1974 Thermoquad, 360 HD Truck , Rebuild adding performance modifications

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Brooks James

Brooks James

VET, CPT, Huey Medevac Pilot
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This is a two fold experiment. Im trying to get more people intrested in running Thermoquads, and building a TQ from scratch, with performance mods, they can rebuild one to stock specs or modify one for their hot rods, with basic tools

Ive been collecting TQ,,'s and info for years
Some of the mods are proven to work like increase in the seat orfice and needle from
.093 (stock) to .130, and others I have read/heard bout that are not substainiated, like milling off the choke/air horn to increase CFM by 50 CFM.
I intend to keep good records, resting after each modification, using 1/4 mile track times to verify

This is for my 360 Magnum in my 70 Dart

Yes, I will be asking for tips, corrections, advice, help, ideas, and would be great

I've been thru plenty of TQ's, this will be the first one I'm building with performance mods. If I screw up and have to start over, no problem I've got over 30 TQ's in the shed

this carb designation HD truck means that from 72-75 HD trucks, motor homes and industrial engines did not have to meet passenger car emissions standards.
It is clearly evidenced here by the single charcoal canister vent tube nipple

In addition, the fuel volume thru the carb was greater, giving more power.
The Vanath TQ guide states that for raw material, non emissions style, that the 72-75
TQ's are the ones to build

Pic #1 is the plan to increase air available in the fuel bowl by adding the large vent tube the the other 3 small ones
I got the idea when I was researching TQ's and discovered that some TQ's
Have an altitude compensation mechanism. (Federal and CA) (See pic).
It is a small electromagnet with a rubber stopper on the end to close the vent that keeps the fuel bowl vapors from escaping during normal operation. The vent opens when you reach X altitude to provide more air flow, I have not been able to find at what altitude it opens.l

After Rumblefish sent me a pic of a Superquad TQ
And the bowl vent was very large, I decide I would add the much larger vent tube to the 3 existing working one's.

It make sense to me, the mods will make it push more fuel thru so I need to make sure there is plenty of air.

even if it doesnt work, i think it looks cool, i havent seen it done before

PXL_20231001_133434237.jpg


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The TQ is one of the most overlooked carbs in the muscle car world. I have used one of these on a Mopar m1 intake on my 512 stroker since 2008. I did the needle and seat mod as mentioned in your post. The only problem I have ever had with the carb is that it has leaked fuel around the gasket on the top rear corners of the plastic fuel bowl. There is an area in the corners where the plastic will warp (After about 50 years of heat cycles) from the pressure being exerted on it from the clamping pressure of the fuel bowl cover. The corners of the bowl had yielded about .005 of an inch. I corrected the warpage with a thin of application of JB weld epoxy around the rear corners of the bowl mating surfaces and then sanded it down level with a good flat metal sanding block. This has been a durable repair and has held up for about 4 years now. My engine runs very well with this carb and gets better than average fuel mileage for an engine of this type. I have never seen any reason to switch to one of the ubiquitous Holleys.

Roger
 
Great carb, and the most misunderstood. Putting a 1000cfm Comp-series on my small-block, with all the S/S tricks.

Krusty Impastato Thermoquad.jpg


Krusty Thermoquad jets.jpg
 
Dodgeboy68 said:
The TQ is one of the most overlooked carbs in the muscle car world. I have used one of these on a Mopar m1 intake on my 512 stroker since 2008. I did the needle and seat mod as mentioned in your post. The only problem I have ever had with the carb is that it has leaked fuel around the gasket on the top rear corners of the plastic fuel bowl. There is an area in the corners where the plastic will warp (After about 50 years of heat cycles) from the pressure being exerted on it from the clamping pressure of the fuel bowl cover. The corners of the bowl had yielded about .005 of an inch. I corrected the warpage with a thin of application of JB weld epoxy around the rear corners of the bowl mating surfaces and then sanded it down level with a good flat metal sanding block. This has been a durable repair and has held up for about 4 years now. My engine runs very well with this carb and gets better than average fuel mileage for an engine of this type. I have never seen any reason to switch to one of the ubiquitous Holleys.
Click to expand...
If you have trouble again with that bowl I'll send you one
 
What number is that 74 carb you're using?
Brooks James said:
This is a two fold experiment. Im trying to get more people intrested in running Thermoquads, and building a TQ from scratch, with performance mods, they can rebuild one to stock specs or modify one for their hot rods, with basic tools

Ive been collecting TQ,,'s and info for years
Some of the mods are proven to work like increase in the seat orfice and needle from
.093 (stock) to .130, and others I have read/heard bout that are not substainiated, like milling off the choke/air horn to increase CFM by 50 CFM.
I intend to keep good records, resting after each modification, using 1/4 mile track times to verify

