Some Tips For Rebuilding a Thermo-Quad Carburetor

Some Tips For Rebuilding a Thermo-Quad Carburetor

Notes:

• This is not a rebuild procedure, but tips for the rebuild work. The tips are loosely in order of rebuild steps.
• The following assumes the carburetor is complete, off the vehicle, and still assembled.
• Carburetors with heavy corrosion or scale require more involved cleaning procedures, if salvageable.

Thermo-Quad Information

For Thermo-Quad information, see my following guides, although they are not rebuild instructions, they can provide repair insight, identification, and other details:

- A Carter Thermo-Quad Guide, Version 2.0 - A pure text form.

- A Carter Thermo-Quad Guide, Version 3.0 - A simple HTML form.

- A Carter Thermo-Quad Guide, Version 3.1 - An alternate HTML form.


See the Vaanth Thermo-Quad Guide Site for the foregoing guides and other Thermo-Quad reference information, including service instructions and guides.

Needed Items

Obtain the following items. Everything may not be needed, and in some cases additional items might be required.

- Carburetor kit - see my guide for possibilities.

- Service manual with rebuild instructions and/or the rebuild kit instructions. Chrysler or International Harvester (IHC) factory service manuals for the specific or similar model carburetor are excellent.

- Tools

• 3/16” straight blade screwdriver
• 1/4” straight blade screwdriver
• 5/16” straight blade screwdriver
• 3/8” straight blade screwdriver
• 3/16” nutdriver
• 5/16” nut driver and 5/16” wrench (need depends on the base attached item screws)
• Needle nose pliers
• Common pliers
• Small punch and small hammer
• Vise-Grip type pliers
• 11/16” or 3/4” wrench to remove the fuel inlet fitting. Note: another size is possible for aftermarket fittings.
• Small ruler with 1/32” increments, sliding T-type preferred. Note: most carburetor kits should have a small paper or plastic ruler, or float setting gauge.
• Soft bristle paint brush
• Trimmed soft bristle paint brush
• Medium plastic bristle brush(es), such as a toothbrush
• Soft bristle brass brush
• Soft bristle steel brush
• Steel or brass wool
• Optional: Parts cleaner stiff bristle brush
• Optional: Float Scale: Walker Products 88-825, GP Sorensen 779-5500, Tomco 13700, SMP Hygrade CT3, Airtex 9F1000, BWD CF107S, Kem F300, Filko 19-200.
• Optional: Air Valve Tool: Carter 109P-397, Miller Special Tools # C-4152B.
• Optional: 1/4” long, slender shaft shaft screwdriver for the air valve tool.
• Optional: Bench grinder with wire brush, or similar

- Chemicals

• Carburetor dip cleaner. If available, use pre-EPA-altered Berryman carburetor dip cleaner. This is the one containing Methyl Ethyl Ketone (MEK). Currently available commercial cleaners are inadequate compared to old, but it is what’s on the market. Alternatively use a cleaner such as Zep Purple Power, Simple Green, or similar diluted based on the carburetor soiling.
• Optional: Gunk engine degreaser or kerosene, if the carburetor has a lot of built up oily deposits on the outside.
• Carburetor spray, with spray tube. Gumout or similar.
• WD-40, or similar
• Penetrating oil spray, PB-Blaster or similar
• 10W oil, 3-in-1 or similar light oil
• Optional: Lacquer thinner
• Optional: brake cleaner
• Possible: Fuel resistant epoxy. Miller-Stephenson MS-907 or grey Marine-Tex

- Other

• Paper towels
• Plastic container(s) for small parts
• Dip basket for carburetor dip cleaner, if using
• Clean, well lit work area
• Optional: safety glasses, gloves, etc.

Disassembly

Note: Pay attention and note how everything is connected and appears as the carburetor and its attachments are disassembled. Note the numbers on the throttle base, jets, metering rods, fuel inlet seats, and accelerator pump squirter for your records. See my guide for information about numbering and sizing.

