Alternative to Using a ThermoQuad

The Question:
What are the options OTHER THAN using a ThermoQuad Carb on my 340 to get the SAME height so my Shaker Bubble does not rub on Hood. I used a Base plate under a ederbrock performer carb and it raised the bubble to high. Do the make a replacement carb for the 71 thermoquad?

Personally I'd make the TQ "work"...but if your seeking a "newer" alternative, there's the Demon

X2 on the TQ find someone to rebuild it.

holley

there was a holley replacment for the thermoquad they turn up now and then .

think i got one layin around somewhere .

Yes this. If the Demon here will work your not only getting a brand new carb but an improved version of the Thermoquad. The Demon incorporates the best attributes of the Thermoquad the Quadrajet and the AFB with some additional improvements. Gentleman who designed it worked for Carter back in the 60s and 70s.

^^^ sounds like a good plan assuming the "height" ain't a problem.

The main fitment problem I've heard about with the Street Demon carb is that a drop base air cleaner can hit the choke body. Folks either go with a flat lower pan or modify it for clearance.

The Street Demon is supposed to work very well out of the box. I was very close to buying one but fell into a good sale on an Edelbrock AVS2.

if you have the room around the valve covers, "sometimes" a 15 or 16" circle track drop base will work for extra choke clearance.
it may need modification however.

Dimensions of the Demon Carb, courtesy of Speedway motors.



Relevant height: This carburetor is 3 1/4" tall from the bottom of the carburetor to the air cleaner surface., In the Q&A section.

good information to know. thanks for posting that !

Best street carb I ever had was a T-quad that Demonsizzler went through for me.
Haven't seen a post from him in years, though.
App was a 406" SB Duster street car with A/C; started quickly after sitting, ran clean, perfect throttle response, 17MPG.
That was 2002-2004.

Worst POS carb I've ever had was a Race Demon 850DP, so I'm leery of the brand even though the one posted is a different animal.

The Q should be .... why do you feel the Plastic Fantastic should be replaced ?

I had several different carbs on my 50 Dodge 4x4. It seems even the best one had to be messed with every year (I live where we have cold winters and hot summers, and the truck was driven year around. The best fix I ever made was installing electronic fuel injection.
That truck had the last carb on anything short of the mower at my house. Gene

shaker base

Holley did make a replacement carb with small primaries and big secondaries for the Quadrajet which I believe would also work as a Thermoquad replacement. Don't know if its still made though. I also agree the Thermoquad is a good carb if you know what your doing with it as they work great when done right.
I believe Mopar Muscle tested the Demon Thermoquad replacement they make about a year or so ago in the mag and it worked good. I have heard that Demon carb shown in this thread is a good working carb. Ron

The Thermoquad was a pretty good carb in it's day, but they were not without their problems. And as with any 50 year old carb, they were designed to run on a very different fuel than they would see today. So, any Thermoquad would need a bit of work.

The Street Demon was designed by an former Carter engineer, which is why it is similar to the Thermoquad. But it has a number of design features that set it apart from any other mild street oriented carb out there including the Thermoquad.

Go with the Street Demon. As mentioned, the only issue that might be encountered is the choke housing.

I have some recent experience trying to get a Street Demon 750 to run well on one of my Mopars. The following is my observation/opinions. The motor is a mild build 400, cold Edlebrock performer intake and E-street heads. Out of the box it was jetted pretty close for this combo but accelerator pump didn’t seem to be working correctly. Wound up having to open it up, found small gaps in lip of the acc pump the lip seal and a tear in the Holley umbrella style check valve. After replacing those parts the pump worked better but still needed some larger nozzle tuning to resolve some slight residual hesitation.

No trouble height wise as a Thermoquad replacement but required extensive throttle lever modifications to retain the original throttle cable, throttle pressure linkage components, and correct overall geometry. The Mopar throttle pin adaptor designed for the Street Demon and installed on the “universal” throttle lever on the Street Demon, was useless in avoiding the extensive mods. Plumbing for bowl vent and charcoal canister purge are not present, as with most universal aftermarket carbs.

Could never get the choke tuned correctly for decent cold start & run. The factory dual snorkel air cleaner had no issues with the choke housing. The documentation that came with Street Demon refers to possible interference with the choke linkage near the air horn and factory style air cleaners. There is nothing the locate, or center, the lower air cleaner other than the air cleaner stud. I could see where the air cleaner could shift out of place and contact the linage.

Another irritating issue was the “goggle” valve, secondary throttle blade, binding in its bore requiring excessive throttle pressure to overcome periodically, especially on first use after warm-up. Tried to reseat the throttle blade on its shaft to correct, helped a little but never resolved the issue completely.

Overall tolerances of the various components are not anything close to the Thermoquad, when in good shape, or new Holleys, ended up going back to the best Thermoquad I have. While a bit complex by comparison, with parts that are hard to come by now, a correctly tuned Thermoquad, in good shape, is hard to beat for performance and all-around drivability.

Holley still does make a spread bore replacement but it’s a 650 CFM vacuum secondary GM replacement design with a universal throttle lever. Ran one of those too, good drivability overall, just not same full throttle punch IMHO. The Mopar specific models are now long out of production.

BTW, I have the Street Demon 750 and all the extra tuning parts for it currently are on E-bay should there be someone here who would care to take it on. Priced well below what I have in it.

Street Demon 750

I also have a '71 340 shaker set up. I eliminated the Thermoquad by going with a '70 factory cast iron intake, so I could use an AVS.

Should be noted too that the Street Demon is not really a Thermoquad replacement as it is a square bore configuration. It likely will not bolt directly on an OE cast iron spread bore manifold, just like the Edelbrock carbs, requires the use of a square bore to spread bore adaptor, that would mean additional height.

I have run many an Edelbrock carb on a spreadbore manifold with no adapter. Just a thick carb to manifold gasket is sufficient.

I know on the factory 340 intakes with the double keyhole arrangement the speed demon will not fit without a spacer. It will fit on an older edelbrock performer (not air gap) though. I don’t know how much that will effect height for you. I think interference with the choke well was the issue on the factory piece.

The Street Demon has both bolt patterns, square and spread bore. Which is why it has the weird (I'ld call it a 3 barrel) secondaries so that it will fit both types of intakes. It is possible that there could be some interference on come parts of some manifolds. I haven't ran into any yet, but it's possible. But it wouldn't be a big deal to trim some metal if there were interference.

That may be, all my LA 4bbl intakes are either very late model without a choke well to speak of (late 80's) or aftermarket.

Another thing to note on the street demon with the accelerator pump. I had a bad dead spot when hitting the pedal. Tried bigger nozzles and increasing pump stroke, but not much improvement. I found the air cleaner base was pushing down on it, limiting the stroke. I found this when I went for a quick drive without the air cleaner. I do have an aftermarket air cleaner though.

For what it's worth: I run the 9800 series (aftermarket with electric choke) version on my 360 motor...very tunable, but as others have stated you really need to know the carb and a good parts selection helps.

For what it's worth, I know you are looking for alternative to a TQ, but why not get one done by Scott Smith (Harm's Automotive) . . . .it will look the business and work ! . . . probably for same $$$$ that you would spend on getting another carb . . .

Just my 2cents worth . . .

Good luck with it . . .

Cheers,

Mark