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Diagnostic Dilemmas: Servicing Quadrajet Carburetors

May 1, 2008

Introduced at the dawn of the emissions control era of the early 1960s, the “spread-bore” Rochester Quadrajet carburetor was considered leading-edge fuel delivery technology for the time. The “spread-bore” configuration consists of two small-diameter primary venturis designed to maintain high primary air velocities during normal driving for maximum fuel economy and low exhaust emissions. The two large secondary venturis are designed to produce maximum intake airflow and fuel delivery for maximum horsepower output.

Shortly after its introduction, mechanics unfamiliar with the principles underlying the design pronounced the Quadrajet a “junk” carburetor. Fortunately, drag racers competing in the National Hot Rod stock engine classes soon found that the carburetor responded well to various “tweaks” and the carburetor soon proved its worth in the performance market. Currently, the Edelbrock Corporation manufactures several different versions of the Quadrajet for performance applications. A number of aftermarket racing carburetor specialists also rebuild and customize Quadrajet carburetors to fit various restoration and performance applications.

Design Features
Unlike any other carburetor on the market at the time, the Quadrajet nestled its float bowl between its two primary venturis so that intake air would cool the float bowl assembly. The float bowl’s fuel volume is also kept to a minimum to maximize fuel circulation through the float bowl. Both features drastically reduce fuel percolation in the float bowl when underhood temperatures reach a high extreme.

The primary venturis feature dual-venturi fuel delivery nozzles designed to achieve maximum air velocity for accurate fuel metering and optimum fuel vaporization. A pair of metering rods is located in the primary float bowl meter fuel to the primary venturis. Idle mixture is adjusted by turning two idle mixture adjustment screws located in a separate throttle plate assembly. Some later models contain an adjustable metering rod stop that allows the rod height to be adjusted for a smooth transition from off idle to main metering. The adjustment is located under a plug located between the idle mixture adjustment screws.

The Quadrajet’s large secondary venturis are controlled by a lower set of throttle plates that are opened by a progressive linkage connected to the primary throttle shaft. The spring-loaded air valves located at the top of the secondary venturis are designed to maintain a constant depression or vacuum at the secondary fuel delivery nozzles as the secondary throttle plates open. This air valve assembly also contains a cam that coordinates the opening of the secondary fuel metering rods with the opening of the air valve assembly.

Although the Quadrajet was primarily designed for General Motors’s engines, the Quadrajet was also used on many Ford and Chrysler engines. Many Quadrajets were application-specific and were manufactured with exhaust-heated and electric chokes, right-hand and center fuel feeds, and a wide number of choke pull-off configurations. In short, few models of Quadrajets are identical because they were produced in an era of increasing exhaust emissions standards.

When restoring a collector vehicle, it’s important to make sure that the carburetor model number corresponds to the application. The fact that the carburetor resembles original configuration does not ensure that the carburetor will perform correctly. In most cases, the metering rods, metering rod spring, main jets and air valve adjustments are designed to correspond to specific applications. Because the secondary throttle is a constant-depression design, airflow specifications aren’t as critical on Quadrajets as on conventional carburetors.

In addition, some late-model Quadrajets were configured to only partially open the air valves or secondary throttle plates on small-displacement engines such as the Chevrolet 305 cubic-inch V8 engine. Such a carburetor will not perform correctly on a larger displacement engine.

Last, some Quadrajets were built with an altitude compensation aneroid capsule located opposite the accelerator pump assembly. These aneroids should not be adjusted or tampered with. A computer-controlled, feedback version of the Quadrajet was also produced during the 1980s. Because only a few of these carburetors are on the road today, suffice it to say that a few special tools available from aftermarket tool suppliers are required to adjust the primary metering rod height and travel during a rebuild. A common ignition point dwell meter set on the six-cylinder scale and attached to the green connector at the  carburetor wiring harness can be used to adjust the idle fuel mixture. A scan tool may also be used to indicate metering rod solenoid dwell. Idle dwell times generally run best at 30°. Most of the basic electronic inputs are familiar to modern driveability technicians, so we’ll leave feedback carburetor technology at that.

Failure Points

Photo 1: Leakage from the secondary metering well plugs are the most common Quadrajet failure.
Because the Quadrajet was a mass-produced carburetor, it suffered from a few failure points in its basic design. The most notable failure in a Quadrajet is the secondary metering rod well plugs leaking fuel into the intake manifold air stream. The symptoms of leaking well plugs are a rich idle air/fuel mixture or a rolling idle caused by a fuel drip into the intake air stream.

  Early Quadrajets used brass well plugs that could be replaced with a more durable O-ringed aluminum plug. The well plugs on later Quadrajets were swaged into the main body and are better sealed with a hardening epoxy like JB Weld or a similar

Photo 2: Float bowl cover distortion is common on well-used Quadrajet carburetors.
hardening epoxy compound. Inserting a lozenge-shaped neoprene plug into the cavity below the secondary metering wells can seal minor leaks. Both products are
Photo 3: Although the process requires care and patience, Quadrajet float bowl covers usually can be straightened.
available from aftermarket suppliers. See Photo 1.

