Fuel drawn upwards from the...
Fuel drawn upwards from the idle tubes mixes with a small amount of air from the upper idle-air bleed and then passes through this down-channel restriction, where it is eventually discharged into the engine. After we measured their initial diameter, both were enlarged to a maximum of 0.052 inch for our combination.
A calibrated amount of air...
A calibrated amount of air supplied by the upper idle air-bleeds mixes with the fuel drawn from the idle tubes. The bleeds are typically located in one of two specific areas on Pontiac castings. Those on early carburetors are located in the air horn, while in later years, the bleeds were relocated to the main body.
The lower idle air-bleeds...
The lower idle air-bleeds are located at the bottom of the main body in the primary barrels. These measured 0.070 inch and rarely require resizing.
Though the air horn and main...
Though the air horn and main body contain provisions for bypass air, the throttle body of this Quadrajet wasn't equipped for it. To add it, we placed a lower main body gasket over the throttle body, used a center punch to mark the location, and used an electric drill to bore a hole through the aluminum throttle body, first to 0.085 for the initial calibration and then 0.100 for the final calibration.
| Original Calibration |
| Upper Idle Air-Bleed | 0.039 inch |
| Lower Idle Air-Bleed | 0.070 inch |
| Idle Tube | 0.031 inch |
| Down-Channel Restriction | 0.040 inch |
| Throttle-Body Bypass Air | None |
| Primary Metering Jet | 0.070 inch |
| Primary Metering Rods | 0.039 inch |
| Secondary Metering Rods | 0.410 inch "CE" code |
With the original calibration recorded, we used the guidelines found in the book to determine which calibration was best suited for the characteristics of the 400ci we were working with. We then carefully drilled each restriction and added bypass air to the throttle body. We also used several components from the rebuild kit to properly prepare the Quadrajet for service including a pair of 0.073-inch primary metering jets, and we replaced the original 0.039-inch primary rods with a pair of 0.043-inch units that we had available.
| Initial Calibration |
| Upper Idle Air-Bleed | 0.052 inch |
| Lower Idle Air-Bleed | 0.070 inch |
| Idle Tube | 0.037 inch |
| Down-Channel Restriction | 0.046 inch |
| Throttle-Body Bypass Air | 0.085 inch |
| Primary Metering Jet | 0.073 inch |
| Primary Metering Rods | 0.043 inch |
| Secondary Metering Rods | 0.410 inch "CE" code |
After priming the carburetor for initial start up and setting the mixture screws approximately 2 1/2 turns out from fully seated, we were rewarded with an engine that fired immediately. It idled much smoother, and the exhaust was much cleaner. The idle-mixture screws had a noticeable effect on idle quality, and manifold vacuum increased towards 12 inches. However, we found that removing a vacuum hose still caused engine speed to increase slightly, indicating that the engine might benefit from additional bypass air.
With the carburetor removed from the intake manifold for disassembly, we proceeded to enlarge the bypass restrictions 0.015 inch to a total of 0.100. Wanting to prevent any fuel-related issues, we also increased the diameter of the down-channel restrictions to 0.052 inch. Together, these mods provide the engine with more air and fuel at idle, yet allow for full control over idle speed and quality with the idle-speed and mixture screws.