Modifying A Rochester Carburetor To Improve Your Pontiac's Performance - Calibration Celebration

The small changes we made were immediately detectible. Trimming the mixture screws pushed manifold vacuum to a maximum of nearly 15 inches at 950-1,000-rpm idle speed, and placing the automatic transmission into gear didn't dramatically affect idle quality nor did the engine want to stall when coming to a stop. Throttle response noticeably improved, and the exhaust tone not only gained a pleasant "tuned" sound, it also lost the persistent odor of unburned fuel. But no matter how well it idled in the garage, the real test would come once it was on the street.

Backing the GTO from the garage, we found that less throttle angle was required to get it moving. The vehicle accelerated noticeably smoother, and there was no detectible transition between the idle and primary circuits. Heavy acceleration from a stop revealed the engine pulled smoothly through the rpm range, and although we had not altered the secondary circuit, the engine seemed to have seamless transitions between the various circuits.

When Fanning returned to claim his GTO, he immediately noted how easily the Pontiac started. After his return trip home and a few weeks of normal driving, Alan says, "One of the very nice things is that it comes off idle very smoothly without any hesitation or bog at all. And it goes down the road at 70 mph nicely. When you step into it, the torque and the tall gears come into their own, and it reminds you how great of a highway car it is." He also reported a sharp fuel-economy increase on a recent road trip of several hundred miles.

What We Learned - After spending a weekend tuning the GTO, it became apparent that this Quadrajet's original calibration wasn't best-suited for the modifications this 400ci engine contained. With the assistance of How to Rebuild and Modify Rochester Quadrajet Carburetors, we successfully richened the idle circuit, which not only allowed us to lean a portion of the primary circuit but also rid the carburetor of its "nozzle-drip" condition. We also learned that each application is slightly different, and similar combinations cannot be tuned identically. In the end, a slight amount of trial and error may provide the best results, but when dealing with carburetors, keep in mind that a change as minute as 0.001 inch can have dramatic effects.

Conclusion
Over the years, we have heard from those who detest Quadrajet carburetors due to the unsuccessful tuning attempts they have made in the past. But after seeing how well the 400 in Fanning's GTO responded to a few simple idle circuit modifications and the resultant increase in engine efficiency, our example further proves the Q-jet's versatility when modified correctly. We also hope that the simplicity of these modifications proves that no casting should be discarded because of its original application, and any unit can contain the potential for high-performance use.

Although idle-circuit modifications are not something most beginners want to tackle, we are confident that after reading How to Rebuild and Modify Rochester Quadrajet Carburetors and then applying the idle-circuit modifications, any hobbyist can achieve similar results. With a little practice, you too can become a skilled carburetor tuner with the ability to turn a swap-meet or salvage-yard bargain into a high-dollar performer for your Pontiac with minimal out-of-pocket expense.

Special thanks to Jim Hand for his contributions to this article.