  A second common failure is fuel-metering problem caused by over-tightening the Quadrajet’s two 5/16” diameter front mounting bolts. See Photo 2.

Over-tightening these bolts essentially warps the bowl cover, forcing it away from the main body casting. The problem can sometimes be remedied by suspending the cover between two steel bars mounted lengthwise in a hydraulic press and incrementally applying pressure until the cover comes back into tolerance. See Photo 3.  Fuel-saturated floats also may cause a Quadrajet to run rich or flood during normal operation. Float level

Photo 4: Float level can easily be measured on the Quadrajet.
can be measured by inserting a special Kent-Moore float level gauge into the bowl vent located just aft the choke valve. See Photo 4.

Measuring the distance from the top of the bowl vent to the float and then subtracting the dimension from the bowl vent to the cover gasket can also determine the float level. If a repair manual with adjustment specifications is available, refer to the float level specifications listed for the model number stamped vertically on the driver’s side of the carburetor main body or on a tab attached above the primary throttle shaft. Although most Quadrajet float levels are set at 3/8”, the specification can vary from 5/16” to 1/2”.  Last, the metering rod piston can stick in its

Photo 5: Metering rod travel can be easily checked by inserting an applicable tool into the front float bowl vent.
bore, causing a lean air/fuel condition. The metering rod travel can be checked by inserting a screwdriver or punch into the bowl vent hole in front of the choke valve to see if the metering piston spring returns the metering rods to a raised position with the engine off. Spraying some penetrating oil into the bowl vent and tapping on the metering rod hanger with a screwdriver or punch can often loosen the metering rods. See Photo 5.

Quadrajet Adjustments
  The Quadrajet has a number of critical adjustments that make it perform better. The initial adjustment for idle mixture screws on most Q-jets, for example, should be lightly seated and then

Photo 6: Metering rod travel can be easily checked by inserting an applicable tool into the front float bowl vent.
backed out three turns. Initial idle speed can be obtained by seating the primary throttle plates and then turning the idle speed screw in about two turns.

Photo 7: The secondary metering rod height adjustment should be checked to ensure delivering a correct secondary air/fuel ratio.
Primary jetting is most easily corrected by increasing the jet size in two-step increments. To remove the primary metering rods, lightly tap the metering rod hanger with a screwdriver handle. Spring pressure will pop the plastic retainer from the main body casting.

  Before installing any Q-jet, make sure that the secondary throttle plates are seated and that the secondary throttle linkage is adjusted to specification. A tab on the primary throttle shaft actuates the secondary linkage. This tab determines the open position of the secondary plates. When the tab contacts the secondary linkage, the linkage pushes against the upper part of the secondary throttle shaft lever. At that point, the linkage should be located mid-point in the slot located below. See Photo 6.

Photo 8: Use a 3/32” hex wrench to loosen the lock screw on the air valve spring preload adjustment.
On the topside, the secondary metering rod hanger should be adjusted to ideally achieve equal metering rod height and to allow both metering rods to touch the bottoms of their respective wells. This adjustment is done incrementally to avoid binding the rods in their hanger and to avoid damaging the hanger itself. See Photo 7.

 Secondary air valve adjustment is critical to Quadrajet performance because correct secondary air/fuel metering requires a constant depression or vacuum in the secondary venturis. If a repair manual with adjustment specifications is available, begin with the applicable spec. If a specification is not available, preload the air valve spring adjustment one-half turn,

Photo 9: Make sure the secondary throttle lockout disengages when the choke valve opens.
then road test the vehicle. If the engine stumbles as it accelerates, increase the spring preload in one-half turn increments until the stumble disappears. See Photo 8.

Last, always make sure that the choke valve lockout is allowing the secondary throttle plates to open. Some Quadrajet models incorporate the lockout at the upper choke housing linkage while others locate the lockout  at the secondary throttle shaft. In either case, the choke valve linkage keeps the air valves closed until the engine warms up. See Photo 9.

Quadrajet Wrap-Up
When serviced correctly, the Quadrajet will prove to be a very responsive and economical carburetor to use for many street driving and racing applications. Although Quadrajet is very dependable, it pays to clean the external choke linkage and air bleeds with an aerosol cleaner. When changing the fuel filter, remember to replace the plastic gasket located on the end of the fuel inlet housing before reinstalling. Cross threading the inlet housing can be greatly reduced by cleaning and oiling the housing threads before installation. If the threads are seriously damaged, aftermarket kits can be used to replace the factory housing. In virtually all cases, a well-maintained late-model Quadrajet will maintain exhaust emissions within legal limits for collector and street rod vehicles while providing a full-bore performance experience.

 

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