"Rebuilt" Thermoquad idle mixture screws have no effect.

[Dibbons]

I personally rebuilt the 1972 factory Thermoquad (6139S) after purchasing it used. Now installed on a fresh rebuild which we have just barely broken in the camshaft for twenty minutes. Idle sounds crappy and mixture screws turned in all the way have no effect (nor does unscrewing them). Putting the gearshift in drive provokes the engine to stall if idle is set to a reasonable speed. I'll be looking into a vacuum leak. May have to do a compression check to boot.

 

[Aron Gleason]

Do thermoquads have transition slots like holley carburetors? If they do, check to make sure they're not uncovered too much. But it could be a vacuum leak.

 

[torredcuda]

Stock cam? Like Aron said are primary blades partially open to get it to idle?

 

[Dibbons]

Stock cam? Like Aron said are primary blades partially open to get it to idle?

Camshaft is Oregon Cam's #2120 which is made similar to the '68 340 hydraulic four-speed profile. When I went to turn the vehicle around in the driveway, it stalled and would not re-start because it ran out of gas (gauges are out of service). Added a couple of gallons of gas, finally got it turned around and part way in the garage (more rain forecast) and called it a day for now.

Not sure about any transition slots yet.

 

[Dave6T4]

On some of those old ThermoQuads, the phenolic (plastic) carb body can warp due to engine heat over the years. When this happens, there can be vacuum leaks, and the mixture screws will have no effect. To check this, take your carb apart again and rub the top surface of the body on 320 grit sandpaper on a pane of glass (for flatness). If this surface is uneven, the bottom of the carb will not be flat, either, causing vacuum leaks. Make sure the epoxied well bottoms are not loose. There are also special square section "O" rings, that have to be in good shape. They can not be replaced with regular round section rings, and expect to seal correctly. The ThermoQuads worked well when fairly new, but they do not age well. I tried to use them as long as I could, over the years, but eventually changed to Edelbrocks.