If you’re using an old carburetor that has sat unused for a long time—such as the time it took to remove, rebuild, and reinstall the engine—it’s pretty common for the gaskets in the metering block to dry out and/or shrink, causing idle problems. If everything else seems to check out, a rebuild of the carb to replace the old gaskets will likely do the trick.
One More Thing: Overdrive Transmission Compatibility
When using a Holley vacuum-secondary carb on a vehicle with a non-electronic overdrive automatic transmission, such as the AOD, a throttle kick-down bracket must be used. Otherwise, the transmission’s throttle valve (TV) cable won’t be positioned properly. When that happens, the transmission will shift too early or too late and the line pressures won’t be correct, eventually (and probably quickly) causing catastrophic (and expensive) transmission problems. The brackets are available from a variety of aftermarket vendors, such as TCI and B&M.
Tips For Setting Up Dual-Quads
If you’re replicating a factory system or going after the style of a period dual-four-barrel induction system, you’re right in thinking it’s trickier to set up than a simple single-carb system—but not that tricky. It’s just a matter of matching your tuning inputs consistently between the carbs. Follow these tips and your dual-quad muscle car should be running crisp and clean, with great throttle response. By the way, we’re starting with the assumption that the carburetors are installed on the intake manifold and have been set up enough to be in running condition.
- Disconnect the throttle linkage between the carburetors.
- Turn the idle speed screw on each carb all the way out and back in, then 1-1/2 turns from dead-closed. Repeat this step a couple of times as necessary to make sure the screws are in the same position on each carb.
- Mixture adjustment--the vacuum method is the most accurate and requires connecting a vacuum gauge to at least one of the carburetors; it doesn't matter which one, but if you can hook vacuum gauges to both carbs, that's all the better for a more accurate overall reading. If the gauge is connected to the correct vacuum fitting, the gauge will register immediately upon engine start up. With the engine running, adjust the mixture screws--a quarter-turn at a time--to get the highest smooth-idle vacuum reading. Make the adjustments on one carburetor at a time rather than making matching adjustments simultaneously between them.
- For properly balanced mixture settings on the carbs, the mixture screws shouldn't be more than 1/8 or 1/4-turn apart from one another. If they are, start over and make finer adjustments to balance the screw positions, but always keep in mind the goal is achieving a smooth idle.
- Next, adjust the idle speed screws on both carburetors equally to achieve the desired idle speed. Check the idle in gear and out--there shouldn't be more than about a 250- to 300-rpm difference for most engines with "small" or mild cams--and no more than a 400-rpm difference for engines with large cams. If the idle speed difference is greater than that, the carbs may not have been adjusted properly. It could also mean the base timing is off and maybe even that the torque converter is too small, but start with the carb settings.
- Connect the throttle linkage and adjust so that it fits snugly between the carbs, but not so tight that it holds one of them open.
- Make sure both carbs open fully when the gas pedal is pushed to engage wide-open throttle.
- For a progressive linkage, adjust the linkage so that the rear carb opens slightly before the front during light throttle application. Typically, this means a slightly looser linkage connection at the front carb, but one that still ensures both carbs go full-open at wide-open throttle. Importantly, a progressive linkage should only be used if the intake manifold has a common plenum, and not if each carb feeds only half of the cylinders directly. Otherwise, a progressive linkage could lead to an engine-damaging lean condition.
7 The jets are numbered,...
7 The jets are numbered, with larger numbers indicating larger-diameter orifices for the fuel to flow through. Removal and installation is as easy as screwing them in or out with a flat-blade screwdriver. When changing jets, experiment in small-size increments, as minor jet size changes can have a comparatively large effect on performance.
8 Changing the power valve...
8 Changing the power valve requires removal of the metering block in order to loosen it from the block.
9 Look closely at the connection...
9 Look closely at the connection points for the electric choke mechanism and you see “-” and “+” symbols. The negative point grounds the choke back to the carburetor, while the positive point is connected to a key-on hot lead.