428CJ carburetion

[wemooz]

I recently started a thread asking about changing my 2 X 4 carburetors with progressive linkage from 390 Holleys to 600 CFM Holleys. This is on my 428CJ that is very mild (9.65 compression, bored to 433 cubic inches, Crane 343941 cam, MSD). After reviewing the helpful comments, I decided to call the Holley tech line. The bottom line of my conversation was that the Holley guy told me:

1) Using bigger carburetors than the 390s would be too much, and
2) NEVER, NEVER use progressive linkage, which will ruin the engine because it starves some cylinders.

When I mentioned that these engines originally came equipped with 600 or larger carburetors, I was told that essentially the manufacturer did not completely understand carburetion. Then, regarding the progressive linkage, I was told that it was so bad to use that Holley did not offer it for sale.

It seems to me that using linkage that opens both carburetors simultaneously is essentially the same as using a single 780 CFM carburetor, and that it defeats the advantage of having a small venturi at low speeds. Obviously I need more helpful discussion. Incidentally I have run my present setup for 21 years and my engine is not ruined – at least not yet.

[mpanten]

Originally Posted by wemooz

I recently started a thread asking about changing my 2 X 4 carburetors with progressive linkage from 390 Holleys to 600 CFM Holleys. This is on my 428CJ that is very mild (9.65 compression, bored to 433 cubic inches, Crane 343941 cam, MSD). After reviewing the helpful comments, I decided to call the Holley tech line. The bottom line of my conversation was that the Holley guy told me:

1) Using bigger carburetors than the 390s would be too much, and
2) NEVER, NEVER use progressive linkage, which will ruin the engine because it starves some cylinders.

When I mentioned that these engines originally came equipped with 600 or larger carburetors, I was told that essentially the manufacturer did not completely understand carburetion. Then, regarding the progressive linkage, I was told that it was so bad to use that Holley did not offer it for sale.

It seems to me that using linkage that opens both carburetors simultaneously is essentially the same as using a single 780 CFM carburetor, and that it defeats the advantage of having a small venturi at low speeds. Obviously I need more helpful discussion. Incidentally I have run my present setup for 21 years and my engine is not ruined – at least not yet.

Im no carb expert but im not sure i buy the argument that they would be to big. Several guys on here are running 1100 CFM's on their small block

[Silverback51]

I run dual 660's on my 460 and I'm not over carburated.

[saltshaker]

When you use two 600 vac carbs remember that you start off using the front two barrels of each carb which equals 600 cfm and as the motor needs more cfm it then opens the secondaries.
Jon

[wemooz]

I understand about how the primary and secondaries work, but what about using progressive or non-progressive linkage? Does anyone have experience in using both? If so, what can you say about oit?

[ERA Chas]

Originally Posted by wemooz

I understand about how the primary and secondaries work, but what about using progressive or non-progressive linkage? Does anyone have experience in using both? If so, what can you say about oit?

Go back to your last thread and read what I wrote.

[DanEC]

It's nice and cool in my house and I'm too lazy to go out and look at my engine that is sitting in the corner of the (hot) garage to verify my foggy memory but I believe the carbs are mounted backwards so that the primary barrels of the front carb are not that far off center of the engine. I can't see a lot of difference with this as compared to a low performance 2 bbl carb as far as fuel distribution to the various cylinders goes. Holley Performance may not be aware of that.

I fail to see how there is much risk of overcarburetion too. Up to about half throttle you run off half the front carb or about 300 cfm max. At more throttle opening it brings in the primaries on the rear carb up to a max of about 600 cfm. And beyond that, engine demand will bring the vacuum operated secondaries in only as vacuum demand and carb calibration require. I doubt the secondaries on the front carb can open at all until your getting into the second carb. I guess it could depend on the springs in the secondary diapragms.

