Looking for some advice on jet sizing- Thermoquad 6139

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hudskit

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All,
I am looking for some input on what sizes to use in my Thermoquad rebuild that i am working on. The goal of this build was to use only parts I have laying around to get the car running decently enough to drive it as a weeekend car until I get the forever motor built. The idea is to spend as little as possible currently and do the least amount of work- with the side benefit of using parts I have laying around that aren't particularly valuable or worth selling.

Car is a 273 2 BBl 66 cuda- still original motor untouched - supposedly has 49k miles on it. Heads are shot - carb is mismatched and it has an orange box electronic ignition that seems pretty new.

I have a set of 302 heads that were freshly done when I bought them- it is intake port matched to an Eldebrock preformer spreadbore intake manifold and I have a very good condition 340 4 bbl camshaft that I will be putting in as well- Hydraulic lifters to go with the stock adjustable rocker shafts/rockers that i also had laying around.

I have a Thermoquad to use - a 6139 series 340 AT carb that I am currently rebuilding but I know that it will be jetted improperly if I put it back to stock- so the question is - what size jets would you step down to?

Please try to stick to the concept- i don't want to pull the engine - I don't want to put something else on there- I really want to use up the stuff i already own until I build the forever 340 for it.

Thanks in advance everyone,

Keith
 
Use TQ as is, should work perfectly. I used that carb all the time with 273's. You may have to adjust the secondary air valve so it won't bog. I'd run hydraulic rocker system simple, no adjustment, bullet proof. 340 valve springs or equivalent, you will also need kickdown linkage or cable if Auto trans. Sell me your heads and adjustable rocker gear ;-)
 
Hey Keith

Here's a pretty in depth site re: Thermoquads. Scroll down where it shows the applications and that info may help you to make your decision. Else, it's trial and error. Good luck.
A Carter Thermo-Quad Guide
 
A popular misconception is that changing engine size requires a change in jet size. Might sometimes, but not always.
I would adjust the air valve spring to 2 turns.
 
Bewy you are somewhat correct! I remember on nmy mates 253 Holden years ago with a 600 holley carb , to get it to run properly I had to increase the jet size .
The only thing I could come up with is the smaller motor did not have enough power to pull the fuel through the jets, not enough vacuum signal so needed a larger jet size than my 265 Hemi engine!
 
Thanks everyone for the useful advice- i printed the guide and it is indeed useful- Thanks! I think I'm going to go with a 94 jet in the primaries - and a 100 in the secondaries to keep it a little more proportional to the reduced motor and compression of this application- the old ones are surprising oblonged inside anyway so they probably should be replaced.
 
The "72" 340 TQ is jetted perfect for a 273. I never had to make allowances. As a matter of fact they run lean but run very well and get great mpg. I would recommend the .095 primary jets and the .137 secondary jets. Don't overthink things. The "66" 273 should have higher compression than a "72" 340 and you are running the same cam. You will need new pushrods to work with 273 rockers and hydraulic lifters. How much air do you think a 340 pulls at idle? The air fuel mixture is the same on a 273 and a 340.
 
66fs said:
The "72" 340 TQ is jetted perfect for a 273. I never had to make allowances. As a matter of fact they run lean but run very well and get great mpg. I would recommend the .095 primary jets and the .137 secondary jets. Don't overthink things. The "66" 273 should have higher compression than a "72" 340 and you are running the same cam. You will need new pushrods to work with 273 rockers and hydraulic lifters. How much air do you think a 340 pulls at idle? The air fuel mixture is the same on a 273 and a 340.
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Understood - I'll leave well enough alone. Pushrods came with the cam and lifters and rockers - since this was a 2 BBl the compression runs light so the 302 heads should be a natural fit...- I thought that the pushrods will be another "exciting" :) step forward on the rebuild once i get there to see how they fit once it's all together. I genuinely appreciate the time you're taking to post this info.
 
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Did you read post #5?
100 sec jets is waaaaaaaaaaaaaaaay too small. Carter changed the size of the air bleeds in these carbs, which affects the jet size. The 6139 had the large 0.039" sec high speed bleed which requires larger fuel jets. Start with the suggested 137 sec jets.

As I said in post #5, jet size does not correlate to engine size. The 6488 TQ had a 143 sec jet, on a 360 engine. The 6410 TQ had a 125 sec jet on a 440.
 
I haven't fooled with a Thermoquad since the late 70's, and the Strip Kits, jets and about anything you needed were readily available. Is there a source for that stuff now?
 
JDMopar said:
I haven't fooled with a Thermoquad since the late 70's, and the Strip Kits, jets and about anything you needed were readily available. Is there a source for that stuff now?
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Big money, like around $500 now for a strip kit. But you rarely need one. Like the old days, you can always redrill jets and sand rods.
 
Yes, Woodruff carbs, the host of this forum. Also The Carburetor Shop, Eldon, Missouri. Owner is retiring, selling off at reduced prices.
 
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