I am having trouble getting my Thermoquad equipped 440 to give a consistent AFR across a range of cruising speeds and am hoping for some Carb guru help!!

I recently changed from a Carter AFB to a rebuilt Thermoquad on my mildly tuned 440 engine, partly to improve fuel consumption as well as the increased cfm.
I have stripped the carb and corrected the many issues it had (linkages and float level) and generally the car runs very well. Idle is perfect and responsive to adjusting screws, throttle tip-in is now good after correcting the accel pump and WOT is fine. The problem is that at steady state cruise the AFR varies from 13 at 40mph to 15 at 80mph, I'd like to be able to get closer to 14.5 throughout the range.Secondaries do not open throughout this range.

I have tried different metering rods (original and some I have made) with the only effect being a gross shift in AFR. I have tried different float levels with the same result. Similarly the metering rod screw only moves the issue around and I can dial in an AFR for a single speed only. I am thinking the issue could be bleed hole sizes? or maybe the taper on the rods(unlikely?), but i have no information what the bleeds should be or what to try and alter.

Other information;
- 18 in hg at idle
- 3200rpm at 80mph
- Timing 12BTDC static - standard distributor 36 total
- "mildly polished/ported" heads, Edelbrock performer intake and headers.

Look forward to any suggestions.

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.

What cam and how much vacuum? Engine vacuum works against the primary metering rod tree spring. You may need a lighter spring to even things out at low vacuum loads. What is your carb number? It should be stamped on the driver side rear bolt mounting flange, facing the firewall and start with either a 6 or an 8.

I tuned my thermoquad with the a/f meter and if memory serves I had the idle around 13:1, the 40mph cruise was around 14.5 and the 80mph would have been more like 15.5. I never got any ping by running a bit lean at high speed so I left it as-is.

Hi, Thanks for your input. The Thermoquad number is 9009 and the top casting 2141. My idle screws are 1 1/2 turns out for an AFR of 13.5. Just to check on terminology, the IFR is what the manual calls the Low Speed Jet?
Interesting point that you are calling the idle circuit the cruise circuit? I had been trying wires in the high speed circuit vents/bleeds with some changes but not fixing my issue - maybe I need to spend time on the Idle side, it could be supplying too much fuel at light cruise causing my rich mixture?
Having just said the above I have found that one of the Idle Air Bypass brass inserts has been ""peened"" over during the re-manufacture process - still a passage bit much smaller than the other side. This could lead to a rich idle circuit on one side?

Another issue on the varying AFR with speed; when I am at cruise and accelerate moderately (not on the secondaries)the AFR reduces by about the same at any speed (about 2.0) so if the cruise AFR is on the rich side I get poor accel from an over rich condition, similarly poor accel if the mixture is lean. This is easily noticed when driving and the other reason I am trying to get a consistent AFR.

Thanks again for your input and the first thing I need to do is somehow fix the one idle air bypass and see where that takes me.

Thanks for your input, see above for part numbers etc.

I don't know what cam - still original, I think it is the 'purple' shaft? Vacuum is 18 at idle, above 12 at all cruise conditions on the primaries. I am actually running a larger spring load to get on the accel portion of the rods earlier without a flat spot. I have tried running without the spring and it makes no difference at steady state cruise speeds.

I have also run as high as 15.5 on cruise with no pinging, but as I mentioned in my other reply, the throttle tip in was poor.

Thanks again.

That is a late emissions era type thermoquad (tq). Many of these had brass inserts to limit idle fuel. For example, the idle mixture screws could be backed out many turns, but mixture would not go richer due to the limiting holes of the inserts. 1 and 1/2 turns out sounds about right. But 13.5 AFR at idle would be too lean for me (again I am going by a very antiquated Heathkit Co meter). But if your engine stays cool (does not puke out coolant when you shut it off), exhaust doesn't burn your eyes, your clothes don't smell like a gas station, and no fuel is dripping out of the boosters at idle, then you are good to go. Does backing out the idle mixture screws make the idle AFR substantially richer?
IFR (Idle Feed restriction) is what some call the idle jet. This IFR also feeds the transition slot. My motor is still mostly controlled by the transition slot fuel (IFR) even at 70 mph. So that is why I call the idle circuit also the light throttle cruise circuit.
I don't use or have any of the late model tq's, but I don't recall idle air bypass holes ever being controlled by brass inserts. Usually they are just as cast holes in the bottom casting (throttle plate). I have also found, like you, that big blocks do not accelerate well on lean mixtures.

