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My Carburetor journey.

Longer duration cams run at higher idle speeds....which means more idle air. Hole could have been enlarged because a bigger cam was fitted.....
 
I do not think it is a hole for a HIC because [ a] the surface is rounded & there does not appear to be any where to mount the HIC.
 
Another thing I found odd on the Six Barrel set up was the metering plate on the rear carb.
The fuel inlet holes were plugged with screws and above them two new holes were drilled?

spack.jpg
 
Those are the main system metering holes, equivalent to main jets in a metering block. Maybe some body wanted to reduce the size & this is how they did it?
 
Jonas Nordstrom said:
A few years ago I discovered that Carburetors are the most fun thing in the world.

...so a deep dive into the carburetor world started, here is the short version:
I started reading all the carburetor books/forum posts/articles, getting all different tuning kits, installing Afr meters, being out with the car every lunch and every evening then home again to change something, doing this year after year.
Things broke, leaked, burned, pinged, backfired but all those mistakes did not matter when you finally got that perfect fuel curve and the smile on your face went from ear to ear.

This is what I found out:

- All carburetors can be tuned to run good. The important part is to choose a carb that fits your goals (Cruise? Street? Strip? etc) and your cars driveline.
(For example: My car is heavy, automatic, 3:23 gears, stock-ish 413 and pulls from idle.)

- The carb size calculator tools that is out there gives you an average in where the carb size will function good in both bottom-end and top-end power.
You can step outside the calculated size in either direction to get the benefits of either a snappier throttle response or a stronger pull up top, but you cant get them all at the same time.
If you are not going spreadbore that is...

- The last 10 percent of tuning a carb are the hardest but makes your car go from "Runs really good" to "Holy shiiiii...!!!".
This last part is the accelerator pumps relation to the rest of the fuel curve.

My journey starts in 2007 when I got my car.
I got all the carburetors in the picture to run good with perfect fuel curves, though there was always something missing that made me continue to the next one:

1. Carter AFB.
A black carburetor that came on my car when I bought it. Looking at the numbers it seemed to be a marine carb. It always had problems with starting and it moved the car forward but not much more.
Later when I opened it i saw that one of the needles were stuck. Not knowing about the treasure that is old carburetor parts i tossed it on the scrap yard. May it rest in carburetor heaven.

2. Edelbrock 750 AFB.
I got a lot of recommendations to put on a newer Edelbrock 750cfm and so i did. It worked fine straight out of the box, the car started and moved, though there was no life in the car.
At this time I did not know what a tuning kit was and I drove around like this for many years.

3. Holley Classic 650 Vacuum Secondary.
The Holleys always looked so good on top of engines so I bought one for the looks alone (...and cos those carb size calculator tools told me to get a smaller one for my 413.)
Those dual fuel inlets looked awesome and the carb weighted a ton, it just felt quality.
I put the Holley on my engine and whoa, it didnt feel like "moving weight" anymore. It had power and broke the wheels loose easily from a stand still.
Here my thoughts started to wonder about how just changing the carb could make my car feel like a different car.
Now I started to dig into tuning kits, jet assortments, pump cams, vacuum springs, power valve numbers etc. I tried everything on this carb, i even put on the most extreme power valves and pump cams to just see and feel what the effect was. I taped a vacuum meter on the hood to read manifold vacuum when out driving. I added double AFR meters and welded in the sensor bungs right after my headers.
....This carb still feels like an old reliable friend and stands on a shelf in my garage today. I could have stopped here and be quite satisfied.

4. Holley 750 VS.
I traded some parts against a brand new Holley 750 VS (Aluminium). It was shiny, light-weight and I wanted to try if those extra cfms made some difference.
It was not as smooth as the earlier 650 to tune, the idle was more sensitive to get right etc. ...But it gave an extra push at Wot that was not there with the 650.
So, the bottom suffered a bit but the top gained some. I noted this down and continued on.
...also, these two Holleys was vacuum secondary. Vacuum secondary is smooth and nice....but they have a small delay before the secondaries kick in with the vacuum and that did not feel....fun.

5. Holley 650 Double Pumper.
After discovering carbs I always wanted to try the Holley Double Pump, the coolest carb out there. Slick, mechanic system with dual accelerator pumps.
And boy was it fun, you could feel through the gas pedal when you opened the secondaries, you were in full control.
I blasted through the neighborhood and was only an inch away from loosing my left tail to a speeding Hyundai. (I started driving more carefully around corners after that...)

6. Holley 750 DP.
I got a 750 DP cheap from a friend. It needed a full rebuild so it was time to start restoring/rebuilding carbs.
Cleaned and with new gaskets, power valve etc it gave a bit more again at Wot.
A Double Pumper was fun.....but you had to drive in a certain way so that you did not open the secondaries too early. You had to adjust your driving to the carb.

