This is the only view that...
This is the only view that most hobbyists have ever seen of the idle tubes. They actually extend down into the main body and draw fuel from the main fuel well to supply the engine with fuel while idling or operating at very low speed.
Idle-tube removal can be challenging,...
Idle-tube removal can be challenging, but the process is simplified when following the suggestions in Ruggles' book. We soaked them with WD-40 and gently tapped the center tube driving it down in the collar. We then threaded a small screw into the collar and slowly pried upwards until the tube pulled out. If you choose to reuse them, gently clamp them into a soft-jaw vice, and tap the collar back down around the tube.
The stepped end of the idle...
The stepped end of the idle tube is where its restriction is located. These units had a diameter of 0.031 inch, which proved to be too small for the mildly modified 400 we were working on. We increased the amount of available idle fuel by drilling the end with a 0.037-inch drill bit.
To combat said situation, Rochester incorporated a fixed bypass that allows a specific amount of filtered air to enter the engine without passing through the primary barrels. The bypass acts similarly to a controlled vacuum leak and allows for a proper air/fuel mixture while maintaining greater control from the idle-speed adjustment screw. However, no matter how much air an engine ingests at idle speed, the air must be combined with sufficient fueling to attain proper idle quality.
What many hobbyists fail to consider is that each carburetor was calibrated for its original application, and the amount of available idle fuel is directly related to the range of requirements for that combination. Simply stated, installing a unit originally designed for a small-displacement, smog-era application onto a heavily modified performance engine may not contain sufficient idle fuel to always yield positive results, but a few calculated modifications found in a newly released book may no longer make that a great concern.
Modifying The Quadrajet
In the past, hobbyists have referred to the Quadrajet by a number of disparaging names, and most of these can be attributed to poor idle and off-idle characteristics. Until recently, only a handful of gifted carburetor tuners had the skills and knowledge required to properly prepare a common Quadrajet casting for a specific application. S-A Design's recent release, How to Rebuild and Modify Rochester Quadrajet Carburetors, by Cliff Ruggles of Cliff's High Performance in Mount Vernon, Ohio, has brought carburetor tuning into our own garages.
Local hobbyist Alan Fanning recently approached us about the poor operating characteristics of his '67 GTO. The car contains many of its original components, including the numbers-matching 400ci block, No. 670 cylinder heads, cast-iron intake manifold, Turbo-400 transmission, and 2.93 rear axle ratio. The engine has, however, been rebuilt and now displaces 406ci and has been fitted with an aftermarket 068-type camshaft featuring 216/228 degrees of duration at 0.050-inch. Though the carburetor is not the original No. 7037262 unit, it is a '67 GTO manual-transmission casting (No. 7037263), so its internal fuel calibration should be close to its automatic-trans brethren.
Fanning complained of hard starting, poor idle quality and fuel economy, and an overall lack of throttle response. Since we had previously calibrated the GTO's points-type distributor on our distributor tester, we directed our attention towards the carburetor. It had been "rebuilt" in the past, but all other attempts at tuning the unit proved futile. Knowing that it likely needed serious attention, we purchased a complete Quadrajet rebuild kit from Cliff's High Performance and made arrangements with Fanning to leave his GTO with us for a weekend.
Upon receiving the GTO, we recorded a preliminary manifold vacuum reading of roughly 8 inches. We noted poor throttle response and an over-rich condition being emitted from the tailpipes. Additionally, the mixture screws had no real effect on idle quality, and engine speed increased when a vacuum hose was removed, indicating that the engine wanted more raw air than the carburetor was supplying. A subsequent test drive revealed that the engine also wanted to stall as we approached a stop sign or stoplight.
With all indications pointing at the carburetor, we quickly removed the Quadrajet from the engine, placed it on the bench, and began disassembly. A close inspection revealed that the casting was very clean and free from any warpage, but we also found that Fanning's Q-jet was still equipped with its original 0.070-inch primary jets, 0.039-inch primary metering rods, and 0.041-inch secondary metering rods, and the throttle body was not equipped with any bypass air.
Prior to performing any modifications, we located and recorded the measurements of the upper and lower idle-air bleeds, the down-channel restrictions, and the idle tubes using a complete set of numbered drill bits. Although measuring the restrictions was straightforward, we did have to remove the idle tubes from the main body. This can be a tricky process, but Ruggles' book offers a few excellent suggestions to ensure safe removal.