Editor's Note: Though I feel that it's important to always use Pontiac specific parts when doing Pontiac tech stories this time we were thrown a slight curve. Ray and I had discussed which carb to use prior to his writing the story. I was going to have a GTO Q-jet sent to him but the company that he was working with assured him that he would be able get Pontiac specific carb to use from a racer friend of his who drove around in an old Pontiac wagon. The friend's name is Bill Jenkins-as in Bill "Grumpy" Jenkins of Chevy drag racing fame. I thought that this would be kind of a cool twist, the racer known for Chevys providing a Pontiac carb from his driver for our story. As fate would have it, though the dimensions of the carb are the same as a Pontiac model, this Q-jet is originally from a '76 455 Buick engine as evidenced by its part number. It won't hurt the story but it is an interesting aside regarding how stories sometimes come together and why everything doesn't always go exactly as planned. Tom DeMauro
A Q-jet has a front mounted...
A Q-jet has a front mounted fuel filter. Early models used a bronze filter that was replaced later with a paper element. The spring behind the filter is used to keep the element against the incoming fuel and force it though the media. No spring, no filtration. The Q-jet is a progressive four-barrel design with an air valve actuated secondary circuit. The idle mixture screws are located in the base plate for superior idle quality. As with any carburetor, cleanliness is essential. Make a habit of spraying the entire carburetor down with cleaner as part of your maintenance schedule along with keeping the bolts sung.
Years back when I first heard someone refer to this carburetor as a Q-junk it bothered me, and since then I have been a champion for this often maligned fuel metering device. Though the Q-jet was intended to be a production carburetor, Rochester engineers made sure their creation would suit the needs of the performance community. But there was a caveat attached: you needed to understand how it worked. To this cause HPP will provide this primer on the Q-jet. For more in depth information, H-P Books offers an excellent text titled Rochester Carburetors. Authored by Doug Roe, a former Rochester and Chevrolet service engineer, the 176 page book covers all GM carburetors and includes excellent tuning tips that space would not allow here.
Mr. Q-jet
If you are looking to have your Q-jet rebuilt and modified to really wake up your Pontiac, you need to speak to Bob Wise of RaceKrafters in Lancaster, Pennsylvania. Bob is recognized as one of the countries top Q-jet tuners. His knowledge has been gathered first hand. For a number of years after leaving the Marine Corps, he was a carburetor and EFI calibration technician at the GM Proving Grounds in Mesa, Arizona. Now he spends his days with his brother Craig building carburetors along with race engines and EFI systems. You do not need to be near Lancaster to have your Pontiac enjoy one of Bob's carburetors, just ship yours to him and let him work his magic. You will be surprised at the difference. -RTB
The Q-jet is considered a...
The Q-jet is considered a three-piece design: air horn, float bowl, and throttle body. The part number is cast into the secondary side of the float bowl (some early carbs had the number stamped on a metal disc instead) and needs to be referenced for rebuild kits and high performance tuning parts. This number is for a '76 455 Buick. Whenever possible, always use a quality name brand rebuild kit.
Some applications from 1967...
Some applications from 1967 to the late '70s used a hot idle compensator that was located in the rear of the float bowl. It was used to compensate for an overly rich mixture from fuel vaporization in high heat conditions. A bimetallic spring that responded to temperature would then open a port and introduce fresh air into the idle circuit to lean the air/fuel ratio.
The accelerator pump is actuated...
The accelerator pump is actuated by a lever that attaches to the throttle linkage. To remove the air horn, the roll pin that holds the accelerator pump arm will need to be driven toward the air horn. Make sure a gap is left to allow prying the pin back for reinstallation. Many Q-jets have had the air horn casting broken trying to reinstall the accelerator pump arm. All pre-electronic control versions held the air horn to the float bowl assembly with nine screws, feedback designs used 13 fastners.
With the choke spring removed...
With the choke spring removed the secondary lock-out lever is in view. It is designed not to allow the secondary barrels to open if any choke spring tension is present. A misadjusted choke spring or a gumed-up lock lever will mean your Pontiac will run on only two barrels.
Early versions used a drill...
Early versions used a drill bit index to adjust the choke break or pull-off opening distance. Later versions employed an angle gauge specification. Some carbs had both a primary and secondary pull-off. Note the linkage connecting the pull-off to the secondary air valve. A swollen diaphragm in the pull-off can either delay or limit the opening of the secondary air valve. For proper cold driveability, it is essential that the fast idle speed and pull-off opening are set to specifications. Q-jets are very sensitive to this adjustment. When out of calibration, either a cold stumble or stall will be present if the pull-off opens the choke plate too much. Conversely, not enough opening and the engine will load-up and blow black smoke when cold.
The secondary metering rods...
The secondary metering rods are attached to a pivot that is operated by a cam connected to the air valve. When the throttle is floored, the butterflies open in the throttle plate and allow air to rush in, lock step with the piston velocity. The amount of air then controls how quickly the secondary air valve is blown open. The metering rods then lift and allow fuel flow from the secondary jets. This is in contrast to a Holley design that uses a vacuum diaphragm to operate the secondary butterflies.