Ask Away! with Jeff Smith: How to Fix a Bogging Quadrajet Carburetor?

I have a small block 350 in my 1972 Nova and I recently picked up a Q-jet to bolt on, along with a new four barrel manifold. The engine runs decently except it bogs when I jump on the throttle hard. I’ve read a lot of comments by guys who say this is a classic problem with the Quadrajet—they call it the Quadra-Bog.

Rather than junk it, is there a way to tune or modify this carburetor to fix this problem? Thanks

M.C.

It’s not unusual for uninformed enthusiasts to throw insults at the Rochester Quadrajet. In my experience this misinformation usually springs from someone who just doesn’t understand how carburetors work.

The solution to your problem is likely a very simple fix.

The secondaries on a Rochester Q-jet are mechanically operated. But instead of using a secondary accelerator pump, the Q-jet uses a very simple air valve door system. If you look closely at the upper air door on a Q-jet, you will notice that the shaft is slightly offset. When the carburetor is opened to wide-open throttle (WOT) at lower engine speeds, the lower throttle blades are open but at first the upper air valve door remains closed.

Then, as the engine demands more airflow through the primary throttle blades, this will place increasing air pressure on the air valve door. At the right time, this air pressure overcomes the built-in spring pressure holding the air valve door closed. As the air valve opens, it also operates an eccentric in the middle of the lid that lifts a secondary metering rod hanger. This lifts the secondary metering rods up at the same rate as the air valve door opens.

Rochester engineers knew that larger engines would want the secondaries to open sooner compared to smaller engines, so they made the tension spring adjustable. This allows the end user to custom set the point at which the air valve door opens. This is accomplished by accessing the air valve door spring adjuster. This is a small, straight screwdriver blade adjuster screw located on the passenger side of the lid in line with the secondary air valve door shaft.

In order to adjust the spring tension, you must first loosen a small Allen screw that keeps the adjuster from moving. Loosen the Allen bolt and then adjust the spring tension on the air valve door.

If you reduce the spring tension too much, the engine will hesitate slightly, or bog, when going to WOT. When this occurs, this is a clear indication that the air valve door is adjusted to open too soon and the spring needs to be tightened slightly. Some people think that a slight hesitation shows the adjustment is correct, but this is not correct. Setting the adjustment spring slightly tighter will allow a smooth transition to WOT, which is the correct setting.

If you want to go farther into Q-jet tuning, you can also try experimenting with the secondary jetting by changing the secondary metering rods and/or the hanger. If you look closely at the hanger, you will notice a single letter stamped on the top. This letter indicates how much the hanger lifts the metering rods out of the fixed jets in the main body of the carburetor. 

As an example, the B hanger will pull the rods out much farther from the fixed jets than an M hanger. Each letter is worth roughly 0.005 inch of metering rod lift. So a B hanger will lift the metering rods 0.040-inch more than a J hanger.

Metering rods are stamped with a two letter code that indicates several specs. The most important spec is the diameter of the power tip at the end of the metering rods. When the air vale is fully open, this will pull the metering rods out of the fixed jets so just the power tip is still in the fixed jets. A thicker power tip will flow less fuel than a very thin power tip. In addition, as the metering rods are pulled out of the jets, the taper of the metering rod will affect the air-fuel ratio during this transition period.

It’s too complex to go into the various metering rods sizes and hanger combinations in this tech forum, but that information can be found in the Rochester Q-jet book written by Doug Roe. The revised edition is still in print. This is the book that I have relied on for 40 years for Q-jet tuning information.