7. Edelbrock AVS2 650.
The new Eddy AVS2 was interesting as you controlled the secondaries just like a Holley DP but it had an air valve smoothing out the transition. It also had Annular boosters that was advertised to improve fuel atomization and more sales talk...
This carb made my car set off like a rocket! The throttle response was unbelievable and it continued up in the mid-range. The Holleys bottom-end was not even close.
The air valve smoothed out the transition to the secondaries opening as I had hoped and it was a joy to drive.
....but at Wot it just stopped accelerating after a while, while the Holleys just kept on pushing as there were no limit in how fast they could go.

8. Custom built Quickfuel/Proform 650 VS.
I wanted that AVS2 throttle response but with the Holley top power. So I decided to build my own carb.
It was built with mostly Quickfuel and Proform parts with metering blocks tuned for 650 where all the bleeds/restrictions could be interchanged. It got a HP choke-delete body that I drilled and plugged for vacuum secondary operation (A DP body from the beginning.) I tapped the holes in the body to fit a screw-adjustable vacuum pod housing.
The HP body got annular boosters just like the AVS2. I tuned it for weeks, getting new air bleeds etc. Emptying full tanks of gas just driving around in the neighborhood scaring those runners and cyclists.
At the final tune it had great bottom response, great mid-range and a strong never ending pull at the top.
......but not as great bottom response as the Edelbrock AVS2... (This was my most expensive carb.....and it drank fuel like there was no tomorrow.)

9. Street Demon 625.
A bit stumped I had a coffee in the garage, sitting there staring at my car. I walked over to the shelf where I still had the Eddy AVS2. I looked it over trying to figure out the secret...and there when I turned it upside down
I noticed the smaller primary throttle blades, they were quite a bit smaller than the secondary blades.
I hadnt noticed this before, all the Holleys four throttle blades had had the same size, same with the early 750 Eddy.
So Annular Boosters was not the thing. The thing was smaller primaries...
I had also grown tired of not having a choke, it was fun in the beginning feathering the throttle from cold starts...but it started to feel cumbersome sitting there looking awkward at car meets. I longed for the instant-start days that my old Holley Classic 650 had with its electric choke. Those were trouble free days.
What carb to try now...?
I stumbled upon the odd looking Street Demon carb from a cartoon ad where the cartoon guys talked about a "Goggle valve"...
It was really ugly, It looked like a cake box...
But it had everything....smaller primaries than the Eddy AVS2, triple stack boosters to increase primary air speed, mechanically controlled secondaries, air valve for a smooth transition, electric choke and a huge hole where
the two secondary throttle blades should be, a 3 barrel. (Like that old old Holley.)
I got a black one that had the plastic body and bolted it on.
Tuning it was a bit of a hassle though as the screws holding the carb together comes from beneath the carb, so you have to remove the carb from the intake when you want to change jets.
It also does not have the quality control as the earlier carbs have, so a inspection before run is needed.
What a carb...... It starts instantly, it blasts away from a stand still, it pulls hard through mid-range and continues into Wot with that smooth air valve. This was FUN.
Surprisingly it pulled harder at the top than the 25cfm bigger Eddy AVS2.
.......but I missed the even stronger pull at Wot that my custom Quickfuel carb had.

10. Thermoquad 800.
Enjoying the Street Demon, I red all articles about the carb and what good times/problems people had with it, here I found an interview with the designer, a guy that had earlier worked at Carter.
He mentioned how the Street Demon had lots of things borrowed from the Thermoquad.
Thermoquad? That old relic from the 70s that had that alien "spreadbore" foot-print and looked like a big bug had parked itself on top of your engine?
Later on....searching for pics and looking at the Thermoquad foot-print I was surprised to see that the primary side blades had the same size as the Street Demon 625....but then the secondaries was HUGE.
This monstrous old thing sparked my interest.....but finding info or parts was easier said than done as it was not produced anymore, you had to find a used one.
After a long winter of studying, joining TQ groups, well, practically breathing Thermoquad...I had all the books, service manuals, hard to get tuning kits, a bunch of actual Thermoquads on my shelf and a by me fully restored 9800 Super Quad (Over the counter Thermoquad 800cfm with no emission stuff on it.)
This carb was more complex than the ones before, both in restoring (Gluing wells etc etc) and tuning (What? 15 steps to adjust it and they have to be in order too!?!) but it started instantly with that old electric choke,
it launched like the Street Demon, it punched the mid-range like the Eddy AVS2 and it passed the Custom Quickfuel at Wot.
So, everything that I had searched for was actually possible to get into one single carb and with the spreadbore pattern it took those size calculators and threw them out the window.

