Re: ThermoQuad tuning at Cruise
[Re: RayR]
#2358872
08/24/17 12:36 AM
08/24/17 12:36 AM
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Joined: Sep 2015
Posts: 8 Michigan
chargerjoert
member
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member
Joined: Sep 2015
Posts: 8
Michigan
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What list number thermoquad are you running and what is the top casting 4 digit number (near the fuel inlet, like 2124 etc...)?? Thermoquads were tuned very lean for each application and they got worse (leaner) as the years went by. Changing IAB's, IFR's, Idle air bypass passages, and adding bleeds to lean the circuits. Not to mention all the emissions junk that was added. Also, how many turns out are your idle mixture screws (from lightly seated closed)?
I do not have, or ever used, a lambda sensor for AFR. But I have used a vintage Heathkit Co meter. My experience has been the AFR will (and should) get leaner as rpm goes up (say in Park, and idle to about 3,200 rpm). I have not actually driven on the road with it to get data. For the gasoline here in the states (E10) I find more idle timing helps a lot. Reduces coolant temp and reduces the exhaust smell. And that big blocks do not like to idle or cruise on lean mixtures. But I also have a single plane intake manifold. My cam (xe268) generates about 16 in-Hg at idle in park.
My thermoquad is a modified earlier 6500 series with a 2124 top casting. This casting in stock form has IAB @ 0.038" and 0.034" IFR. I typically run 18 to 21 degrees at idle and 32 total. The modifications to the thermoquad cruise circuit (currently) is IFR's @ 0.040" and IAB's have 0.011" guitar wire in the holes. And idle air by pass holes drilled to 1/8". Idle mixture screws are about 2.25 turns out. Modify at you own risk (and I would go in small steps to see what works for you). My car runs nice and cool and does not stink. My Co meter reads approximately at idle 12.6 and goes leaner to about 14.1 at 3,000 rpm (in Park). Meter could be off, as I suspect the real readings might be slightly leaner.
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Re: ThermoQuad tuning at Cruise
[Re: RayR]
#2359893
08/25/17 09:32 PM
08/25/17 09:32 PM
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Joined: Sep 2015
Posts: 8 Michigan
chargerjoert
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member
Joined: Sep 2015
Posts: 8
Michigan
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Good, I'd say your idle is responding to the mixture screws and looks like you are happy with it.
Yes, light throttle cruise is all about the "idle circuit".
Tq's have what they call a five point idle system circuit. Low speed jet (IFR), By-Pass, Economizer, IAB, and idle port. This By-Pass is not the same By-Pass (as what I was referring to as idle air by-pass). Sorry, the terminology is confusing. There is another circuit that just routes air to the intake manifold. On an early model tq there is a small rectangular passage from the throttle bore front between the throttle body gasket and phenolic main body and then it routes the air down towards the shallow passages at the very bottom of the casting (towards the intk mnfld gaskt). Sorry, I don't have, or now how to do pictures. Latter tq's have the intake holes higher in the air horn, and route down via the phenolic body to the bottom casting. This is exactly like the old Holley trick of drilling holes in the throttle butterflies, to get more idle air. It is a good trick if the transfer slot is over exposed.
Good that you have another top, especially an early one like a 2124. But I believe that top will only fit 6500 series type phenolic bodies and throttle bodies. Later phenolic bodies have more space between the primary and secondary bores to accommodate an extra bleed circuit. But you can measure the IAB diameter of it, and compare to the current carb you have, for grins.
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Re: ThermoQuad tuning at Cruise
[Re: RayR]
#2360466
08/27/17 04:20 AM
08/27/17 04:20 AM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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I am just curious as to how well the car actually runs for you with the readings you have. Does it run good or does it have any stumble or surging ? Chargerjoert is right about the wording they use on them between idle air bleeds and idle fuel restictions and idle bypass and econimizer which most are more or less an idle air bleed other then the idle fuel restriction but not all are all the time. Some carbs feed extra air to the eng in the idle circuit and others right to the intake and called them perculator ports years back. I am guessing this Thermquad you have dont have the emission idle enrichment system on it that later years used as they had leaner idle systems and would richen the idle on only cold eng starts with vacum to the idle enrichment valve by blocking off an idle air bleed. If it has that and the valve has no vacum to it then it will have a leaner idle. But it sounds like you know the Thermoquad good enough to know if it had that valve. Good luck with it as they can be a pain since many were used in the emission years and ran very lean. I would think one of the best ones would be from the 1971 Darts , Dusters and Demon 340 cars. They can be alot of work to get right but they can work very good when set right. Many racers in Stock Eliminator run them and run very good with them. Ron
Last edited by 383man; 08/27/17 04:22 AM.
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