What's Your Problem?

Curing Thermo-BogI have a '73 Plymouth Duster with the original 340 and a TorqueFlite automatic trans. The car is a mostly original nice-weather driver, but since it sits for the winter, I often have to tend to tuning issues in the spring. My most recent trouble is with the carburetor. The original carb is a Carter ThermoQuad, and it seems to be the cause of the hesitation I've been experiencing lately when I step hard into the throttle. I was going to have it rebuilt, but a lot of people are telling me that the ThermoQuad was trash, and not to invest any money in it. Since my car is fairly unmolested, and not a racer, I'd like to avoid major changes. What's your recommendation?Jim StanfordYonkers, NY

The ThermoQuad has gained a bad reputation among motorheads mostly because it is generally considered to be a "smog-era" carb. However, as you've found, the ThermoQuad was actually in use prior to the heavy onset of emissions-control equipment. When functioning properly, the T-Quad is actually a great street carb, offering the same basic advantages of the General Motors Rochester Quadra-Jet: small primary bores for improved cruising economy and brisk throttle response coupled with large secondary bores for big-cfm power under full throttle. Often referred to as Thermo-bog, the T-Quad is also known for full-throttle hesitation. This is usually the result of a ruptured choke pull-off diaphragm, which may sound odd, but the same diaphragm that opens the choke slightly after the engine initially fires also serves as a damper for the secondaries. These are commonly damaged, particularly now that nearly all of them are a couple of decades old. When the diaphragm is bad, the secondaries basically flop wide open under hard throttle, rather than gradually opening. If your pull-off diaphragm is bad, you're probably also experiencing driveability issues during cold starts; in fact, the car may not be driveable.

A thorough rebuild of your current carb followed by the proper tuning should have you back on the road, running smooth and strong, but the key to success here is finding someone knowledgeable in ThermoQuads and the proper adjustment procedures. As an alternative, Edelbrock offers a GM-style Quadra-Jet carburetor as a direct replacement for Chrysler ThermoQuad applications (PN 1905).