Problems with Thermo-Quad

Discussion in 'Carter' started by batsong, Jul 14, 2009.

  1. batsong

    batsong Well-Known Member

    I've got a Q-Jet on my 455 now. Its is well set up and I like it, but I would also like to run my 850 comp series Thermo-Quad. I find I can get slightly better mpg and has great performance at WOT.
    The problem is that I get a bad surge at cruise, and it is really sluggish for normal driving around town.
    It is hard to find info on these carbs, but it is so simple, and this is a good clean unit, that I can't imagine what I am missing.
  2. 71stagegs

    71stagegs bpg member #1417
  3. batsong

    batsong Well-Known Member

    Thanks. I've checked out that site. I'd like to get parts from him.
    I know a lot of people used to run T-Quads on 455's in the early days. My motor is starving for fuel at high rpm.
    Last edited: Jul 20, 2009
  4. doc

    doc Well-Known Member

    Be advised,,,, back in the day,,, thermoquads were known for the main body warping....
  5. batsong

    batsong Well-Known Member

    Turns out the problem with my Thermoquad was primary rods were not seating properly at cruise.
    Last edited: Jul 26, 2009
  6. 71stagegs

    71stagegs bpg member #1417

    Gabe let me know how it works.I spoke to Dan@ thermoquads he has a nice 1033cmf model i might try.
  7. batsong

    batsong Well-Known Member

    I really like the Competition Series Thermoquad. It's an elegant design, gets mpg on par with a Q-jet, and great performance with the secondaries.
    The problems I've had are the availability of parts (mine have had the press-in jets) and finding others with T-Q experience.
  8. No Lift

    No Lift Platinum Level Contributor

    I've run the Comp series TQ forever now in both the 850/1000 sizes. Right now I'm running the 850 on my 455. Who's the Braniac who came up with press-in jets? They should have fired him!

    I played around a bit and actually converted 2 of my carbs to screw-in Holley jets on the primary. I'm not much of a machinest but I got it done with not too much JB Weld. It has been working well for years now.

    When I run maximum performance timing and a vacuum advance I get some cruise surge on the highway. With the 850 I'm at .086 primary jets but I'm not sure what rods. That's higher than I'd like to run for max MPG. I run a 1" 4 hole spacer which seems to give me max performance at the track but the 1" open spacer seems to give the smoothest cruise performance. This is all on a B4B intake.

    My mpg has usually been better with the TQ because I usually have the Q-Jet jetted up pretty high.
  9. Scott Smith

    Scott Smith New Member

    Yes they were but mistakingly so. Many "mechanics" hired to work on them used the "warping/cracking" story to get off the hook for breaking them telling people that the phenolic/plastic portions of the carbs were warped or cracked because upon dissasembling them said mechanic missed a couple screws and pryed them appart accidentally breaking the plastic.

    Thermoquads are a wonderful design, and once you become familiar with how tunable they can be you'll never look back, EXCELLENT carbs for a street strip car or even an all out drag car depending on the application.


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  10. Cliff R

    Cliff R Well-Known Member

    Agreed, the TQ's are a very good design. They are extremely wide carburetors, and every peice of linkage on them is an afterthought. Among the most complicated linkages to get everything adjusted correctly as a street carburetor.

    If you take the choke out of the equation, they become a lot easier. The primary side uses the same set-up as a q-jet, so they will be equally as smooth at light throttle openings and for "normal" driving with correct tuning.

    There are several variations of them, pressed in jets as mentioned, and some have HUGE jets screwed into the bottom of the airhorn instead of being located in the main casting.

    They had MAJOR troubles with sealing up between the main casting and airhorn. In most cases the leaks were nothing more than the small seals used between the two components. They redsigned this seal decades ago, and it solved the issue. I suspect truckloads of them were scrapped due to this problem, folks thinking the castings were cracked instead?

    I don't mess with them too much here, not really all that much interest, at least in comparison to the q-jet. The TQ offers two needle/seat assemblies and two fuel bowls in comparison, so it will support big HP without a garden hose for fuel delivery. Even with that said, we have customers running q-jets deep into the 9's, and my daily driver runs nearly into the 10's with a very basic fuel delivery system feeding a later model 1977 Q-jet.