The end result for my engine turned out to be a custom 850cfm Thermoquad that has a 6545 base and bowl with a 6322 top and retrofitted original Super Quad electric choke and 9800 primary throttle shaft.

...maybe someone thinks: -You should try Efi!
Well, I have tinkered with a Efi car and it is not for me, some people enjoy Efi but I love the simplicity of a carb. It can run like crap but still limp you home. (Dont ask how I know but that story includes blown head gaskets and a mechanically failed ignition....)

Coming out from this deep dive into the carburetor world I noticed that it is a dying art to fully tune a carb?
I meet many people that just puts the carb on the engine, directly out from the box and are satisfied with that.
Well, I finished my journey and now it is time to help others.
With the experience I gathered from this journey I am now helping my Dad with his Cuda and my sister with her Duster, tuning their Street Demon carbs.


I hope you guys have as fun as I do and I want to thank this forum for helping me with all the questions I had and still have. I also want to thank my B-body Charger that always got me home.


p.s. The longer story includes intakes like the Eddy performer 440 and the CH4B that I changed between. It also includes lots of testing with different carb-spacers, 4-hole, wood, plastic, super sucker. Ignition curves for the different carbs etc etc....but that is for another day. d.s.

View attachment 1520293

View attachment 15202946
Click to expand...
Love this detailed write-up! Thanks for sharing.

I am a total rookie when it comes to carb tuning. I used to have a 67 Chevy 327 with a spread-bore quadrajet and loved how that carb ran for the 20 years I drove it. I never messed with it. It was always reliable. In 2010 when I got my first mopar it had an edelbrock 600 VS on top of the 360 motor. That carb started every time and had decent gas mileage. Then when I had the mild 451 built I opted for a Quickfuel SS-735-VS that is on the car now. The car is a 68 Satellite with Eddy Performer RPM intake, tri-y headers, Eddy Performer RPM heads, 727 Automatic and 3.55 gears. The motor only has 474 lift and 220/224 at .050 but pulls hard. My friend took me to the dyno shop and in the end the carb was jetted up 4 sizes and the air bleed size changed to smooth out a tiny rough spot in the idle. The Quickfuel starts almost instantly and has good pull in the mid-range and top end, but off-idle does not feel as crisp as I'd like and drinks fuel like there's no tomorrow. I get around 8 mpg average and on the highway maybe 10. I feel like with a different carb I could do better on throttle response as well as fuel mileage.

Have you also experimented with different carb spacers? I have a tapered 4 hole Wilson spacer I bought to try. Not sure if I'll feel any difference on the street around town.
Edelbrock also released their new VRS 4150 carb that I'm curious about: Edelbrock 1307 Edelbrock VRS-4150 Race and Performance Carburetors | Summit Racing
Your write-up has got me wanting to try a Thermoquad:)

By the way, that last photo of your charger taken at that angle looks bitchin'!
 
^^^^ If you want better performance....& mileage.....put a TQ on it!!!
 
Orig69 said:
That hole looks like the right spot for a hot idle compensator.
Click to expand...
The hole is indeed related to a hot idle compensator valve system mounted to the back of the center carb if the list number called for it. Otherwise, the needed machine work for the passages, valve and cover would be omitted.
2300 series hotidle comp.jpg
 
68 Sport Satellite said:
Love this detailed write-up! Thanks for sharing.

I am a total rookie when it comes to carb tuning. I used to have a 67 Chevy 327 with a spread-bore quadrajet and loved how that carb ran for the 20 years I drove it. I never messed with it. It was always reliable. In 2010 when I got my first mopar it had an edelbrock 600 VS on top of the 360 motor. That carb started every time and had decent gas mileage. Then when I had the mild 451 built I opted for a Quickfuel SS-735-VS that is on the car now. The car is a 68 Satellite with Eddy Performer RPM intake, tri-y headers, Eddy Performer RPM heads, 727 Automatic and 3.55 gears. The motor only has 474 lift and 220/224 at .050 but pulls hard. My friend took me to the dyno shop and in the end the carb was jetted up 4 sizes and the air bleed size changed to smooth out a tiny rough spot in the idle. The Quickfuel starts almost instantly and has good pull in the mid-range and top end, but off-idle does not feel as crisp as I'd like and drinks fuel like there's no tomorrow. I get around 8 mpg average and on the highway maybe 10. I feel like with a different carb I could do better on throttle response as well as fuel mileage.

