The Carter TQ (Thermoquad) - a performance carburetor

General information

The Carter TQ is a spread-bore air valve carburetor, first released in 1969, and production continued through 1983. The carburetor is named for the center section (bowl) which is constructed of a high quality thermoplastic. The TQ contains 2 large capacity fuel bowls, eliminating one of the issues with the Rochester Q-Jet. The thermoplastic bowl allows the fuel in the bowl to maintain a lower temperature. The TQ is the only carburetor of which I am aware, that was designed as a race carburetor, and then additionally detuned for street use. According to Carter, an independent laboratory performed tests on the fuel temperature in 1969, and found the fuel temperature would run from 25~28 degrees cooler than carburetors with metal bowls. This feature is important for two reasons:

(1) There is an unwritten rule of thumb that a 10 degree reduction in carburetor fuel temperature is good for 1 percent increase in performance, both power and fuel economy. Assuming this rule of thumb is true, then the TQ, all other factors being equal, is from a little more than 2 percent more efficient than an equivalent carburetor with a metal bowl.

(2) But even more important, especially with the fuel of today, is the reduction in hot soak issues, both during city traffic, and hot restarts. Think of the heat benefits of a 2 inch spacer under the carb without the associated idle issues.

History

The thermoquad was released on 5 February 1969 in two sizes: number 4846s as an 850 CFM, and 4847s as a 1000 CFM. The first release contained press-in jets. It was thought the press-in jets would be easier, thus quicker to change at the race track. Sadly, this did not prove to be the case, and due to customer complaints about the press-in jets, modified versions 4846sa, and 4847sa were released with screw-in jet on 27 May 1970.

Later in 1970, numbers 4972s and 4973s, both 800 CFM, would be released for use on 1971 performance MoPar 340 CID engines. These were still what was known as the "air bleed" design. Air was mixed with the fuel prior to the mixture being released into the venturi area.

In 1971, the air-bleed design was discontinued, and solid streams of fuel were discharged into the venturi air stream.

The 9000 series "Super Street Thermoquad" with electric choke was released as two models, one with GM linkage, the other with MoPar linkage, both rated at 800 CFM, on 3 July 1975.

Performance models

There are only a few aftermarket models which are of interest to the general enthusiast; with many other models which will not be covered here of interest to the MoPar enthusiast.

4846s - 850 CFM, manual choke, press-in jets, initially released for racing or very high performance street (CFM 250 / 600)

4846sa - 850 CFM, manual choke, screw-in jets, initially released for racing or very high performance street (CFM 250 / 600)

4847s - 1000 CFM, manual choke, press-in jets, initially released for racing or very high performance street (CFM 350 / 650)

4847sa - 1000 CFM, manual choke, screw-in jets, initially released for racing or very high performance street (CFM 350 / 650)

9800s - 800 CFM, electric choke, screw-in jets, GM linkage, released for high performance street (CFM 200 / 600)

9801s - 800 CFM, electric choke, screw-in jets, Chrysler linkage, released for high performance street (CFM 200 / 600)

9810s - 800 CFM, electric choke, screw-in jets, GM linkage, AND EGR port, released for high performance street (the EGR port may be plugged) (CFM 200 / 600)

9811s - 800 CFM, electric choke, screw-in jets, Chrysler linkage, AND EGR port, released for high performance street (the EGR port may be plugged) (CFM 200 / 600)

As the TQ is an air valve carb, the CFM's mentioned above are in the format fixed primary / variable secondary.

Parts

The TQ may be thought of as 4 distinct and separate generations, some parts interchange, many do not; ALWAYS best to consult a parts breakdown for both carbs if one is wishing to interchange parts.

Generation 1 - 4846s, and 4847s

Generation 2 - 4846sa, and 4847sa

Generation 3 - 4972s, and 4973s original equipment on 1971 MoPar 340 engines

Generation 4 - all other thermoquads

In addition to rebuilding kits, Carter released "strip kits" (a plastic box, with a number of different metering rods, metering jets, and larger flow fuel valves) for the TQ's

10-104 - 4846s, 4847s

10-105 - 4846sa, 4847sa

10-110 - 4972s, 4973s

10-203 - 9800s, 9801s, 9810s, 9811s (kit 10-203 is a generic TQ kit, the jets and fuel valves are applicable to these carbs, the metering rods will fit, but generally the desired calibration is not included in the kit). The parts in 10-203 will NOT interchangeable with the parts in 10-104, 10-105, and 10-110.

Problems

I personally started working with TQ's about 1975. Since then, I have had my hands on close to 1000 of the models mentioned above. With these models, ALL of the problems I have seen have been inflicted upon the carburetor by individuals that were too lazy to read the instructions, or too cheap to buy the better rebuilding kits, or ignorant of the existence of either instructions or better kits. Lets look at the "internet known" issues:

(1) "The carb doesn't leak right after I rebuild it, but will shortly (2~3 months) later". The first street production carbs came with sealing O-rings in the bowl (those familiar with Rochester Q-Jets may liken these O-rings to Q-Jet secondary well plugs). Carter QUICKLY found that even though the O-rings worked fine in the test lab, the O-rings failed on the street! About 3 months into production, the O-rings were replaced with what Carter called "Quad X-Rings". An O-ring is call an O-ring because the item is a ring with the material cross-section round like the letter O. So obviously, the quad X rings are rings with the material cross-section in the form of the letter X. ALL future production contained X-rings. Now, the problem. Carter placed X-Rings in the genuine Carter kits, AND the kits Carter sold to Chrysler. In manufacturer quantities, O-rings of this size are about $5. per thousand; Quad X rings about $2.50 EACH! Guess what you get in a FLAPS kit!!! Not the carb's problem if incorrect parts are used.

(2) "The *&^%$#@ bowl warped". Again, will use a Q-Jet analogy. Remember those two screws hidden beneath the choke plate on a Q-Jet? Well, guess what? So the first time one repairs a TQ, and doesn't read the instructions, one removes the 8 large screws one sees. One then shakes the carb, but it doesn't come apart. One now hits the top with a plastic hammer, and it still doesn't come apart. Now, completely frustrated, a screwdriver is used to pry the castings apart. AND THE BOWL SPLITS! The individual now takes the carb to someone who knows and shows the 2 hidden screws. So when talking with one's buddies, what story is told? "My bowl warped"! Not saying it has never happened, but I have yet to personally see a warped bowl!

(3) A possible issue is trying to use the TQ with an original GM air cleaner. As previously mentioned, the TQ contains 2 large bowls, one on either side of the center casting. When replacing a Rochester Q-Jet with the single bowl in front, one needs to inspect for clearance from the top of the carb to the bottom of the air cleaner. Many of the O.E. GM air cleaners do not sit on top of the Q-Jet, rather, the air cleaner wraps around the Q-Jet. It is sometimes necessary to include a 3/8 inch riser for the air cleaner to clear.

(4) Another issue when replacing a Q-Jet is that the fuel inlet on the Q-Jet is in the front, whereas the fuel inlet of the TQ is in the back; a custom fuel line will need fabricating.

(5) If one is placing the TQ on a Ford with the auto transmission requiring the kick-down arm on the carburetor, none of the performance TQ's have that arm. Ford did use a TQ on their 460 with the arm, model number 6568s.

(6) Flange mounting gasket - the original Carter mounting gasket should ALWAYS be used, or there is a possibility of vacuum leaks(CFM 200 / 600)