    For tuning, the Q-jet is MUCH easier, as any and all metering changes with the TQ involve removing the top, unless you are just going to tune the part throttle A/F to a different range by changing the metering rods.

    Since the q-jet uses an external hanger for the secondary side with removable metering rods, we can change the fuel curve at the track in less time than it took to type this, without taking anything apart.

    Several years ago we were donated a factory high performance TQ for testing. It came up short on power production at the dyno back to back against my q-jet. The A/F curve was fine, but it was WAY down on power.

    After the session was over, I discovered the the throttle plates were only opnening to about 80 percent as the linkage hit the stop. This problem was not easily corrected, again, going back to the way the linkage was designed. I ended up selling the TQ, as it was just too wide and I didn't like the fuel inlet location and "cobbling" it required to use with my drop base Shaker assembly. It hit in several places and required the fuel line to come in from underneath and still the carb really needed a spacer between it and the Shaker base.

    Anyhow, I still consider them nice units, and the aftermarket support has improved considerably in recent years.:TU:

    Beware of the new replacment castings that are available. They often require a few holes to be plugged and some Marine Tex epoxy to make them work with some applications. Had a Mopar unit in here recently that had been in many hands before we got it, with zero success. Nothing more wrong with it than HUGE internal vacuum leak as it was not a DIRECT replacment for the original unit......Cliff

  11. this is an old post but worth revisiting. I have recently obtained several 800, 850, and 1000 cfm TQ carbs, both the Competition Series and later Superquad 9800 series.

    the 1st gen CS carbs have the pressed jets in airhorn
    the 2nd gen CS carbs have the large screw in jets in airhorn
    the Superquad (9800 series) have small screw in primary jets in center section float bowl floors, and long secondary jets theaded into top air horn roof. they also have different thead on the needle valves than CS series- the CS series have HUGE needle valves and brass floats.

    the CS series have no idle air bypass for primaries. this is they reason they idle so lousy. add idle air bypass passages to one, it will idle better. some of the passages are already there, just not drilled to vacuum source in base, or blocked with gaskets. the 9800 Superquads already do have idle air bypass.

    on the 800 CFM 9800 series Superquads, the secondary AIR VALVE does not open as far as the earlier 850/1000 CS series, and the front tabs on the air valve flaps are bent downward to partially block and limit airflow even more, at WOT. they also have smaller 1-3/8" primary throttle plates. if you had one of these, that would explain why power was down compared to a Qjet.

    the 1000 cfm units have a different airhorn, not only is the outer booster ring removed- there is less casting material on the sides of the primaries, that supports the booster, so the 1000 cfm units have a very unique top air horn. much of the material that would support the outer booster ring was removed from the casting, because there is no outer ring to support.

    the Qjet is a fine carb, I used/rebuilt/bought/sold them for 30 years, the TQ is basically a Qjet on steroids. the beauty of the QJ was at one time, they were $10 each or free, and gave a budget rodder 750/800 cfm for basically lunch money. Carter took the Qjet and increased the CFM and fuel delivery, but errantly removed the easily changed secondary rods in the Qjet. that may have/probably was done to get around the Rochester and GM patents, or licensing fees- considering where the carbs were made. some major design changes must take place to stop patent infringement lawsuits. these companies would often steal shamelessly, and fight it out in court later if need be. removing the 2ndary rods and hanger is definitely a setback in the TQ, compared to the original copied Qjet design. let's cut to the chase, Carter shamelessly copied the Qjet, and modified it. the TQ took 2 steps forward (dual needle valves, plastic center) and one step back (no secondary rods). the primary rods being replaceable from top on the TQ, is not as handy as the secondary rods being the same on the QJet- the 2ndary is 3/4 of the carb, and where most of the power is.

