During its years of creating mechanical fuel-delivery systems for GM vehicles, The Rochester Products Division certainly turned out some misunderstood creations. Early Rochester fuel injection almost reached red-headed-stepchild status as more than a few frustrated Corvette owners replaced these unique but complex systems with carbs. Of course, at least one of Rochester's carburetor-type offspring also developed a bad rap. Ever hear of a "Quadraflush" or a "Quadrabog?" Naysayers notwithstanding, the Quadrajet four-barrel carburetor has proved its mettle, doing its duty on hundreds of thousands of GM vehicles, from the '60s all the way into the computer-controlled '80s.
CHP's as-yet unnamed project '74 Z28 is topped with a Q-jet, and we decided to yank the old mixer off and get it back into shape for service on whatever hot little small-block we decide to drop into our Camaro. We tabbed Sean Murphy, proprietor of Sean Murphy Induction in Huntington Beach, California, to do our rebuild and tweaking for us, and he discussed this particular carb's peccadilloes. "The Quadrajet got a bad rap," he told us, "because they're misunderstood. They're sophisticated, but have proven to be a good carb, especially in heavier vehicles with automatic transmissions." This sophistication is the Q-jet's advantage and its Achilles' heel.
In this photo, Murphy has already removed the idle-speed solenoid, as well as the accelera
"There's more tunability with a Q-jet," Murphy continued, "which means that they can be calibrated more accurately." On the other hand, that also means this carb is more sensitive to cam and head changes. There's more than just a set of jets to deal with here. Among its many adjustable features, the Quadrajet uses a power piston primary metering system that can be fitted with a wide variety of metering rods and piston springs, an equally large selection of secondary metering rods and rod holders, and an air-valve system on the secondaries that is very sensitive to modification. Oh, and there are main jet sizes to be chosen, of course. The combinations aren't endless, but there are certainly many to chose from.
That being said, Murphy gave us a few general Q-jet tuning tips gleaned during his 15 years of working on the little beasts. Not surprisingly, these pointers center around the metering rods. In general, we were told, the home tuner should stick with main metering rods within the same letter family, i.e., if your carb came with 43B rods (as ours did), and you change your setup, try a 44B or higher. That being said, the more effective way to tune the primary circuit is with main jet size. "Metering-rod changes will only affect performance at light- to mid-throttle," Murphy told us. "Jet changes will affect the entire power range."
...as well as the vacuum break assembly (which must actually be pried off) and its rod.
With the secondary system, things get a bit more nebulous. In general, more aggressive cars (lighter weight, more horsepower, stiffer gearing, and so on) should have a more aggressive (i.e., higher letter) secondary-metering-rod hanger. As for the rods themselves ... well, there are many, and they're all interchangeable across the several Q-jet variations. The best resource is Rochester Carburetors, by Mark Roe (available from your local HP Books dealer). This treasure trove of Q-jet info lists secondary metering rods and their various dimensions, and explains how these tapers and tips will affect your carb's performance. Haynes' Rochester Carburetor Manual is also worth a look for the D.I.Y.'er.
In our case, we had two items on our agenda. One was to end up with a reconditioned, rarin'-to-go, and just-like-new Q-jet; the other was that this ready mixer be tweaked just enough to work with our project wreck's intended powerplant: a 355ci small-block making in the neighborhood of 400 ponies. The first goal was no sweat for Murphy, and he was even able to help with our second goal despite the paucity of info we provided about our intended engine combo. Follow along, and we'll show you how this Q-jet got to be the best looking--and performing--part of CHP's Z28 beater.