carb suggestions

[toryminx]

I just got off the phone with the guy who is building my new 383. He told me that my new 750 edelbrock may be to big for the motor. I was wondering what the suggested carb for this motor would be. I am stuck cuase I just bought this thing. He suggested that I go with a barry grant demon carb. i do not like the looks of this and it looks like a pain to tune. He said the 750 would wash out the spark plugs. any truth to this ? Help>>> http://www.donzi.net/ubb/frown.gif

[Forrest]

A 600 CFM Holley Marine carb (http://www.holley.com/HiOctn/ProdLine/Products/Marine/FMS/FMSC/0-80551.html) will work just fine.

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Forrest

[Riley]

There has been a lot of discussion on this forum about carb size, with the general consensus being that smaller is usally better. However, there were a couple of good article in Car-Craft's May issue regarding carb replacement, and in both stories they used a carb most would deem to large for the given application.

In one story they put a Holley 670 cfm Avenger on a box stock 327, and in the other story they replaced a 750 cfm quadrajet on a 355 small block with an Edelbrock 750. Both carbs ran well.

Heck, if you already have the Edelbrock 750, I'd run it and see what happens. "Back in the Day" my dad and I ran a 750 Holley vac sec carb on a hopped up 350 in our '56 Chris-Craft, and had no problems at all. And this motor would spin to 6 grand all day long.

By any chance does your engine builder sell the Demon carbs?

[toryminx]

He does sell demon carbs from 350 for a 625 cfm. He is real good and knows his stuff like nobody I have heard before. He built 3 750hp sbc for a guy to go in his 30ft cat.
Single carbs with a small whipples. http://www.donzi.net/ubb/smile.gif

[This message has been edited by toryminx (edited 06-26-2001).]

[This message has been edited by toryminx (edited 06-26-2001).]

[mikev]

650 is about the max you need for a small block unless you are turning over 6000 rpms. but all quadrajets are 750 or 850's depending on the application. as long as they have vacumm secondaries you will be ok because it will only suck what it needs and will not be over carbed. It should run fine with a 750 if tuned right but you will sacrifice a little low end power.

[Jamesbon]

I think you'll be fine with what you have as long as it's jetted correct.

I don't recall the formula, but there's one where you take your max rpm's, cubic inches and do some math/conversions to determine the cfm intake of the engine. The cfm intake will determine what carb size you'll require. Anyone recall?

Personally I have always liked the QUADRAJETS http://www.donzi.net/ubb/biggrin.gif

Good Luck.
Nate

[toryminx]

This is actually an edelbrock marine carb I just bought. I think that I will try it for now and then switch but I thought there marine carbs were ok. It looks nice...

[jaroot]

TUNABILITY... that is the key, especially on any performance engine... and i would avoid the holley marine carburators unless you don't care about tunability... don't know much bout the edelbrock/carter design... quadrajets are tunable though... you are likely or in better words almost certainly going to have to jet whatever you put on it in order to take full advantage of what you have... especially if it is a modified motor. I'd personally find someone that modifies dual feed, center hung float, adjustable float holleys to USCG standards and put one of those on it.. 650 or 700 CFM... too big and you lose your vacuum signal and hurt your low end and mid range power. The secret to everyhing though is tuning.. and jetting... you likely won't wet plugs if you have your jetting correct but you might suffer on power due to poor atomization of fuel which can also wet plugs if are too big on the flow side and dont atomize the fuel into the air. That's why quadrajets have small primaries and honkin big secondaries... give it to it when it needs it kind of thinking... I reckon i could go on and on and on into carburator theory and fluid and thermodynamics and sound like a geek but i won;t... sometimes it just sucks being an engineer... you give yourself too many headaches thinkin bout stuff...hope this helps.. anything else just ask eh.

Jim

[woodsy32]

Tory,

If the 750 is new, return it and get the 650 cfm. You did not say if it was a vacuum or mechanical secondary. If it is a mechanical secondary carb, no question you will wash your cylinders down with gas... if its vacuum secondary, you might be able to use it. A 650 cfm mechanical secondary carb is just about perfect for your application. Just my .02 worth.

