How to Overhaul and Tune your Carter Thermo-quad Four-Barrel Carburetor

How to Overhaul and Tune your Carter
Thermo-quad Four-Barrel Carburetor
©2004 David Kovac, All Rights Reserved, This booklet is intended for the private use of the purchaser,
and may not be reproduced or transmitted by any means, electronic or mechanical, without permission
in writing from the copywrite holder.
Carburetor descriptions and overview
The Carter Thermo-quad has gained a rather unpleasant reputation for being
difficult to rebuild, and even more difficult to tune. While it isn’t as easy as the AFB or
the AVS, it can be done, with patience and attention to detail. The first thing you’ll
notice about it is the molded plastic fuel bowl. This, as well as it’s suspended metering
system, result in lower fuel temperatures, meaning more precise fuel metering and better
emission control. Twin metering rods are operated by a combination mechanical-vacuum
system, according to engine load. A cam on the primary throttle shaft raises the rods as
the throttle starts to open; then the vacuum control takes over. It’s this mechanicalvacuum integration that is present in all the systems on this carburetor that make it so
difficult to work on.
The airhorn, or upper body, contains all fuel metering circuits: the choke valve
and shaft assembly; secondary air valves that control the fuel inlet system, the low-speed,
high-speed and accelerator pump circuits, a vacuum-controlled step-up piston; fuel
discharge nozzles and all air bleeds. The throttle body contains the hot –idle
compensator, the throttle valve and shaft assemblies, and the idle mixture adjusting
screws. The main body is simply the molded plastic fuel bowl.
I set this guide up in four sections. The first section, this one, is a FAQ type of
section, and contains general carburetor and performance information. Section two is a
parts diagram of the Carter Thermo-quad 4-barrel carburetor. Section three is a step-bystep guide to rebuilding your carburetor. Section four is a general troubleshooting matrix
that I have found to be very useful. The reason I set up this guide is because I was never
able to find what I was looking for when rebuilding carburetors. The literature that was
out there had either too little info, as in the brochures that come with the rebuild kits, or
way too much, as in some of the SA books available. I tried to come up with something
in the middle. The goal of this guide is not to provide professional level instruction on
carburetor theory, operation and maintenance, but simply to try to explain rebuilding and
adjusting carburetors to the home do-it-yourselfer who might not have a lot of experience
in dealing with carburetors.
Do I really need to rebuild my carb?
Many times, I would even say most times, problems related to carburetors are
actually problems with something else. Before you take your carb off and rebuild it, try
the following adjustments and see if they solve the problem.
a) Ignition Timing
b) PCV system
c) Distributor vacuum advance
d) Idle mixture and speed
e) Fuel and fuel filter condition
f) Cylinder compression tests
g) Manifold heat control valve
h) Vacuum line connections (especially on Thermo-quads, take the time to check this one
out.)
These are all easy checks and may save you quite a bit of work.
OK, OK, but I really do need to rebuild my carb, now what?
If you do actually need to rebuild your carburetor, the most important thing you
have to have is a large clear space on which to work. There are many, many parts that
need to come off, and having a clean area in which to organize them will be crucial in not
ending up with “extra pieces” at the end of the operation. In addition, you will need the
following:
a) A rebuild kit for your particular carburetor, found at your local auto parts store.
b) A large container of solvent, for cleaning the parts.
c) A couple of different sizes of needle nose pliers.
d) A good set of screwdrivers, possibly with Torx heads.
e) A compressed air source is nice to have to blow out hard to dry places on some parts.
When you rebuild your carburetor, take your time and follow the step-by-step
instructions in Section 3. Clean parts as needed in your solvent, and make sure they are
completely dry before reassembling them. Use all new gaskets when putting your carb
back on the engine.
A little personal experience here.
1. If you put your carb back together and it stalls out every time you give it gas,
check the accelerator pump transfer tube. The rebuild instructions mention that it
is plastic, I have also seen them metal. They can crimp when you put back on,
and when that happens it restricts the flow of gas during the accelerator pump
shot. Be careful with this piece when rebuilding.
2. I have never had good luck modifying Thermo-quad beyond their factory set
specifications, unlike AFB’s. Give it a shot, if you want, but in my experience,
stick with the factory set jets, metering rods, and accelerator pump settings.
What kind of adjustments can I make to my carburetor?
Adjusting your Thermo-quad can, and probably will, be a lesson in patience and
endurance. It is unlike any other Carter carburetor, and is quite a bit more involved,
mostly because of the various vacuum circuits involved and the unique overhead fuel
metering circuits. I cannot go into detail on all of the adjustments in this format, however
I’ll cover the important ones. For a very detailed guide to adjusting your Thermo-Quad,
go to http://www.imperialclub.com/Repair/Lit/Master/290/intro.htm. This website is
a copy of the Chrysler Master Technicians 1972 Thermo-Quad Reference Guide. It is an
excellent resource, fairly easy to read, fully illustrated, and comes straight from Chrysler.
a) Idle speed adjustment. On the diagram, the curb idle screw is called the T.S. “off”
adj. screw. The difficulty is in coordinating the electric throttle stop solenoid, and the
curb idle screw.
1. Disconnect and plug the carburetor hoses from the vapor canister, the EGR valve, and
distributor vacuum advance. If the car has a vacuum-release parking brake, disconnect
and plug the hose.
