Advice on Bolt-on Replacement for my Original 71 Cuda Carb.

Time to replace that 45 year old 340 Thermoquad and would like some advice on a direct bolt on replacement. I would rather stick with a Holley or Edelbrock. Car still has it's original intake as well.

Thanks.

Age doesn't make it obsolete, what's your reason for replacing it instead of rebuilding it?

Not knowing anything else about the issues you are having and assuming a replacement is the only thing you'd consider, I'd try the new Demon carb (basically a modern Thermo-Quad).

I've rebuilt it a couple of times in the 25 years or so that I've owned the car. Beyond it being a 45 year old carb and the phenolic body issues this style carb is known for, I would prefer a more modern carb for a car I've been driving to Mopar events extensive distances. My issues with it over the years have to due with leakage in the gasket areas below and above the body.

Have Scott rebuild it, it will be better than new. Other than that I would go with the one pictured above, or a 750 vacuum secondary holley or the new 800 cfm AVS.

The plastic body issues you have heard of are primarily a myth. Most plastic body cracks or other issues are related to a ham fisted mechanic trying to pry one apart and cracking it without removing all of the screws first (one is a bit hidden). The leaking issues you have had sound float level or needle and seat related (over full bowls cause fuel to purge out of the top and other areas), a problem you can have with any carb.

I agree that could be. Giving more thought this morning I'm going to go ahead and rebuild the original. Any idea on kit # for the 71 Cuda carb which I believe is an 800 CFM not 850 as the later years are? Thanks.

Here's a good read on Thermo-Quads, the Vaanth Guide offers a good baseline education on their history

http://members.shaw.ca/crussel/thermoquad/tqguide.html

In answer to your questions, there were two production TQ sizes, 800 & 850, (this excludes the Competition Series TQ's produced in 1969 and 1970 which were 850 & 1000cfm, sold over the counter only typically for racing).

Basically 800's were used on factory small blocks and 850's were used on factory big blocks.

I recommend kits from either Daytona Carburetor or www.walkerproducts.com

Thanks for the info. I've been going back and forth on this, especially when I discovered today that I do not have the original carb on my Cuda. Over the years I thought I did. In looking at my records I last rebuilt that carb in 1994. It's actually a 9014S, an 850 big block thermoquad for a 74. Thinking about that Demon you suggested. Was also looking at a 600CFM 1406 Edelbrock square bore.

I have a Holley 82751 750HP that I bought through Autozone when they had a 20% off sale (the code only works on that carb). My street 383 runs great with the carb right out of the box.

The 82751 has no choke with mechanical secondary's. Autozone has a $35 gift card with every $100 spent with free shipping. Yes the carb is $549, but with $175 in gift cards (if you like shopping at Autozone) brings it down to $375.

Jegs or Summit should match that price.

https://www.holley.com/products/fuel_systems/carburetors/hp/street_hp/parts/0-82751

There are several carbs that would work well for that combo but the spreadbore intake is a complication. (I'm assuming it is a spreadbore intake)

So you might want to change intakes at the same time then use a small Edelbrock AFB/AVS carb.

You realize that nothing is going to be a "bolt on" replacement except another Thermoquad. Choke and fuel lines come to mine as well as linkage adapters and if you have cruise control you have to mod that too.

Someone linked Walker Products above, they sell brand new plastic bodies if you really need one.

Yes I realize nothing is going to be a total direct bolt on. My comment with that was to get something that would bolt to the intake without an adapter, as I would have to use with a square bore carb. Any carb I use may or may not need linkage adapters or fuel line fitting mods. Choke isn't a problem as I would use an electric on a new carb but would have to run some wire to hook that up.

When I looked up a rebuild kit for my present quad I did see that Walker sells the body for around 100 bucks. I was surprised at that. If I run into an issue with mine, that would be a good alternative as well.

Yes it is the original spread bore intake. I got a Walker rebuild kit but I'm still mulling the 625 Demon carb. Looks like a good possibility for a bolt on replacement being spread bore.

As mentioned there are several good choices. You brought up the 1406 eddy. I had one on a DD 318 & it ran flawless. you DO need a strip kit to go with it (it's lean as is) & the flat plate (cheap) to cover the intake spread bore flange. Only caveat with em, reg the psi (if needed) to ~5 or so

Looks like the Demon needs a throttle adapter but I didn't see one when I got on their site. They had them for Fords and GM's.

https://www.demoncarbs.com/124007.asp

I see. Thanks. I was looking for a bracket similar to the Holleys and Edelbrocks.

I've been curious about the Street Demon. Has anyone here tried one?

That doesn't even look close to right for the kickdown. Throttle only, yeah.

Looks like you need a Holley kickdown linkage adapter for the demon.

https://www.holley.com/products/fuel_systems/carburetor_components/brackets_and_linkage/parts/20-7

Demon talks a game, but can't deliver for Chryslers, something as basic as that should be there on the Demon site.

Good question. Can anyone give any feedback, positive or negative, who has one of these Demon Carbs.

