Everyone vividly remembers their first vehicle. Mine was a 1973 Camaro. As we all know, a 17-year-old with a Camaro is often like a monkey with a machine gun. But that's a story for another day. One of the things about that Camaro that I remember most was trying to tune the Quadrajet carburetor that never seemed to work, so I enlisted the help of a friend.
At some point in our lives most of us have had a "friend" who claimed to know how to fix our car-but in that special, not-at-all-helpful-preemptive-strike kind of way that usually ends up with disastrous results. Such was the case with my Camaro. It ran worse after he played with the carb. My buddy informed me that my carburetor was junk and that I needed a new one. That sounded reasonable to me at the time. After saving all my hard-earned minimum wage money, I was off to the local speed shop to buy my new intake and shiny new performance carb. That tired old Q-jet was history!
By a show of hands, how many have had a "pal" tell us that the OE Quadrajet is junk? Generally speaking, the Q-Jet was never meant to be a performance carburetor, though it was installed on thousands of factory musclecars, including 327-, 350-, 396-, 427- and 454-inch Chevys, the 350/350 and 427/390 Corvette engines, and the Pontiac 455 H.O. and 455 Super Duty mills. And during its 25-year life span the Q-jet carb was installed on more four-barrel GM vehicles than any other model of carburetor.
During those two decades of production, the Q-Jet underwent several changes, far too many to list here. Sometimes the changes were good and other times ... not so good. The unique design of the small primaries and larger secondaries stayed pretty much the same throughout its casting history.
While the performance aspect of the Q-Jet debate goes on, just ask any NHRA or IHRA racer how well they can really perform. Those racers who compete in the Stock and Super Stock class can attest to the fact that the Q-Jet carburetor is very capable of producing gobs and gobs of horsepower, enough to go 9s in a quarter-mile. It should also be noted that Edelbrock manufactured its own version of the Quadrajet, which was more performance oriented and flowed up to 850 cfm.
To find out how to make one of the puppies bark, it was decided to make a trip to the experts at The Carb Shop in Ontario, California. The technicians there have cut their teeth building and rebuilding performance-oriented carbs of all makes/models for years. In the process, they have created a rock-solid reputation among racers and horsepower junkies alike. When they told us they could prove the Q-Jet's value on a dyno and compare the numbers to other popular models, we jumped into the briar patch to take a look.
The numbers speak for themselves. The Quadrajet, a carburetor that was never intended to produce these types of numbers, can indeed run with the big dogs. If it's possible to keep your foot out of the secondaries, there just might be an added bonus of saving some mpg. It should be noted that our testing was done with the supercharger disconnected.
From the factory the carburetor was designed to operate as efficiently as possible across a broad power band for the average consumer-ultimately it was used at the same time as the ever-restrictive emissions laws set up by the U.S. government came into play. Throughout those years, GM altered the Q-Jet in good and bad ways by trying to satisfy those government demands. What makes the Quadrajet a unique carburetor is its amazingly large secondaries (as seen here). When those open up, that's where the power is made. Take a stock engine equipped with a Q-Jet and you have an efficient engine. Once the engine is altered with a higher lift cam or heavy breathing heads, the fuel demands of that engine have just been changed, and that Q-Jet is now inefficient. The tips and tricks that turn a Q-Junk into a Q-Jet are in the fuel metering.
These secondary metering rods are one of the reasons that a Q-Jet will run efficiently or inefficiently. Fortunately the metering rods on the secondaries are easily accessible and therefore simple to change. Depending on the power curve of the engine, the secondaries can run rich or lean. From top to bottom these metering rods are rich all the way to lean. Not shown in this photo is the power piston and spring. Along with the metering rods and jets, the power piston is another key element in adjusting fuel curve.
The base plate and butterflies can affect the carbs performance in a number of different ways. It's possible on some carbs for the base plate to be warped, in which case it will have to be machined to make it true again. The same can also apply to the butterflies and the possibility of their not being true, or how they are adjusted.
One of the things the guys at The Carb Shop pointed out to us that can affect performance is the link from the choke to the secondaries. If bent, this linkage can adversely affect when the air flaps on the secondaries open and close.