'd suggest modifying the Buick casting for maximum performance potential. As you'll learn in the book, when increasing available idle fuel, it also changes the primary fuel curve, oftentimes necessitating a leaner primary jet and rod combination. Use How to Rebuild and Modify Rochester Quadrajet Carburetors' list of "recipes" and select the calibration that most closely resembles yours, and you'll find just how well a properly-calibrated Quadrajet can perform.