Rochester Quadrajet Carburetor - Getting To Know The Rochester Quadrajet

The metering rods are changed to tune the WOT mixture. Thinner rods will allow more fuel to flow through the secondary circuit. When tuning a Q-jet you can also alter the hanger ratio and the cam profile that lifts the rods for faster response. If the cfm rating of the carburetor is too large for the engine, the secondary will never function since the depression created will not be strong enough to blow the air valve open.

Some models have a fuel discharge hole that wets the top of the air valve and acts like a accelerator pump for the secondary barrels as they break open. The fuel is discharged through this hole as the port becomes uncovered and a low pressure is created by the rushing air.

The secondary opening rate is tuned through a spring tension adjustment. There is a small allen-head set screw under the air horn and a screw that is used to wind the spring. Remember, the choke pull-off also has control over the opening rate. Many Pontiac tuners use a choke pull-off from a '70-71 454 Corvette, since it was designed to allow the secondary to open the quickest. Try to avoid auto parts store one-size-fits-all choke pull-offs if you want the best performance.

Do you want to increase a 650-cfm Q-jet to a 750-cfm? Simply grind some material from the secondary stop lever. If your secondary air valve does not open perpendicular to the air horn, then the air flow is limited. Grind this tang to make sure the air valve opens fully. There was no internal difference between a 650- and 750-cfm Q-jet other than the factory set the air valve to open all the way.

With the air horn removed, the secondary emulsion tubes and fuel discharge ports are shown. The fuel discharges are the two large metal pipes that are pointed toward the air valve. When the air valve is open, the fuel will dump from these pipes into the secondary venturi due to the low pressure created by the rushing air.

All Q-jet carburetors are a spread-bore design. This means the primary side not only has smaller throttle plates, but is closer to the center than the secondary. Note how the casting bows out as it approaches the secondary butterflies. To the right is the fast idle speed screw.

One of the many attributes of the Q-jet is a three-ring booster venturi cluster in the primary. This provides excellent fuel atomization and driveability over the common two booster design. The ring-within-a-ring design is the trademark of a Q-jet. The round cavity to the top right would house an aneroid valve on some models for altitude compensation. As seen here, the primary jets are threaded-in, but the secondary jets are sleeve pressed into a cast orifice. For this reason, many tuners found favor with Holley's four-jet design since it was easier to tune.

A common problem with all year Q-jets is wear of the throttle plate bushings. When this occurs, an air leak is created and an unstable idle condition occurs. This can be repaired by a specialty shop (see sidebar).

Another problem is warping of the air horn. When this occurs, the gasket does not seal against the float bowl and fuel is then pulled from the intermediate or main metering circuit at idle. If you have a Q-jet that is very rich at idle no matter what you do, check for a warped air horn. If a replacement is not offered, a machine shop can mill the part to correct for this condition.

To the side of the accelerator pump bore is a metal plate and below that is the discharge check ball. If either one of these is missing, the engine will have a tip-in stumble since the accelerator pump will not function properly.

The accelerator pump uses two springs: one on the assembly and another underneath the plunger cup in the bore. The pump is designed to deliver fuel over time, not all at once. That is why a specially calibrated spring is used.

It is very common to find that the secondary fuel well feed tubes have fallen out of the air horn. When this happens, a WOT bog will occur due to a delay in delivering the fuel to the secondary barrels. This is often diagnosed incorrectly as an air valve that is opening too quickly, but is really a lack of fuel. You may need to stake the tubes or spread them out slightly to recreate a tight fit.

During the '70s many Q-jets had porous floats that would absorb fuel and create an overly rich mixture. Float level is an extremely critical and overlooked part of tuning. The float level is measured from the rear of the float to the top of the float bowl casting. To adjust the float, it needs to be removed and the lever gently bent.

Unlike a Holley that uses a power valve, the Q-jet has primary metering rods and a power piston, which is sometimes referred to as an adjustable part throttle (APT). The piston travel is altered with a threaded double-D screw that is barely seen here. Note the aneroid cavity plug and the float baffle. The baffle is used to control fuel movement and stabilize the float and should be retained on all street applications.

The problem that Q-jets are most known for is hard starting cold, or a start and stall when cold. Often the main well plugs leak and allow the fuel bowl to empty when the engine is shut off. During manufacturing this passage is drilled to size the main well and then sealed with a lead plug. A quick set epoxy is all that is required to solve the problem.

These are the main well plugs that leak. If the carburetor is going to be used on a high horsepower engine, it may be necessary to drill these passages larger and then seal them with a steel plug and epoxy. Even if your Q-jet gives you no problem, when you have it apart place a dab of epoxy on the plugs to keep the well from leaking.