2. Connect a tachometer to the engine. It can be done by ear, if you don’t have a tach. Set
the parking brake and block the drive wheels.
3. Start the engine and warm it to normal operating temperature.
4. Follow the manufacturer's instructions regarding transmission position and air
conditioner or headlamp operation.
5. With the engine warmed up and running at idle, adjust the solenoid adjusting screw to
get the specified slow-idle speed.
6. Adjust the carburetor throttle stop screw until it just touches the stop.
7. Back out the screw one full turn to obtain the shutdown idle setting.
8. Reconnect all hoses disconnected in step 1.
b) Idle mixture screws. Basically, idle mixture is adjusted by first seating the mixture
screws lightly and then backing them out until a specified engine speed is reached.
Turning the screws clockwise seats the screws and leans the idle mixture. Turning them
counter clockwise backs them out and richens the mixture. Do not over tighten the
screws when turning them clockwise or you will damage them and make proper
adjustment impossible. What works best for me is leaning the idle mixture until you
notice a drop in RPM, and then back it out a quarter turn. Both screws should be backed
out at the same level.
d) Fast idle adjustment screw. Turn this screw to fine-tune the idle RPM when the
choke is engaged. A good Rule of Thumb is between 1500 and 2000 RPM.
e) Float level adjustment. Fuel control, or lack thereof, can make even a perfectly
functioning carburetor seem like a candidate for the scrap heap. This adjustment should
be done during the rebuild, as it does require some disassembly to get to the floats. With
the bowl cover inverted, gasket installed and floats resting on seated needle, the
dimension of each float from bowl cover gasket to bottom side of float should measured.
Consult your specs chart for measurement. To adjust, bend float lever.
f) Accelerator Pump Stroke
1. Make sure the choke valve is wide open and the fast-idle cam is in the off
position.
2. Turn out the throttle stop screw so the throttle valves can close completely.
3. Make sure the hooked end of the throttle connector rod is in the specified hole on
4.
5.
6.
6.
the pump arm.
Close the primary throttle valve completely by lightly applying force on the
throttle lever .
Measure the distance from the top of the plunger shaft to the top of the bowl
cover.
Specs should be on your rebuild sheet that comes with your rebuild kit. If your
measurement does not meet the specs, bend the throttle connector rod at the angle
to correct.
ON MANUAL TRANSMISSION CARS ONLY: Continue past the basic pump
stroke setting by opent the throttle slowly until the secondary throttle shaft begins
to move. Hold this position and again measure the stroke as before. If adjustment
is needed, bend the pickup arm on the primary throttle shaft dog.
Check accelerator pump setting with primary throttle valve closed.
Pickup arm on manual-shift models only.
Where can I get more information on these carbs?
One indispensable website is:
http://www.imperialclub.com/Repair/Lit/Master/290/intro.htm
This website is a copy of the Chrysler Master Technicians 1972 Thermo-Quad Reference
Guide, and is an outstanding guide to the different circuits on the carburetor, as well as
very specific adjustment techniques. It does not, however, cover rebuilding much.
SA Design and Hayes also have very good books on Carter carburetors. They are,
however, very in depth, and digging up the information you need out of them can be a
challenge
In closing, I hope this helped. I could write for days on how to fine-tune these
carburetors, especially the Thermo-Quad, but hopefully this guide was able to take some
of the mystery out of rebuilding and fine tuning this carburetor. If you have feedback on
this guide, feel free to email me at [email protected]
Troubleshooting Chart
Possible Solution
Carburetor
doesn't
adjust
properly
Carburetor
floods
Fuel
Internal Misses Stumbles Engine Rough Backfires
leaks at fuel leak or
or loads won't
idle
shaft
surges up
idle
Check for air leaks. Make
sure carburetor and
manifold gaskets seal
properly. All outlets must be
plugged or connected.
X
X
X
X
X
Check ignition system.
Replace parts as
necessary. Adjust timing to
proper specifications
X
X
X
X
X
X
X
X
Check choke adjustment.
X
Change carburetor fuel filter
and/or in-line fuel filter.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Check for dirt or metal in
needles and seats. Needles
and seats may need
replacement.
X
X
X
X
X
X
X
X
Increase accelerator pump
stroke. Pump squirter size
change may be needed
Check for dirt blocking the
low speed circuit. Clean
unit and apply air pressure.
Install in-line fuel filter
X
Engine stalls Low fuel
on hard
economy
braking
X
X
Too rich.
Off-road vehicle may need
spring-loaded needle and
seat kit.
Lack of Pinging at
power moderate
cruise just off
idle
X
X
X
Check float level and drop.
Hard
starting
X
X
Too lean.
Check idle mixture screw
adjustment.
X
Bogs
X
X
X
X
X
X
X
X
X
Check floats for leakage.
Replace if necessary.
X
X
X
X
Check air horn gasket.
Replace if deterioration or
breakage is present.
X
X
X
X
Fuel is boiling due to
excessive under hood
temperature.
Fuel pressure too high.
Fuel regulator may be
needed.
Fuel pressure too low.
Check fuel system.
Check secondary latching
device and rod for proper
movement.
Check Float level
X
X
X
X
X
X
X
X
X
X
X
X