[saltshaker]

Each motor is different depending on heads,cam,etc....try and see what works best for you. As for your 390's, they are vac carbs? With a stock cam they could be just right

[wemooz]

I went back to my previous thread as Charles suggested. When I read it a week or two ago I reviewed what I had on the car. The carbs had been set up by a professional mechanic who owns an original 427 and who does work for Jay Leno. The linkage was set as Charles suggested, and the recent note regarding the fact that the carbs are mounted backwards makes sense. But I am left with two questions. The first is the kind of change I would experience if I switched to 600 CFM carbs, keeping the progressive linkage. The second question is what kind of change would I see if I kept the 390s and used a linkage that opened both carburetors equally. Frankly that seems kind of foolish, since it seems to mimic a single 780 CFM carb with the two 390s, and it would seem to lose the advantage of sequential opening. But that is what it seems to me and I am not a carb person.

[saltshaker]

What are you trying to do or get out of the motor with a carb or linkage change? What's your cobra not doing for you?

[wemooz]

It is the question of the unknown. Here I am with my ERA cobra that I have been happy with for 21 years. But, since I have a pair of 390's and the original cars had a pair of bigger carburetors, I wonder what the difference would be if I changed from 390's to 600's. It is an academic question, and if someone tells me that using 600's would be far more satisfactory as a daily driver, I might be inclined to make the change. The subject of progressive linkage entered this question from left field, but it is there and it is a valid consideration. So I am not saying that I am unhappy with the 390's, but I am curious about what it would be like with 600's (probably with progressive linkage). Thanks for your questions, but this is an interesting learning experience for me.

[ERA Chas]

Originally Posted by saltshaker

What are you trying to do or get out of the motor with a carb or linkage change? What's your cobra not doing for you?

Exactly the point.
Unless you have an aggressive (not race) cam and heads that flow 280-300+ CFM just stay as you are.
A. It works and is not problematic.
B. You have no demand from the other systems in the motor for more carburetion.
C. It's the least expensive thing to do.
Take the advice from Holley man with a boulder of salt.
Progressive linkage alone will not lean certain cylinders. That's more a function of which intake you use.
Larger than 390's are NOT too much for your combination. Vacuum simply allows the engine to only use the CFM it wants, making larger carbs a bling thing.
You're fixated on progressive-or-not linkage.
A. Leave it as is.
B. Experiment and vary the rear carb feed-in in small stages.
C. Experiment and make a 1:1 link and see how it runs. I did, it does.

[Zoom This]

Progressive linkage no good says Holley? Hell, the 2x4's on my '70 Hemi Cuda in 1970 had progressive linkage. 13.6 at 106 in the quarter with street Polyglas tires and a stone stock motor. In your face Holley.

[elmariachi]

You going to listen to a guy on the other end of the phone that wasn't even born when the engine was originally designed???? I've called them a few times and the answers vary every 15 minutes, depending on who picks up the phone.

I am running a pair of 715 cfm OEM bone stock Holleys on my mild 427 (list 3300 and 3301). They are the matched set Ford BC/BD set designed for the medium riser application. And I am using the stock progressive linkage. The spark plugs, which are the defacto indicator or what's going on inside, look like they were sent down from God himself. All 8 of them, absolutely identical in toasty tan color and no indication of running lean whatsoever.

Send Carl's Ford Parts $1100 for a set of BC/BDs or BJ/BKs and bolt them on. Go run the car. If its better keep them and if its not, put them on eBay. Maybe you cost yourself $200 (knowing oblivious and blind eBayers you'll probably MAKE money) but then you can quit speculating.

[Rick Parker]

Whats the typical CFM of a set of Webers

[saltshaker]

Might I suggest you buy Jay Browns book the Great FE Intake Comparison ....
It might explain what different gains some intakes and carbs have on a motor. As for making it a better daily driver there were never any complaints about the original 715cfm carb....but with your combo you get a few extra "wow's"
when they see a 2x4 setup
Jon

[ERA Chas]

Originally Posted by saltshaker

Might I suggest you buy Jim Browns book the Great FE Intake Comparison ....
Jon

That's JAY Brown.

[saltshaker]

That's funny that I did that...Going back to Cleveland tomorrow and we were talking about Jim Brown last night...correction made!
Jon

[wemooz]

I went to Amazon.com and the book by Jay Brown does not show up. Where is it available?

[saltshaker]

Google his name and the name of the book and it will come up