Rebuilding and adjusting the Quadra-Jet Carburetor.
Photos and Text
By Ken Styer
There have been many arguments, both pro and con about the Quadra-Jet carburetor. Some hate them, and some love them. Much of the hate comes from lack of understanding. This is to clear that up.
First, what are the advantages and disadvantages of the Q-jet? Unlike most other 4-barrel carburetors, the Q-Jet has a drastically different size primary and secondary bore sizes. The much smaller primaries act as a small two-barrel carburetor until you press the throttle enough to start to open the secondaries. Any carburetor works most efficiently at wide open throttle, or WOT. The small primaries allow the primary throttle plates to be opened wider, and thus making the carburetor more efficient than a standard two barrel, or larger primary four-barrel. This also creates better throttle response at part throttle application.
Odd as it may seem, the Q-jet on a standard engine will supply better fuel mileage the standard two-barrel for the same engine, as long as you don?t open the secondaries. Easier said than done. Most of us like to get into the throttle hard once in a while. Once the small primaries are open the huge secondaries take over and the volume is the same as other 4 barrels. This makes the Q-Jet an excellent performance carburetor for street use.
The disadvantage is also found in its unique design. Fuel flow and air volume is a bit greater to the rear of the carburetor. So the rear of the engine has a tendency to get just a bit more than the front of the engine. A 4 barrel with equal size primaries and secondaries don?t have this problem. But for street applications, it really is not noticeable. If you are running your car on a drag strip and watching for hundredths of a second change, something else may be just a bit better.
There are basically three versions of the Q-jet. The first and second versions are visually different, but are basically the same function and rebuild procedures. The third version is a computer controlled electronic version. The electronics showed up in 1981 on General Motors cars. While very similar in many ways, there are huge operation and adjustment differences. We will go through the second series, and make comments for the others. The carburetor in the photos is on/off a 1975 Chevrolet Corvette L-48 engine. Specifications vary for different applications.
Just keep in mind it is up to you to follow all safety procedures, don?t hurt yourself or your car.
Before you can rebuild it, you have to get it off of the car.
Be careful to mark all vacuum hoses for location. If you need to, draw a diagram to help keep track. Use a piece of tape on each line and mark it. It?s difficult to find vacuum diagrams for older cars if you lose track.
Disconnect the fuel line from the carb. It attaches to the fuel filter housing. You need to hold the filter housing still while you loosen the fuel line, or the line will twist.
Disconnect the throttle linkage. The link is actually a cable held by a snap clip. It can be seen in the photo attached to the same lever the springs are attached to. Remove the spring as well. Make sure to reattach these springs before trying to restart the engine. They are the throttle return spring, and without them the engine car race and create damage.
At the rear of the carb is a steel vacuum line if you have power brakes. Again, hold the fitting the line goes in to, or you will twist the steel vacuum line.
This car is missing the heat line from the manifold to the choke housing. You can see a red cap where the line belongs. If you have an electric choke it won?t be there. Directly below the red cap you can see an empty port on the intake. This is where the other end of the heat tube belongs. Just below that is a port for the air inlet to the manifold. This is just a tube inside the manifold. The exhaust port for the EGR heat the tube and supply hot air it warm the choke coil. This one will be reinstalled.
Now there are just four bolts left. Two at the rear of the carb, and two at the front. The front ones are quite long and go all the way through the carb.
You can see on this car the fuel line has been damaged and replaced with a piece of neoprene hose. It works but it?s not pretty. When you lift the carb off of the intake make
sure to keep track of the gasket. You may need to compare it to the new one to select the
Off the Car! This car has not been in operation for 5 years that the car has been sitting. It?s bound to be a mess, although it does not look too bad from here. Notice the large secondary air flaps are not shut tight. They are sticking and not moving freely. This is a problem we will need to address as we go. The choke plate on the primary is not tight either, but that?s okay. It does move freely. Notice where the fuel line was hacked and twisted from improper removal sometime in the past. Remove the gasket where the air filter sits on the carb. You may need to scrape it off. You will have a new one in the carb rebuild kit, so don?t plan on using the old one.
Two screws hold on this primary vacuum pull off. It mounts on the front side of the carb housing and has one vacuum line from the carb to the pull off. With the screws out the link can be removed. Look at the previous picture to see its mounted location. Push in on the lever and press it back into the housing. Plug the vacuum line port and release the lever. Vacuum should hold it from coming out all of the way. It will come out some. If it does not hold in a least part way, it will need to be replaced. This is a big factor in cold operation. Without it, the car will flood or run very rich when started cold. As soon as the engine starts, manifold vacuum is applied to the pull off. This opens the choke a small amount within a few seconds after start up. The choke must shut tight to start, but must open a small amount to prevent flooding or fouling spark plugs.
The secondary pull off is mounted toward the rear of the carb. Like the primary, it is held on by two screws and a link. It also has a vacuum line. Do the same test as with the primary pull off, and check the vacuum diaphragm for leaks. If it does not hold, don?t throw it out right away. Check to see if there is a small bleed hole in the housing. The bleed hole makes the pull off apply, or pull off, slower. If it has one, plug the hole and repeat the test. It should hold now. Not all Q-Jets use the secondary pull off.
Now we start taking apart the carb itself. The small screw pointed out may be a Phillips, straight slot, or torx like this one. It?s very small and important, don?t lose it. It?s hard to find something to fit. When the secondary air flaps open due to your right foot and air flow, this screw and the lever it is holding lift up. The lever raises the secondary metering rods that control fuel volume to the secondary section of the carb. With the air flaps open and the rods lifted slightly, this car could change the fuel mixture, make the car idle too fast, or hesitate on acceleration. Notice the dirt build up on the top of the carb and air flaps.
There is a throttle plate under these air flaps. The throttle plates are opened by you right foot through linkage. The air flaps open gradually as air flow is increased into the engine. This keeps the engine from bogging by opening the secondary section too quickly at low rpm.
With the screw out, lift the bar and secondary metering rods out of the carb. The metering rods only slip through the holes on the holding bar, and are not secured with any clips. Be careful not to drop them. Just pick them up again, right? Dropping the rods may bend or damage them. Damaged rods will have a huge effect on secondary operation. These rods are selective sizes. You can find a number on them. Selecting a different rod will make the secondary richer or leaner, depending on the rod you select. They have no affect on the base fuel mixture or primary fuel mixture. A smaller rod creates a larger opening in the jet, allowing more fuel to pass, and creating a richer mixture. The other way to richen the mixture would be to install a jet with a larger hole. As you lift them out note the holes where they came out. They pass through the top section of the carb, or air horn, through a gasket, and down into the secondary jets.
This roll pin is a pivot for the accelerator pump lever. When the throttle is opened the link at the right pushes the lever up, which in turn pivots and pushed the accelerator pump shaft down. It can be seen on the left, going into the top of the carb. Note which hole the link goes into the pump lever. This will affect how much fuel the accelerator pump delivers. Use a flat punch and drive the pin back far enough to release the lever. Don?t use a pointed punch. It will mushroom out the pin making it difficult to remove. Be careful not to damage the mounting for the pin. Too large a punch and a heavy hand will break the support, and destroy the top (air horn) of the carb. Now lift the lever and disconnect the link. Computer controlled carbs will have a green pin under the lever that
operates the Throttle Position Sensor. Be sure to remove it or you may never see it again.
This is a non-computer car, and doesn?t have the TPS pin.