ThermoQuad info

Contents...
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= Introduction
= History and Description
- General
- Identification
= Subsystems
= Port Diagram
= Choices and Adaptations
= Numeric Listing
- Thermo-Quad
- Rochester Quadrajet
= Modification of Subsystems
= References
= Acknowledgements

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Introduction...
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The Carter Thermo-Quad is a four-barrel carburetor with a spreadbore
throttle bore configuration. It was designed as an emissions capable
carburetor that retained or surpassed secondary throttle performance of
earlier Carter carburetors, while delivering superior primary fuel
economy. The Thermo-Quad consists of three main sub-assemblies,
an aluminum fuel bowl cover, a phenolic resin main body, and an
aluminum throttle base assembly. The Thermo-Quad derives its name from
the phenolic main body. Due to the material of the body, the carburetor
bowl can stay 20 degrees cooler than an all metal carburetor in the same
environment. The Thermo-Quad was used by Chrysler Corporation in cars
and trucks 1971-1984. Aftermarket versions were also produced by Carter.

This guide is intended to provide information for identifying Thermo-Quad
carburetors and related items. It provides a basic history and basic
descriptions of the carburetor and its subsystems. It is not intended
as a full theory of operation manual nor a repair manual. Service
documentation should be consulted for repair procedures and service
details. The factory service manuals provide good service procedures
and theory for specific models. Other sources may be consulted for
general repair and modification procedures as well as theory of operation.
See the list of references for sources.

This guide is written with carburetor 'swapping' in mind. Thus, certain
items are discussed with modification consideration (ie. emission subsystem
disablement) with the intent for adapting a non-original carburetor and/or
enhancing performance. The modification and adaptation of the carburetors
may present legal issues, so consider the interest of any appropriate
government(s).

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History and Description...
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General:
- - - -
The Thermo-Quad (TQ) was initially released for competition in 1969. Chrysler
introduced the TQ on the 1971 340. The first series of TQs including the
71 340 version and the Competition Series TQs were air metered units
unlike like the 72 and later TQs which were solid (liquid) fuel metered.
The Competition Series (CS) were available in 850 cfm and 1000 cfm ratings.
CS units use a manual choke and have a minimal amount of external attachments
compared to OEM production units. The CS was discontinued in the mid-70s.
Carter released the 9000 series in the latter 70s as replacement carburetors
for Chrysler and GM Quadrajet applications. The 9000 series was very
similar to the production Chrysler Thermo-Quads.

In 1972, the OEM Chrysler TQs changed to the solid fuel metering type. The
TQ coverage was expanded to include the 400 engine. By 1973, all Chrysler
4-bbl applications were TQs (except some 413 truck models which continued to
use a Holley carburetor). As the years progressed, the TQ evolved to meet the
continually tightening emissions requirements. The changes were numerous.
Many features were added or modified externally and internally. The late
70s contained many variations for the various geographic regions, the
various features included/excluded, and the range of applications and engines
produced. Into the 80s, the TQ became more complex, but year to year major
variation lessened somewhat. In 1973, TQs received a port on the base for
canister purge and a port on the main body to provide a venturi vacuum signal
for EGR applications. 1975 saw the introduction of the Idle Enrichment system,
Altitude Compensator on some models, and the Throttle Position Solenoid for
the new catalytic converter equipped cars.

In 1976, Chrysler introduced Lean Burn ignition and the TQ was modified
to produce and run on a very lean air/fuel mixture. An external idle stop
switch and throttle position tranducer were added. In 1978, the TQ bowl
vent was modified with the addition of an electric Bowl Vent solenoid. An
additional rear base port for the vent hose replaced the bowl vent port. The
fuel inlet moved to the rear center of the carburetor from the previous
rear side location. Lean Burn became Electronic Spark Advance (ESA) in
1979. The very lean mixture idea was abandoned, but the electronic control
of the ignition advance was retained. Hidden mixture screws were a feature
starting with some 1980 model TQs. 1981 introduced a riveted cover for
the choke pull-off linkage to prevent tampering and the oxygen feedback
solenoid on some models. Idle Enrichment and Altitude Compensator was not
used on feedback models. 82-84 did not change much more in a major way.
The canister purge was eliminated by 84 in some applications and a power brake
port was added to the rear base.

After 1984, Chrysler stopped using Thermo-Quads. Instead, the Rochester
Quadrajet was used through 1989 on cars and until 1988 in trucks when
Electronic Fuel Injection replaced them. Carter continued to supply the
fuel pumps for the Quadrajet equipped vehicles. Although Chrysler was the
primary manufacturer to use the TQ, International Harvester (IHC) used them
in the late 70s and Ford used them in 1974.

The Thermo-Quad was available with two primary throttle bore sizes, 1-3/8"
and 1-1/2". Flow ratings (CFM) vary depending on the source, but
the TQs with the 1-3/8" bores are listed as 750-800 CFM and those with the
1-1/2" primary throttle bores are rated at 800-850 CFM. All TQs have the
2-1/4" diameter secondary throttle plates. The primary bore size depended
on application. In general, all 78 and later 318s and 360s and all 340s had
the smaller bore. Earlier 360s varied depending on application, most 400s
and all 440s had the large bore. The 9000 series have the small primary bore
and were rated by Carter at 800 cfm. Later TQs (ie, Lean Burn and ESA
applications, feedback systems) are quoted with less flow ratings, but
this is due to the control of the carburetion system, not the inherent
flow capability of the basic carburetor. The internal metering is set for
leaner running conditions for Lean Burn. Either bore size can be tuned
to run well on most engine combinations. The smaller bore offers a slight
increase throttle response but less overall flow. The different bore sizing,
ie. speadbore, is an aspect that can lead to increased fuel economy while
delivering similar wide open throttle (WOT) performance to an equivalent
standard bore configuration. The adjustability of the TQ and the
spreadbore configuration allows the use of a large CFM carburetor on
a small displacement engine.

The TQ gets its name from the phenolic resin main body. This is "sand-
wiched" between the aluminum bowl cover assembly and the lower throttle
flange assembly. The plastic body keeps the fuel ~20 degrees cooler
than an all metal carb in the same enviroment, leading to less percolation
problems and increased performance due to a denser charge. The TQ is a
metering rod based carburetor like other Carters (AFB, AVS, BBD). The
primary jets are housed in the plastic body, the primary metering rods
are suspended from the cover mounted step-up piston assembly into the
jets (in the 71 TQ and the CS, the primary jets are also housed in the cover).
Engine vaccuum (related to load) controls the position of the metering
rod in the jet, metering the fuel flow. The secondary jets are suspended
from the cover. Secondary flow is controlled by variable venturi effect in the
secondary bores. The TQ secondary throttle plates are controlled by direct
mechanical linkage, air flow is controlled by a secondary, spring tension
resisted, air valve. The valve is further controlled and damped by the choke
pull-off diaphragm.

The OEM TQs have a divorced choke (73-up with electric assist in most
applications), the 9000 series have an integral electric choke, and the
CS were equipped with a manual choke. All OEM TQs and later CS have
screw-in jets. Early CS retained the jets via O-rings.