Have you also experimented with different carb spacers? I have a tapered 4 hole Wilson spacer I bought to try. Not sure if I'll feel any difference on the street around town.
Edelbrock also released their new VRS 4150 carb that I'm curious about: Edelbrock 1307 Edelbrock VRS-4150 Race and Performance Carburetors | Summit Racing
Your write-up has got me wanting to try a Thermoquad:)

By the way, that last photo of your charger taken at that angle looks bitchin'!
Click to expand...
With my 850 TQ I got 16.8 mpg (Not opening the secondaries on that test though.... :) )

I did some testing with carb spacers on the holleys, like with four holes, super sucker etc, The super sucker made the top end a little happier. (Barely noticable.) But my engine seemed to like straight four hole spacers so I made a custom phenolic one for my TQ, the response is awesome but it also helps keeping that heat away from the TQ....and together with the TQ plastic body I am no way near any heat problems. (And I dont have a return line)

The VRS 4150 is cool and If I got one I would test it.

Yes, the Thermoquad is a monster, I still cannot believe why nobody is reproducing it. The big hurdle going TQ is finding an actual TQ in good condition and parts. (And having lots of patience learning how to tune it.)
Thanks :thumbsup:
 
If it wasn't for the Thermoquad since they're not easy to find in good condition, from your writeup it seems like the Street Demon 625 was your 2nd favorite? Compared to my Quickfuel SS 735 VS, would you recommend that one? I wonder if I should go for the bigger 750 cfm for my mild 451 motor? Demon is now owned by Holley aren't they? I wonder how the QC is?
Street - Holley
 
Last edited:
I have been going through my old MM magazines & found an article by Steve Dulcich on the TQs.

He says they flow tested them & they flow 900 cfm....
 
Geoff 2 said:
I have been going through my old MM magazines & found an article by Steve Dulcich on the TQs.

He says they flow tested them & they flow 900 cfm....
Click to expand...
And they are so versatile. Cut off those outer rings and you are up to 1000.
 
68 Sport Satellite said:
If it wasn't for the Thermoquad since they're not easy to find in good condition, from your writeup it seems like the Street Demon 625 was your 2nd favorite? Compared to my Quickfuel SS 735 VS, would you recommend that one? I wonder if I should go for the bigger 750 cfm for my mild 451 motor? Demon is now owned by Holley aren't they? I wonder how the QC is?
Street - Holley
Click to expand...

I love the Street Demon, I have installed three of them on my dads cars, one of them being a -70 Cuda 440. And this summer I will install a 625 SD on my sisters -71 Duster.
For a smooth - easy to tune idle, bottom end response and mpg you want the SD 625.
The 750 will give you more up top, so the question is how much do you drive at Wot compared to cruising around?

The SD is made by Holley and you want the one with the plastic body.

The QC is the achilles heel of the SD, its a hit and miss.
Get one and try it out first, if it is hard to tune in or it runs uneven or for ex the choke doesn't want to let go, then yes it is time to open it up and do some clean up.
 
The Competition Series TQ 1000 cfm model has the outer rings cut off. So did a few QJs. All the reports I have seen say the performance of the primaries suffered as a result. I am not sure if cutting off the outer rings on the production carb.....makes it 1000 cfm. The CS model had a bigger venturi, 1 3/8" versus 1 3/16" for the production 850.
The pri bore is 1 1/2"; the 1 3/8"/1 1/2" venturi/bote difference gives little pressure diffetrential & might explain the poor primary performance.
 
Yes, mess with the primary to get more up top and you will get a weaker primary.

SD note:
The SD 625 primary is the same size as the 800 TQ, they tried to open up the secondaries like a three barrel to get more cfm but being restricted by the squarebore pattern they could only reach 625cfm.
The SD 750 primary are the same size as the secondaries (If they were round) like a standard squarebore holley pattern, so some of the velocity is lost. (Though it still has the triple stack boosters.)
 
In my opinion, the SD carbs fail by not having an air controlled dashpot to control the secondary opening rate. They got away without the dashpot with the AVS because the area was much smaller. The sec area of the TQ is a 61% increase over the sec area of the Carter AVS.
The sec area on the SD is also quite large. Maybe a dashpot would have eliminated a lot of the bogging that people have had with the SD.
 
Geoff 2 said:
In my opinion, the SD carbs fail by not having an air controlled dashpot to control the secondary opening rate. They got away without the dashpot with the AVS because the area was much smaller. The sec area of the TQ is a 61% increase over the sec area of the Carter AVS.
The sec area on the SD is also quite large. Maybe a dashpot would have eliminated a lot of the bogging that people have had with the SD.
Click to expand...
Yes, the SD is great but is missing some of the TQs best functions.
 
For someone living in a non-English speaking country, your English is excellent!
 
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