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.