Dave

[toryminx]

I just installed it before and the motor was done after 10 min of tuning so now it is now used. It is a vacuum secondaries though

[Forrest]

CFM = ((CID)x(RPM)/3456)x(%Volumetric Efficiency)

Also, I've never seen a Holley carburetor, marine or not, that you couldn't tune - including the no longer made 4010/4011 series. In fact there are so many adjustments available on a Holley carb you can mess yourself up in a hurry if you don't take it one step at a time. Most Holley carbs you can make changes to accelerator pump cam, pump discharge nozzle size, accelerator pump size, enrichment circuit (power valve), vacuum-secondary opening point, as well as changes to the main circuit jetting. You do have the most flexibility with 4150 series, since that model allows re-jetting of the secondaries where as the 4160 series just has a metering plate that is not adjustable.

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Forrest

[TORYSMINX]

So I think my 750 vacuum secondary carb may be too much for my 383. The guy that is building the engine said I should shell some cash and get a demon carb 625 road demon? I am wondering if it would just be a waste if I put this thing(750) on and ran it.

I have set a aside a couple hrs with him so we can tune and run it on the stand and I will see how it goes.That will be the true test while I get the diagnostics correct. Do you think my merc thunderbolt with a good coil will do the job or should I just shop for a billet distributer. thank you very much for all the good posts and I am learning as I go. All money is paper at this point. All I want is 70mph!

[This message has been edited by TORYSMINX (edited 06-26-2001).]

[jaroot]

think i need to rephrase that before i get flamed to death on the holley marine carb... only thing i really don't like is the inability to externally adjust float height which is the first adjustment that should be made on any carburator. I'd definitely opt for center hung over side hung floats also... but yes you can change primary jets, accelerator pump cams, discharge nozzles, secondary springs and power valves just like on all the other holleys... don't mind a quadrajunk cept i think it is overkill unless you can really take advantage of the secondaries which requires higher rpm;s... they are economical for cruising with the small primary venturis...

[Randy Hibbard]

Tory, you and I have the same goal for our Minx, and I have been running that same 750 for awhile, no problems with it yet, and only on my mild 350! I think the Stainless marine manifolds and then MSD system is what the Minx doctor would recomend! By the way, the preformer RPM manifold works great as well. I try to learn from the SBC car guys and then adapt thier info for marine use. they run this combo and it works, even at the lower RPM. Let us know as allways. Goodluck

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Randy H

[BigGrizzly]

You engine builder is correct. 750 IS too big. 600cfm is fine. I have done many engines and the only time I use a 750 on a small block less than 450cubes and cranking less than 6500rpms is when I don't have a 600 or a 650cfm. I know you guys say jet it and it will work. This is not true, My blown 502 with 698.5 hp (on the dyno) has a 850cfm Demon and twists 5600->5700 with a 27 inch prop. My Corsican with a 351C and big port heads turns 5600 with a 600cfm Holley 4010. It will do the same with the 750 but hesitates and drinks moooore fuel. My suggestion is run it until you can afford the Demon or the like and then put on a 600->650 cfm.
The Eldbrock will not work well unless you use the marine springs and needles. You will think you have it untill you start jumping around then it will not pull. I have one sitting on the shelf. If you don't believe me call Eldbrock.
Forrest Overton's still has a stock of 600 4010, bough all that Holley had. Mp the road Demon is not a marine carb but they do make a marine version. they are made just up the road in Dahlonega Ga. I still would opt for the 4010 Holley .

Randy

[This message has been edited by BigGrizzly (edited 06-27-2001).]

[toryminx]

i think that i will shop for the demon.But I am pissed that the 750cfm marine carb that I bought from the guy is too big. He told me go with this no matter what. Oh well I will have to either trade it or return it.

Thank you for such great posts