Quadra-Jet Operation

[BadAssEddie]

Here is an article my dad wrote for the Silverton Yacht Club he belongs to because alot of guys were mouthing off on the forum about misguided and inccorect information. I thought it was pretty cool, so I figured id share it with anyone else who cares.


The Quadra-Jet carburetors are fairly simple devices. Because they are mechanical devices, they require many pieces to accomplish the many required tasks. This discussion is based upon working on the 1960s-1980s vintage Q-Jets including feedback carbs. I have not seen any of the newer models, but I assume they are basically the same.

There are several circuits built into the carburetor (this does not totally apply to feedback carburetors) that accomplish very specific tasks. They are: 1)idle circuit; 2)off-idle circuit; 3)acceleration circuit; 4)high RPM, secondary circuit or wide open throttle; and 5)choke. Following is a simplified overview of how each circuit works and how to adjust it.

The idle circuit for the Q-jet has two fuel mixture needles located at the front base of the carburetor and the two throttle plates, located at the base of the two primary bores. For the idle circuit to function properly, the throttle plates must be almost closed. This causes a differential pressure across the throttle plates (atmospheric pressure on one side and the engine vacuum on the other) which forces fuel from the carburetor bowl through the idle circuit. No other fuel is delivered to the engine at idle.

To adjust the idle circuit for maximum performance, you first need to warm the engine up so that the choke is fully open. Then determine where the idle mixture screws are adjusted by threading them in until lightly seated (count the turns!). Then adjust them out about 1 ½ turns. Then adjust the idle speed using the idle adjustment screw on the side of the carburetor where the control cable is attached. Set the idle speed per specifications. At this point, adjust only one mixture screw out until the idle speeds stops increasing and just begins to fall. Stop and turn the screw in about a ¼ of a turn. Repeat this process on the other idle mixture screw. During this process, you may need to reduce idle speed to keep the idle circuit operational (remember that if the idle is set too high, the off-idle circuit will control fuel flow).

If there is no change in idle speed while adjusting the mixture screws, then the idle circuit is not operating and you will need to adjust the idle speed or determine if there is a malfunction such as a vacuum leak.

The off-idle circuit mixes fuel through the venturis located in the two primary bores. These are the parts you see whenever you look down the bore. This controls fuel flow from above idle up to the point the secondary circuit kicks-in. This circuit is not adjustable unless you open the carburetor and change out the primary metering jets. The main thing here is to ensure that you have good fuel atomization. Peer down the carb throat (using safety goggles and nothing more than a flashlight in case of a backfire) and you will see the steady spray of fuel leaving the center of the venturis. It should look like the fluid leaving a spray can. If there are steady large drops of fuel mixed in, then you have a malfunction that needs correction.

The venturis cause a pressure drop across their length which creates a differential pressure between the venturis and the carb bowl, thus forcing fuel out into the air stream.

The acceleration circuit provides an additional shot of fuel when you press the throttle down quickly. You can check this without running the engine. Peer down the two primary bores and have somebody push the throttle from idle to the full throttle position. You will see a strong steady stream of fuel enter each of the two bores just above the venturis from a little hole in the water pump end of the carb. If the stream is weak or missing, then the accelerator pump is suspect. There may also be a blockage due to debris or gum.

The secondary circuit was mentioned in an earlier thread. This is the two plates at the back of the carburetor that open up at high RPM or load. These are large bores and dump a lot of fuel into the engine. This is what causes that lovely throaty growl when you push the throttle down.

The opening of these two plates determines how much fuel is delivered. Notice a two prong bracket on top of the carb attached to these two plates. This is connected to the secondary metering jets.

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