Troubleshooting a Quadrajet Carburetor

by Don Bowman

Introduction

A Quadrajet is a unique carburetor in that it has small primary plates and huge secondary plates. It is a good carburetor for fuel economy and still has stellar performance when required. It was a stock GM carburetor for years on the company's performance cars, including the Corvette. It is a good carburetor for up to close to 400 horsepower. Its limit in horsepower applications and ability to be modified was its downfall at the racetrack when horsepower climbed. The Quadrajet uses many moving parts, unlike most carburetors.

Diagnosing

Remove the air cleaner. Inspect the choke system while the engine is cold. The choke mechanism is the plate on the top of the carburetor. It has a spring located in the manifold with a rod running up to the plate. There will be a vacuum-operated servo on the passenger side rear of the carburetor, with a rod also attached to the choke mechanism. When the engine is cold, the spring contracts and creates a pull on the choke to close. The throttle keeps it from closing as long as it is not disturbed. When the engine is started, the throttle should be tapped just a little to allow the choke to snap shut. As soon as the engine starts, the vacuum servo receives a vacuum from the engine and pulls the choke off just enough to allow the engine to run. However, it still creates a partial choke condition.

As the engine heats up, the spring in the manifold expands and puts pressure on the choke to open the choke. On all carburetors, the vacuum created as air is moved into the engine sucks the fuel out of the passages in the carburetor. When the engine has just been started, there is obviously no vacuum--so no fuel is sucked into the engine to start. The choke, when closed, creates the suction to pull fuel into the engine without pumping the gas. Pumping the gas a couple times can also start the engine, since the carburetor has an accelerator pump. However, it's easy to flood the engine this way. Move the throttle by hand just a little and see if the choke snaps closed. It should go all the way closed, or the rod is bent or the spring is maladjusted. Start the engine and make sure the choke opens a quarter of the way. If not, pull the hose off the servo and see if the arm extends and contracts when the hose is reinstalled. If not, the servo must be replaced because it operates with a rubber bladder inside and is leaking. This means that not only does it not work, it's leaking vacuum and interfering with the mixture in the carburetor. This causes the engine to run poorly.

With the engine idling, look down into the carburetor with a flashlight. A couple of jets can be seen in the front primaries. If any fuel can be seen dripping into the engine from the carburetor, the float is stuck or defective and must be repaired. The fuel is too high in the float bowl and is being sucked out of the jets into the engine, thereby flooding the engine.

Warning and Conclusion

If the engine runs OK except at an idle, the idle jets may be out of adjustment of dirty. Using a small screwdriver and starting on the driver's side, adjust the screw in until the RPM starts to drop. Turn the screw out just until the RPM stops rising and turn the screw one half of a turn further. Do the same to the other side and then repeat the process one more time. If the engine idles fine but will not accelerate when the pedal is floored and just falls flat, the accelerator pump is not working and the carburetor has to be overhauled.

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).