"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.

 

[72sunroof]

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.

Thermoquad bodies don't warp. You have one of 3 issues
1 the front throttle shaft is a vacuum leak, test with carb cleaner
2 the fuel wells need tk be glued with a new glue, I use loctite marine, but there are others that are ethonal, heat and fuel resistant.
If those are leaking when you take it apart there will be a puddle under the fuel well
3 the needle and seat are not seating. Either pump is above 6 psi and pushing open or they can be sticking.
3a the floats are too high they need to be 29/32 if they are replacement metal floats.

 

[pnora]

Thermoquad bodies don't warp. You have one of 3 issues
1 the front throttle shaft is a vacuum leak, test with carb cleaner
2 the fuel wells need tk be glued with a new glue, I use loctite marine, but there are others that are ethonal, heat and fuel resistant.
If those are leaking when you take it apart there will be a puddle under the fuel well
3 the needle and seat are not seating. Either pump is above 6 psi and pushing open or they can be sticking.
3a the floats are too high they need to be 29/32 if they are replacement metal floats.

Oh yes they do warp.

 

[WileE]

Thermoquad bodies don't warp.

I learn something new every day. Except today.

 

[Geoff 2]

[1] Is fuel dribbling from the primary booster nozzles?

[2] You need to disassemble the carb & check that the o rings are in place. I NEVER use the factory O rings, they are too flimsy. Instead, get fuel resistant standard O rings, 3/8" OD & 1/16" wall thickness. Get those first.

[3] Fuel pressure. These carbs will withstand 10 psi of fuel pressure; unlikely to be the problem.

[4] The black, epoxied, pods underneath the main body. They sometimes leak but will only cause an idle problem if the lower gasket is broken/leaking.

[5] Make sure you have a washer under the needle/seats & that they are sealing. Invert the airhorn & suck on the fuel inlet; if you can suck air, n/s are leaking. Make sure floats move freely & are not fuel logged.

[6] This could also be a cam problem. Faulty grind, excessive duration that requires the pri t/blades to be open a long way which makes the mixture screws ineffective.

[7] Make the sec t/blades are fully closing. Because they are big they will let a lot of air in even with a small opening.