• Follow the kit instructions or service manual(s) for disassembly with the following tips in mind…

• If the carburetor has a lot of oily dirty buildup, pre-cleaning with engine degreaser or kerosene before disassembly. Remove any electrical parts such as solenoid, TPS, bowl vent, etc. before cleaning and clean separately with solvent as needed. Use the parts cleaner brush for heavy deposits. Rinse with water.
• Pre-soak all visible threaded fasteners with penetrating oil to facilitate disassembly, including the bottom of the throttle base where screws are threaded.
• Remove any attached components such as solenoids, TPS, dashpot, electric bowl vent, etc. before main carburetor disassembly and cleaning.
• Remove the air cleaner stud using pliers or Vise-Grips in the middle where threads are not present or not used. Alternative, place two jamb nuts on the upper threads and remove to prevent marring the stud. Thread size is 1/4-20.
• Remove linkages that are clipped including the choke pulloff link and the accelerator link, noting the hole position.. Remove the S-clip from the accelerator noting the way it is inserted.
• Disassemble the carburetor into its three main assemblies: throttle body/base, bowl, and bowl cover (top).
• Place small parts in small containers.
• As parts or assemblies are removed, save the gaskets, especially the float cover and throttle body gaskets for comparison to new. The kits have multiple gaskets to fit various models.
• Use a screwdriver blade that fits the screws to avoid slippage and screw head damage.
• Use the 3/16” nut driver to remove the choke link bracket. Note the placement of the bracket.
• Remove the accelerator pump squirter under the choke. A check valve is in the hole under it. Invert the carburetor and catch it as it falls out. Penetrating oil and/or solvent may be needed to free it. If it seems stuck still, lightly tap it with the punch to break it free.
• Remove the metal covers around the metering rods, and the screw retaining the cover on the metering rod tree. Lift out the tree and rods, noting the spring under it.
• When removing the 10 cover screws, there are two under the choke plate.
• When lifting off the bowl cover, the gasket may tend to stick in different spots. Use a small blade to slide under it to free it from the main body. Also not that the accelerator tube may hang on the gasket, but freeing the gasket from the body will free the tube.
• On the left of the carburetor is the choke secondary lockout/fast idle rod. This can be removed without disassembling the other linkage by lifting the cover, twisting./turning it over, and removing the rod end from the slot on the lockout/fast idle link.
• Place the edge of the float cover on a solid surface such as a vise and using a punch, lightly drive down on the accelerator pump shaft to knock the check valve cover out. Note the spring inside for the pump.
• Remove the tube, the float pivot pins, and floats, from the cover, then set the gasket aside.
• Remove the main body from the throttle base, saving its gasket. Remove the step-up piston lift lever and pivot pin.

Cleaning

Note: Ultrasonic cleaning can be used if available, but adjust the cleaning chemicals accordingly.

Note: Do not get solvent into electrical or vacuum diaphragm components (ie. pulloffs).

• Clean the main assemblies and small parts using a dip bucket and the dip. If the floats are nitrophyl, don’t place in the dip chemical. Dip the bowl for a short time, not more than 1/2 hour if using an MEK based dip chemical. Mechanical cleaning action with brushes might be required.
• If not using carburetor dip, clean the assemblies and small parts by hand with brushes and spray or liquid carburetor cleaner as required. Lacquer thinner can also be used.
• Note that brushing the bowl cover or throttle base can scratch or remove the original surface finish if still intact under the dirt, etc. Do not brush the phenolic main body with metal brushes in general. It will scratch. Also do not brush or remove the green coating on shafts, if still present.
• After cleaning in the cleaner(s), rinse with water. Careful use of a moderate pressure washer can hasten the removal of dirt. Multiple passes through the dip/clean/rise cycle might be required.
• Dry the parts in a clean area on paper towels. Rinsing with carburetor spray, brake cleaner, or lacquer thinner can hasten drying and helps remove water from internal passage. Blow drying with low pressure is okay.
• Clean electrical and vacuum diaphragm components (ie. pulloffs) by hand with a little solvent as needed.

Inspection and Spot Cleaning

• Inspect the assemblies and all parts for damage or corrosion.
• Spray all orifices, bleeds, passageways, ports, and other similar areas in the float cover and throttle base. Spray into the accelerator pump squirter and ensure it is clear. Use clean out rods or stiff wire, if needed, to clean an orifice, but do not damage or enlarge it. Exercise the throttle plates while spraying. Note: Be careful of back splash from the spray. Wear safety glasses if needed.
• Check the throttle shafts for excessive play. Excessive play will dictate bushing the shaft bores or base replacement.
• Spray the jets and jet wells in the main body to ensure they are clear. Ensure the jets are tight with a screwdriver. Do not over tighten.
• Check the body for cracks or breaks. One or more locating pegs on the bottom of the bowl may be broken. These are an assembly aid and if no further cracking is evident in the area, are not a problem. More care in gasket alignment is only needed.
• Check the top of the body for flatness of the top mating surface, particularly the rear corners. Any light deviation can be cleared with light sanding on a flat surface.
• If the jet well plugs are broken, re-gluing is required. Wiggle the wells by hand to see if they will come loose. If not, check for leakage with the following:

1. Dry the bottom of the bowl and wells completely.
2. Place a clean, dry paper towel on a work surface.
3. Place the bowl, bottom down on the towel.
4. Shim the bowl to sit level and stable.
5. Add a little lacquer thinner or other solvent into the jet until covered.
6. Lightly cover the top of the bowl with cardboard or similar.
7. Observe the wells and towel for a while to see if there is any leakage.
8. If there is any leakage, re-gluing is required.

• To repair the wells, do the following:

1. Remove the wells from the fuel bowl.
2. Clean the well and the fuel bowl of the old epoxy.
3. Dry the area and reattach the wells with a small bead of epoxy.
4. Remove any surplus epoxy that might interfere with reassembly. A small amount of epoxy protruding is acceptable.
5. Test fit the fuel bowl on the throttle body and file away any epoxy that prevents seating of the bowl.

• For nitrophyl floats, check for saturated material with a float scale, or place on a level surface, tip the float back on its pivot. If it stays, the float is okay. If it drops down, the float is saturated and requires replacement.
• For brass floats, shake and listen for gas inside. If any liquid is detected, replace the float or repair it by opening, drying and re-soldering.
• Use steel wool to polish the metering rods, the step up piston, pivot pins, and brass floats if needed. Also polish the accelerator pump bore and step-up piston bore lightly. If the wool is not sufficient, use 600 grit sandpaper.
• Clean and brush all threads on fasteners as needed.

Assembly

• Follow the kit instructions or service manual(s) for assembly with the following tips in mind...

• Stage the clean parts for efficient location and assembly.
• Match the gaskets removed with new gaskets from the kit.
• Spray all gaskets with WD-40 or similar.
• Lubricate the throttle shafts and work it in by actuating them.
• Installing the choke pull off now makes it easier than with the carburetor assembled.
• Spray all screws and other small components with penetrating or light oil.
• Lubricate the accelerator pump, bore and shaft with oil. If the pump is leather, work the oil into the inner edge, and “flare \” the lip out.
• Insert the accelerator pump with the spring, and the S-link. The S-link is positioned so when facing the front of the carburetor, it forms an “S”, not a “Z”.
• Put the bowl gasket on before the floats. Measure float height from the gasket. In general set the float level to 29/32” for nitrophyl, and 1” for brass for 1972 and later production carburetors. Follow the kit instructions for float height changes. Note: do not press down on the needle and seat while float setting, but ensure the needle is seated before measuring.
• Don’t forget the accelerator pump tube. Lubricating the end can ease installation.
• Use the quad X-rings from the kit, lubricate and install squarely into the main body.
• Install the bowl cover in reverse of removal for connecting the choke link to the lockout/idle link. Be careful of the floats. If the cover has a mechanical vent lever, ensure it is aligned with the base linkage if it was not removed.
• Wiggle the cover to align and seat. Note the X-rings will hold it up a little until screwed down
• Install the ten screws in the order and torque listed in my guide. Go through the order sequence at least twice for better seating.
• Install the step-up spring and piston/metering tree with the rods installed while gently and carefully guiding and aligning the rods into the jets in the main body. Due to the fragility and blind alignment, this takes a little finesse. Alternately, install the tree and one rod at a time, but this requires some force and can bend a rod, plus still requires finesse to keep the first rod in place while doing the second.
• Install the lubricated accelerator pump check valve. Lightly tap it with a punch. This helps it seat better.
• Lubricate the mixture screws. A tiny amount of anti-seize can be added to the threads. Install with the springs. Tighten the screws until they just seat. Then turn slightly more, less than 1/8 turn, to seat the tapered points. Then back out 2-1/2 turns as a starting point for tuning the mixture on the vehicle, ideally using a vacuum gauge.
• Complete the assembly following the kit instructions while watching for any misalignment or binding of linkages.
• Lubricate the air cleaner stud, apply a small amount of anti-seize to the lower threads, and re-install.
• Cover or wrap the carburetor until ready to use.

When installing the carburetor, use the thick, preferably open, base gasket. Wet it with WD-40 before mounting. Also wet the air horn gasket with WD-40. See my guide, if needed, for gasket information.