This is for my 360 Magnum in my 70 Dart

Yes, I will be asking for tips, corrections, advice, help, ideas, and would be great

I've been thru plenty of TQ's, this will be the first one I'm building with performance mods. If I screw up and have to start over, no problem I've got over 30 TQ's in the shed

this carb designation HD truck means that from 72-75 HD trucks, motor homes and industrial engines did not have to meet passenger car emissions standards.
It is clearly evidenced here by the single charcoal canister vent tube nipple

In addition, the fuel volume thru the carb was greater, giving more power.
The Vanath TQ guide states that for raw material, non emissions style, that the 72-75
TQ's are the ones to build

Pic #1 is the plan to increase air available in the fuel bowl by adding the large vent tube the the other 3 small ones
I got the idea when I was researching TQ's and discovered that some TQ's
Have an altitude compensation mechanism. (Federal and CA) (See pic).
It is a small electromagnet with a rubber stopper on the end to close the vent that keeps the fuel bowl vapors from escaping during normal operation. The vent opens when you reach X altitude to provide more air flow, I have not been able to find at what altitude it opens.l

After Rumblefish sent me a pic of a Superquad TQ
And the bowl vent was very large, I decide I would add the much larger vent tube to the 3 existing working one's.

It make sense to me, the mods will make it push more fuel thru so I need to make sure there is plenty of air.

even if it doesnt work, i think it looks cool, i havent seen it done before

View attachment 1716148383

View attachment 1716148384

View attachment 1716148385

View attachment 1716148386
Click to expand...
 
Cool TQ brothers, I’m in. I have about a dozen TQ’s , one competition series, a 71 340, several later 340’s, 3-4 9800 series sold through direct connection and others.

BCAA1055-A301-4DA7-A26A-E2BB0DF4186C.jpeg


5D8B1A5C-475A-4E25-868D-66C8F3304853.jpeg


This is my 9097 experimental carb that I milled the choke horn off of. Haven’t worked on it for a while, but next steps are smoothing sharp edge’s and casting parting lines.

CCA762A7-4D17-44C1-9CC9-E2BB6F63200A.jpeg


After that,making a carb hat somewhat like the K&N stub stack for Holley’s. Trick will be how to have air access to the air bleeds. There is so much more that can be done, but I’m leaving it at that for now.

There should be a TQ sticky thread.
 
ch1ll said:
Cool TQ brothers, I’m in. I have about a dozen TQ’s , one competition series, a 71 340, several later 340’s, 3-4 9800 series sold through direct connection and others.

View attachment 1716148693

View attachment 1716148694

This is my 9097 experimental carb that I milled the choke horn off of. Haven’t worked on it for a while, but next steps are smoothing sharp edge’s and casting parting lines.

View attachment 1716148695

After that,making a carb hat somewhat like the K&N stub stack for Holley’s. Trick will be how to have air access to the air bleeds. There is so much more that can be done, but I’m leaving it at that for now.

There should be a TQ sticky thread.
Click to expand...
Awesome, I'm drooling !!
Please keep posting
your work !!!!
Carb hat ?? Same as a velocity stack ?

Access to air bleeds, please
educate me on that
 
084AA9D2-8641-4155-9405-A02F19FFFF28.png



This is a K&N Holley stub stack. It’s not a velocity stack, but I have seen them put into velocity stacks. The problem is one would cover the air bleeds into the primary boosters. Air bleeds are above the venturies in a Holley.
 
ch1ll said:
View attachment 1716148709


This is a K&N Holley stub stack. It’s not a velocity stack, but I have seen them put into velocity stacks. The problem is one would cover the air bleeds into the primary boosters. Air bleeds are above the venturies in a Holley.
Click to expand...
That is sick, cool
I flipped thru my notes I made a while backthat said air bleed enlargement, but didn't say how much based on what criteria or how to do it
 
ch1ll said:
Cool TQ brothers, I’m in. I have about a dozen TQ’s , one competition series, a 71 340, several later 340’s, 3-4 9800 series sold through direct connection and others.

View attachment 1716148693

View attachment 1716148694

This is my 9097 experimental carb that I milled the choke horn off of. Haven’t worked on it for a while, but next steps are smoothing sharp edge’s and casting parting lines.

View attachment 1716148695

After that,making a carb hat somewhat like the K&N stub stack for Holley’s. Trick will be how to have air access to the air bleeds. There is so much more that can be done, but I’m leaving it at that for now.