    I still have my best 800 cfm Qjet, Buick 1971 center section. The TQ is the same design albeit modified. Up to the limits of the single needle valve on the Qjet, an 800/850 cfm TQ would no better, or only slightly better- it's the same design. The Qjet has a finely adjustable secondary circuit, whereas the TQ does not, it's fixed by jets that require top removal to change. in this respect up to the needle valve flow/HP limit, the Qjet is easier to adjust at WOT than a TQ- as you have stated. one little screw removes the hanger and 2ndary rods on a Qjet, and as you have said, I've change them many times at the track in about 10 seconds.

    but that too, is not saying much. we totally REMOVED the 2ndary rods/hanger from a 750 CFM Qjet, on a light 455 race car (2650 lbs.) at my suggestion, and it ran the SAME TIME as with the rods. the mph picked up 1mph, but the e.t. dropped off a few hundredths. it gained at end of track with more fuel, lost a little to half track. this was a car that ran 10.50's with a Qjet, 9.80's with a Dominator 1050, 9.60's with 2-660 Holleys, and eventually went 9.40's with 2-1050 Dominators. I vividly remember watching the runs using the dual Dominators. the 468 CID engine sounded like it was 600 CID- because actually, that's what the big carbs do- they make the engine "feel" bigger at high rpm, the volumetric efficiency is higher with more carb. its getting more air, more of the air it needs at that high rpm, or think of it as getting "all" its air at the higher rpm. he sacrifice is low rpm. street engines are tuned for 0-5000 rpm. the ones winning races are tuned from 4000-10,000 rpm. you may have a 455, but with a small carb, you're actually only using about 400 CID of it. the VE is down a higher rpm, with a small carb.

    I saw that first hand and suggested doing it, to prove a point to the driver. I've also run Qjets with no metering rods in primary. point being, beyond a certain level, for WOT, the metering rods don't matter. this is why and where a Holley shines, at high HP levels, it has no metering rods, and can put more fuel in the engine at high rpm.

    point- both TQ and Qjet are STREET designs. they have limits. I've built an engine in my driveway, that sucked the Qjet 800 fuel bowl dry at high rpm, even when using 2 fuel pumps. It would stutter, stammer, and break up. A Holley 780 dual feed vac sec fixed it. A TQ would have done better than the Qjet as well in that app, but the issue is, a Holley will outdo them both in that app. those rods and hangers were being used as primitive emission devices, and to adjust the carb to many various engines precisely for part throttle mileage.

    a Holley was designed to dump it all in at WOT, like an on/off switch. the part throttle driveability was a compromise, an afterthought, with the power valve thrown in for WOT enrichment.

    i.e. where the TQ will really shine, is a 550HP-up build, where the big 1000 cfm capacity can be utilized, and supported by the dual float bowl inlet arrangement. tQ may also be worth some slightly quicker times even in a lower HP build, because the larger CFM will carry the HP to a slightly higher rpm, even in a 400-500 HP engine. that phenom does occur and exist. I've run quicker times with a 400 CID engine, using an 800 cfm Qjet after a 750 CFM Qjet, back to back tests, only changing the center section, same top/base/jetting/rods, same day at the track. this was street 9.6CR motor making 400 HP. the carb and centers were the early 1968-71 GM series, Pontiac and Buick. The difference in the mph and e.t. was dramatic.

    but this is also a mute point, as a good square bore Holley 850 or 1050 Dominator will outdo both TQ and Qjet above 600HP.

    this relegates both Qjet and TQ to street/strip cars up to about 550-600HP, where the small primaries give good gas mileage, or "class cars" where they must be run per NHRA rules. (SS, S classes, etc,) there isn't a single class car running that would not go faster with a modern Holley or Holley clone type design carb bolted on.

    that's why you'd get dq'd if you tried to tech through a new Demon, Holley, or QuickFuel carb in SuperStock. they aren't dq'ing you because the modern carb would be slower or make less HP. they know it will make more.

    isolated e.t timelips in the 1/4 mile don't mean much, car weight can be varied. it would have to be back to back tests, on the same car. the "Qjets running into the 9's" referred to, may very well run into the 8's with a Holley or Dominator. see ? "running into the 9's" but compared to what other carb, back to back, same day, same carb ?

    there's no doubt in my mind, a tweaked Holley on those same cars, would run faster, because any engine builder knows, 4 equal butterflies will feed an engine better, than a spreadbore design. 4 equal holes will feed better at WOT regardless of cam, compression, or being "trailered". it's basic airflow and physics. the spreadbore was a factory design compromise to get a 4bbl carb more precise, leaned out, and in line with emissions standards. it was not a max all out high HP racing design. proof the design IS a compromise, both the TQ and QJet require idle air bypass to "fix" the idle circuit. that's because the small primaries would require so much throttle opening to idle, the velocity would be so high, they would start pulling fuel from the main nozzle, just like a 1000 CFM CS TQ does.