Thanks for the reply. Idle AFR seems good to me and meets all your comments and if I back the screws out another full turn the AFR will go down to about 12.
I hadn't realized the idle circuit was playing a big part so long into the cruise region, I will have to put more effort into that area.
It also seems I still have a bit to learn on the terminology, from web diagrams I have, the idle air bleed and bypass holes are in the Carb Top by the metering rod area. They both use brass inserts and the bypass one sticks out about .060" from the casting. I also have the original TQ carb from this engine ('73 with a 2124 top) that hasn't been used for 20 years, and it has the same inserts.
I think my first step is to fix this hole and then see where I am. I would send a picture but couldn't get it to attach!

Ray

The idle circuit plays a big role in how the carb works at cruise so you can make a lot of adjustment with the idle circuit. At part throttle the engine can be run fairly lean (14:1 or 14.5 or even higher). You only need to richen it up to 12.5 range at WOT.

If the carb is getting leaner as the speed goes up then that is a fuel curve issue and you have to mess with the air bleeds to correct it. Changing the air bleeds (or air correctors) is fairly easy on a newer Holley carb but I'm thinking that the Thermoquad used pressed in bleeds.

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.

On the early tq that passage is on the bottom casting. Bottom casting can also be referred to as the throttle body. A rectangular port is formed with the gasket (gasket between the lower casting and phenolic body) in the lower casting. If that makes sense...Then routes down...

Thanks Andy, Chargerjoert.
I will pull my top and give it a comparison with the old 2124 top and bring the IAB the same to at least have a grounded start point. Then see what it goes like. Will post when I have done that..

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

Hi 383man, first the carb doesn't have the idle enrichment valve. It looks very similar to the original '73 carb I have with no additional valves etc. When I pull the top again I will measure and compare all the bleeds/restrictors I can.
As for how does the car go? basically it goes quite well, no serious stumbles or surges, idles well, doesn't get hot (winter here!!). The issue i have is the medium throttle response from cruise speeds - what I would call normal road driving and not using the secondaries. Because the cruise AFR changes with speed the response to the throttle also varies, so at some speeds the response is great with a nice immediate kick yet at other speeds it just feels dead, either from a lean or rich condition. If it is a lean condition I can watch the AFR until it gets back to 12.5/13 and feel the kick again. Hope this makes sense? Ray

I'm having another issue with my Thermoquad and this discussion is helpful.

Does someone have a photo or a diagram showing the positioning of the idle air bleed and the idle bypass bleed that they could post? I'm probably not the only person following this thread that would be helped with this visual aid.

Thank you

Read through the help section and have added some images.
First is a diagram that I have used to name parts in the idle circuit, although not sure if this is the common names!
Second is a picture of my original '73 TQ showing idle air bleed
and by-pass (protruding tube)
Third is my "new"TQ showing a modified By-pass, looking carefully this is on both sides of my carb.
Hope this helps.

Third image of new tq below;

Thank you RayR. Mine is idling very lean and your photos help.

Rather than muddy the waters in your thread, I'll start another one, and I might plagiarize your photos and diagram if that is OK.

No problems, go for it!!

Success!!!
I have pulled my TQ top, measured the bleeds, 'fixed' the idle bypass and road tested the car - AFR's now close to perfect!! Image below shows the bleeds as measured on my parts with the Idle bypass being way different and different left to right - this is the bleed that had been modified, also below. I elected based on advice above to try and correct this bypass. I dremeled off the swaged tube and opened out the tube to the original size as best I could - I ended up with 0.050 both sides (tricky job as you cannot get a drill in) image also below.
After reassembly the car started immediately and needed another 3/4 turns out on the idle screws to get the idle AFR back to 13.5, confirming I had leaned this circuit. On the road with no further adjustments the car ran great and AFR steady at between 14.0 to 14.5 from 40mph to 80mph. Metering rod still working well with a 1/2 turn leaner taking the AFR to 14.5 to 14.7 over the same speed range. Also as I hoped the throttle response is now the same at all speeds with a solid push at all speeds and AFR 12.5 to 13.0 (primary only accel).

My conclusions,
1/ Thanks to the Gurus who helped!!
2/ As the replies said the Idle circuit is playing a significant part well into the cruise range.
3/ If buying a used/remanufactured TQ have a really good understanding of if and how it has been modified before getting into tuning.

Ray

Good for you as thats good to hear. I always felt the wording on them can confuse many. When I look at that pic the first thing I notice is the inset pic that says Idle Air which looks to just be extra air bypassed around the throttle blades almost like when drilling holes in the throttle blades on Holleys. Then I see the idle air bleed and then what they list as a bypass which looks to just be another idle air bleed. Then the economizer which is just anothe idle feed restriction or idle low speed jet. But its good to have the pic as you can look at it and understand where all the jets , restrictions and air bleeds are. And you did a good job as you stuck with it to tune the carb in to work how you wanted it to for you. Nice going on a very good job. Ron