I rebuilt my Thermoquad and still have a bit of fuel oozing ever so slightly around the upper seal, plastic body to upper assembly, along both sides adjacent to the bowls. I double checked my float settings and they are exact as the Walker rebuild kit instructions call for. I've cranked a little more on the bolts when the car has gotten to operating temperature to see if I can get a bit more squeeze out of the seal. That bowl area on the quads just seems to need another screw in that spot on both sides. There is quite a bit of area between the nearest screws. To me anyway.

It appears you only need the throttle stud. Their throttle stud looks like anything you would have to use on the brackets themselves for Holley or Eddy. Zooming in on that area, it appears you don't need a bracket, just the standard stud. At 28 bucks they sure did like that allot.

If I was a betting man I'd bet serious money that as designed the Demon will not support the Chrysler kickdown linkage without an adapter. If you closely compare the linkage to a Holley they are pretty much identical.

Could be. Before I buy one though, I'd really like to know what the Mopar users of that carb think of em. Looking on the net, it appears the Chevy guys aren't very impressed with the Demon.

Demon Install - Mopar

Good info. Now are there any testimonials on how well the carb performed.

Scott, any ideas on what might be wrong with my Thermoquad? Seems like a plastic body issue to me. I've covered all the other bases that I'm aware of.

According to Demon's customer service too.

Sure lots of ideas but targeting anything specific beyond a problem with the normal stuff like bad needles & seats, sunk floats, etc. it's is a crap shoot based on a description only, not to mention the potential for non carburetor related issues (ignition, etc.).

My issue is fuel leakage at the upper seal to plastic body at the bowls on both sides.

I ordered a Polymer 625 Street Demon from Jegs today. I'll find out soon enough how well it works, short term anyhow. I'm tired of monkeying with this leaky Thermoquad.

Leaking from the upper orifices of a TQ is almost always due to improper fuel level (bad or misadjusted float, needle & seat stuck, etc.) if there are no broken or missing parts.

I double and triple checked all those things. Made sure the seats were good and needles were sealing off. Double checked that the float heights from the gasket to the top edge of the plastic floats were 1" when inverted. Made sure the floats float. I just don't think my plastic body is worth a darn anymore. It doesn't leak allot but you can see the fuel weeping from the gasket area.

The Demon carb you suggested should be here tomorrow. I'm anxious to try that out.

Did you say you set the float levels at 1.00"? If so you've just found the problem.

From a popular TQ website:



Float Level Adjustment

Adjustment of the float level requires the removal of the carburetor top cover. Since this operation results in almost complete disassembly of the carburetor, it's a good time to consider fully rebuilding the carb. Usually float level adjustment is only part of a full carb overhaul. If the carburetor is old enough, and worn enough to have developed float level problems, it's probably time for a full rebuild anyway.

The Thermo quad has two separate fuel bowls within each side of the carburetor body. There is an individual check-valve (needle and seat) for each bowl, and each valve has it's own float by which it is activated. The check-valves are located within the top-cover, and the floats also hinge from the top-cover. (The majority of hardware in a Thermo quad is mounted to the top cover.) Before attempting to adjust the float level, it's a good idea to fit new check-valves, as the float level is meaningless if the check-valves aren't sealing.

To set the float level, hold the top-cover, with floats installed, upside down and measure the clearance between the top-cover gasket surface, and the underside of each float. (By "underside", I mean the surface that would be the top of the float when the carb is assembled and sitting right-side-up, but is the underside now since you are holding it upside-down. Get it?) This clearance should be about 0.75mm, or 0.030". The idea is that the check-valve is fully closed JUST BEFORE the float contacts the top-cover.

If this check indicates the float level requires adjustment, it's necessary to remove the float and bend the float arm about 1/3rd of the way along it's length from the pivot end. Bend the arm enough to obtain the required clearance.

That's interesting. 1" is called for from the gasket to the TOP (or underside when held right side up) side of the float, not bottom. I'll measure the float and see what that distance would be but we're not talking apples to apples. The Rebuild kit instructions have you measuring to the top of the float not bottom when it's turned upside down. The kit has an illustration so as not to confuse their instruction. I've used this in my rebuild 15 years ago with no issues till now.

Installed my new Demon carb the other day. It was not just a bolt on to my stock 340 manifold the Thermoquad came on. I did have to use a spacer as described in the installation instructions. The carb works great. I am considering a Chrysler throttle stud Demon recommends. I do see a couple of different versions however of the part #124007. The one talked about earlier in this thread and another version which I would prefer for my 4 speed Cuda. Anyone know which version you actually get for that part #. I've attached one of the versions. Is this the actual one you get or the version Scott mentions in the earlier thread? That one appears to be more for an automatic.

This one in my previous response is correct. The Demon tech says the pictorials that their catalog and the distributors are showing is a generic Holley stud. They are in the process of fixing that. Just FYI for anyone else wanting to use a Demon. You need this stud. Neither the original or the Holley will fit.