    the relatively generous idle air bypass in both TQ and Qjet carbs, allows closing the primaries to the point, they only feed from the idle discharge circuit. it's a factory Band-Aid fix on the spreadbore design, nothing more. I've already drilled idle air bypass to 1/4" both sides on my Qjet, with 1/4" holes in both sides of carb center to feed it. the carb idled at 800 rpm in drive, on a tunnel ram, with a 250 duration solid flat tappet cam, with no main nozzle drip. it also had no main metering rods, only jets. with the factory idle air bypass passages and primary metering rods, it required more throttle opening to idle, had a high idle, and the main nozzle was drizzling at idle. idle air bypass is a necessary fix and trait of the TQ and Qjet spreadbore design.

    the dilemma with spreadbore carbs has been permanently fixed, with modern fuel injection. the throttle body only controls air, the fuel is injected at the end of intake manifold passage. with a spreadbore carb, the air in the carb bore is relied upon to do 2 things, feed the engine, but also pull the fuel in. going to a small primary increases velocity and pulls the fuel in from the main nozzle too soon. so idle air bypass was the fix. it's nothing more than a drilled intentional vacuum leak ! now that the fuel circuit has been separated from the air intake on modern cars with fuel injection, controlled by sensors and a computer, this is no longer a problem- provided all the inputs and outputs work correctly. fortunately most have a "limp mode" open loop that will get you home, or to the garage, if a sensor or computer malfunctions. that is fixed timing and fuel with very little variation to just get you home.

    TMK, the fuel leakage problem was between the center section and baseplate on the TQ, not the top airhorn and center section. The TQ shared the same leaky main wells that the QJ had, but the TQ being plastic, with well covers glued on over the main wells, tended to leak even more than the metal/plugged Qjet wells.

    I had to JB weld a ton of Qjet wells, it was common practice to drill those passages to enlarge fuel feed to the secondaries on the Qjet. I also drilled a ton of Qjet secondary tertiary circuit discharge passages, to enlarge them to 1/8"

    now, don't you so-called "authors" out there take what I posted here now, and put it in your new revised "books" or magazines, as lifted second hand information, to sell to the general public, like you've done a few times in the past with my net posts. yeh, I saw that. (not referring to you, Cliff) this is copyrighted and my own personal information and thoughts for this site only, for FREE to anyone, and I want to keep it FREE. not peddled for 9.95 or 19.95 at Barnes & Nobles, which is what a few of you guys did. I'm serious. net posts are not for you to plagiarize into those half-aszed "books" and magazine articles. anyone can easily notice MY terms and writing style, in your books and magazine. I've already seen my posts nearly word for word in the editors column of one car magazine, and earlier in an article in same magazine. my buddies emailed me to tell me, they noticed it blatantly. I've also seen my posts in another engine book about Qjet carb mods. the book author had close ties to the magazine people. not hard to figure out.
    well, NEXT TIME I see my net posts jacked into a book or article, and sold for profit, you're going to be paying me royalty money- because it's obvious you guys are shameless, troll the net for info, and will hack for profit. you guys know who you are. I just dumped over a grand on vintage carbs to learn about them, and share the info freely here, not for you to profit by it. if you want to learn, get out of the office, into the garage or shop, and ante up, and break new ground for once. don't lift my posts for convenience, out of laziness. if those days are over and gone for you, oh well- it's still no excuse to steal my posts and tech info, to sell it.

    then again, I don't think you'd have the balls or guts, to drill 1/4" idle air bypass holes through the side of your Qjet or TQ, or actually write a book suggesting it. and your ego, buying habits, lack of common sense, and engine rpm/HP/airflow limits won't let you admit a Holley is a better WOT carb then a Qjet or TQ- just because you're running and/or selling a spreadbore carb. or you'd have a hard time selling that drilled carb. but you did shamelessly lift my secondary Qjet tertiary circuit mod, and put it in your book as your own, and sold it. shame. all you had to do was ask, and give the proper footnote credit. instead you hacked.
    Last edited: Oct 8, 2014
    87GN_70GS likes this.

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