Carter Thermoquad Troubleshooting

Dies at a Stop

Your Thermaquad TQ dies when hot and comes to a stop.

The Carter Thermaquad was Carter’s answer for Rochester’s Quadrajet. The thermal plastic main body stopped percolation, but preformed like a Quadrajet. The problem was the Plastic main body cracked because the long screws that held the Air Horn and Throttle Body (through the main body) caused it to crack when tightened incorrectly. They used an Epoxy to hold the plastic main well caps to the plastic main body and it held up. Carter Tech line said to replace the Main body on each overhaul and bodies were available for about 45 bucks then, but no longer! They would leak fuel into the intake manifold, raw fuel smell, hard hot start, idle rough, die at stops. Some other possibilities: A damaged air horn gasket High float level (heavy floats or incorrect adjustment) Is the brass float measured the same as the nitrophyl? Yes it is.

Hesitation, or Bog

  • Distributor advance mechanism not working correctly.
  • Accelerator pump cup dried out, damaged by ethanol, or just worn.
  • Accelerator pump passage plugged (ethanol will do this).
  • With engine off look down the carburetor and pump the throttle once. You should see 2 good squirts of gas from the main discharge. If not, then your problem is in the accelerator pump circuit.

Carter TQ Carburetor Manual

Carter TQ, Thermoquad Carburetor Manual


  • Cups may not be ethanol ready. Be sure you are buying non black cups that are being currently produced, like our cups.
  • High concentration of ethanol can cause even the best cup to fail.
  • Too much chemical being added to the gas. We recommend our Ethanol Defense.
  • Accelerator pump well may need polishing. Today’s gas has much less lubrication these days. Polish the well with crocus cloth to provide a smooth glide.
  • Letting the engine sit for prolonged periods will cause ethanol to sit at the bottom of the pump well.

Runs Lean

What indicates a lean condition?

Spark plugs are burning a light, or white color.
Pops back through the carburetor.

A lean condition as well as a rich condition will ruin your engine.

What causes a lean condition?

  • Jets too small. Now if you have been running the carburetor and it started to run lean, then it isn’t the jets. Jets don’t change over time.
  • Moved to 5000+ feet elevation requires the jets to be reduced in size by .002.
  • Look for kinked fuel line. Make sure any rubber fuel line is injector fuel line. Ethanol will create havoc on regular rubber.
  • Check the float level. Set to specifications.
  • Look at the float drop. Is it letting the needle drop out of the seat?
  • Test the fuel pump. Check your motors manual for correct pressure, but should be around 4.5 to 5 lbs. Taking off the fuel line and cranking the engine is not enough. You need to test it with a gauge.
  • Hot rodding the engine may require more CFM. The Thermoquad is plenty of carburetor, but the jets might now be too small. The current size will be stamped on the jet.
  • Popping through the carburetor might be the accelerator circuit not working correctly.
  • Float needle could be sticking closed and not keeping the float bowl full.


A surging condition at part throttle, or constant speed can be caused by several things.

  • Look for vacuum leaks – Carburetor mount, vacuum lines, intake manifold.
  • Be sure the mounting gasket is not leaving any openings.
  • If by chance you are not using an air cleaner, then plug the air cleaner stud hole in the base of the Thermoquad.
  • If large vacuum take-off in base of Thermoquad is not used, then plug with pipe plug in box.
  • If vacuum tube for spark advance is not used it should be plugged.
  • If no air leaks then the primary side of the carburetor is probably too lean.
  • If wide open accelration is good then change to a metering rod with a richer part throttle stip.

Updated on 01/10/2021

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