There should be a TQ sticky thread.
Click to expand...
I think one was started but I don't know how to find it
 
Brooks James said:
That is sick, cool
I flipped thru my notes I made a while backthat said air bleed enlargement, but didn't say how much based on what criteria or how to do it
Click to expand...
You got to be careful enlarging air bleeds as they are cast into the aluminum top. Then again you could hard solder them shut and redrill them, but that would be a pia.

To dial in a carb, it should be done on the dyno anyway. Reading plugs is like reading a crystal ball.

You would need a roller dyno and an operator/tuner with good TQ experience.

I have the idea of drilling and tapping for Holley air bleed jets. Also I can’t figure out where the emulsion tube is. That there is a big part of tuning out lean/rich spots in your fuel curve. I wouldn’t say I’m real knowledgeable about exactly how a TQ works as there few in depth books about it. I’m on the learning curve here.
 
ch1ll said:
You got to be careful enlarging air bleeds as they are cast into the aluminum top. Then again you could hard solder them shut and redrill them, but that would be a pia.

To dial in a carb, it should be done on the dyno anyway. Reading plugs is like reading a crystal ball.

You would need a roller dyno and an operator/tuner with good TQ experience.

I have the idea of drilling and tapping for Holley air bleed jets. Also I can’t figure out where the emulsion tube is. That there is a big part of tuning out lean/rich spots in your fuel curve. I wouldn’t say I’m real knowledgeable about exactly how a TQ works as there few in depth books about it. I’m on the learning curve here.
Click to expand...
Yes, 100% correct that a Dyno tuning would get max hp
For street the cost of a Dyno With an expert is not worth the $ per horsepower for me. If I werea steeley eyed NHRA Class racer that would give anything for another tenth of a second yes.
There's a ton of info out there on TQ's, a LOT of it is repeating.
I have had to seriously dig to get performance mods/tuning info.
I looked at my notes again, I jotted down from three sources .096, .098, and anywhere in the
.090 to .100 range.

This is probably a dumb question. Is it a different concept, but would drilling 1/16 holes in the primary throttle plates do the same.as opening up the air bleed hole ?
I'd like to try something even call .095 good enough for now and move on
If I screw it up I've got 30 more in the shed
 
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Drilling holes in the primary plates is an old trick to add to the idle circuit when you have a cam that idles at too high an rpm that the idle circuit can feed it. Air bleeds add air to emulsion tube which controls the fuel curve through your rpm range in a Holley, not sure about a TQ.

You have to look hard for a dyno guy thats reasonable, they are out there. In the long run you will be money ahead.
 
Very cool, Brooks. Thanks for this. I'll be watching!
 
My secret-squirrel 1000cfm, has screw-in air bleeds.
 
Interesting thread. Two things have always been kind of intimidating to me with our old Mopars, and that is tuning Thermoquads and rebuilding Torqueflites! I plan to learn how to do both, so this thread should help with the one fear....lol. Somebody told me, or I read it somewhere, that you can use jets from a GM Quadrajet carb in Carter Thermoquads. Is that true or just some BS?
 
I knew of several 340 engines that ran Quadrajets from big block chevys
 
I went the opposite way. I had a 69 Firebird with a 400 four speed, and it had a quadrajunk that leaked like a sieve because it was cracked where the fuel line screwed in. A buddy had given me the Thermoquad off his 73 Dart Sport 340 after he built the motor and put a 780 double pumper on it. I bolted it to the 400 and was just going to use it while I saved up for a new spreadbore Holley. I was afraid my car was gonna go up in flames cause the Quadrajet was leaking. It ran so much better with the Thermoquad, that I forgot all about buying a Holley.....lol.
 
Yup, a TQ is a QJ on steroids.

As for QJ jets fitting TQ, nope. The QJ does not have removable sec jets & the pri jets are totally different.
 
ch1ll said:
Drilling holes in the primary plates is an old trick to add to the idle circuit when you have a cam that idles at too high an rpm that the idle circuit can feed it. Air bleeds add air to emulsion tube which controls the fuel curve through your rpm range in a Holley, not sure about a TQ.
Click to expand...

Brooks James said:
This is probably a dumb question. Is it a different concept, but would drilling 1/16 holes in the primary throttle plates do the same.as opening up the air bleed hole ?
I'd like to try something even call .095 good enough for now and move on
If I screw it up I've got 30 more in the shed
Click to expand...

Here is the reason for drilling the plates. You have installed a bigger cam and now the engine idles with less manifold vacuum. To compensate for the lost vacuum you have to open the throttle up more to obtain sufficient idle rpm. This uncovers more of the transition slot that is cut into the throttle body causing the engine run excessively rich at idle. So drilling the plates introduces more air into the engine and leaning the air fuel ratio so the throttle can be closed to something closer to normal.

Roger
 
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