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Old 03-28-2011, 08:42 PM
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Blower Carb Experts I need help

Over the weekend, I landed a great deal on a pair of 1407, Edelbrock 750 cfm carbs. They're in great conditon. I bought them at the Louisville swap meet for $135.00. My intentions are to use them on my 6-71 blown, 392 Hemi which is now really 404 cubes. 7.8 compression, street engine with 8 lbs. of boost, blower cam, max rpm 5500, 4-speed, 3:33 gear and less than 3000 lbs.

Below you will see the Edelbrock recommendation right from my carb manuals. They say to use modified 1405, 600 cfm carbs. I checked the specs on the 1407, 750 cfm carbs and they basically, off the self, already have all the jets, rods and springs called out for the modified 1405. Please read below and let me know your thoughts. I'm really confused because I've read about boost referencing issues with blower carbs. Apparently these carbs work because they have a piston that operates metering rods verses lets say a power valve in a Holley. Help!

From Edelbrock Manual- Blown Engines
Two model #1405 carburetors should be used on engines with positive displacement superchargers, such as GMC 6-71 blowers or equivalent. The following calibration changes make an excellent starting point: Primary Jets -.101" (#1429); Secondary Jets - .101" (#1429); Metering Rods - .070" x .042" (#1450); Step-up Piston Springs - 5" (orange, stock); Needle & Seat
assemblies - .110" (#1466). This calibration has been tested on engines ranging in size from 350 c.i.d. Chevys to 440 c.i.d. Chryslers with good results.
* = Carburetor is certified to meet U.S. Coast Guard Requirements
CARBURETOR SPECIFICATIONS - ALL MODELS
Carb

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Old 03-28-2011, 09:07 PM
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This all makes sense...how do I modify the power piston for lower manifold vacuum source? Thank you!
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Old 03-28-2011, 09:09 PM
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Even though the Edelbrock Owners Manual says that the 600 carb is recommended, Holley recommends dual 750's even w/a 250 sized blower, so there's no reason you cannot use the carbs you have on the 6-71 blower. The carbs will possibly need to be dialed in (usually the stock calibration is pretty close), but otherwise will work fine.

From here:

Quote:
If you are using two carburetors on a 250 Powercharger, we recommend that you use two 750 CFM vacuum secondary carburetors for 350 cid engines and two 750 CFM double pumper carburetors for engines up to 460 cid. For larger engines two 850 CFM carburetors should be used.
You can do a search for more info, like how to boost reference an edelbrock
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Old 03-28-2011, 09:28 PM
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Guys I really appreciate all the help. Glad I didn't pay a whole lots for these things especially if I butcher them up. lol There has to be away to do it. I say that because in the 60's I saw all kinds of Carter (eddy) AFB carbs on blower motors. Holleys have been around since 57 on the Y block Fords but they didn't really get popular until the mid 60's. I really like the AFB and I have several on the shelf so I will look them over.

How in the heck can Edelbrock say they will work out of the box in a blower appilcation????
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Old 03-28-2011, 10:17 PM
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F bird I will take a look at my other carbs and see if I can figure out how to modify them for the lower manifold signal. Stayed tuned and thanks for the advice.
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Old 03-29-2011, 02:31 PM
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All, I spoke to an Edelbrock representive today by phone. He said my #1407, 750 cfm carbs will work fine on my 671 Blower. His only recommendation was to up grade to the high flow needle and seat. He also said there is absolutely no reason to modify the carbs for a boost referenced signal from the lower manifold. He said bolt them on and tune the motor to fit the need.

Thanks for all your info. gang!
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Old 03-29-2011, 02:40 PM
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Quote:
Originally Posted by 1930u
His only recommendation was to up grade to the high flow needle and seat.
That pretty well sums it up.
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Old 03-30-2011, 06:08 PM
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Hey all, I did a little more homework today regarding the Edelbrock carb. blower application. I called Big Al's Toy Box. Much to my surprise Al the owner answered the phone. I called because he sells a 750 cfm Edelbrock blower carb kit. No boost referencing needed. It's all in the jetting and tuning. Please see pic of the kit below. He seemed like a sharp guy and said he has more AFB carbs running on blowers than the Holleys. He also said that they dyno very well and that he has about 9,400 set ups in the market and 6,000 of them have the Edelbrock 1407, 750 cfm carbs.

I will also add that if you send him your 750 cfm edelbrock he will rebuild it and blower calibrate it for $100.00 each including all the parts. I also incuded a picture of a couple old school Carter (Edelbrock) AFB's on a 671 blown, 394 Olds. Now that's old school!
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Old 03-30-2011, 06:47 PM
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Quote:
Originally Posted by 1930u
I will also add that if you send him your 750 cfm edelbrock he will rebuild it and blower calibrate it for $100.00 each including all the parts.
That sounds reasonable. It seems if he were just out to gouge people, he'd be adding boost referencing even if it wasn't needed. But because he doesn't- and has an excellent reputation among members w/blower engines on this board- I can only conclude the info is valid. Not arguing, just saying.

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Old 03-30-2011, 08:37 PM
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All, they sell the Holley carbs too. See pic. I also looked around on the web and found a few more forums and it seems this whole boost reference thing is a hot topic everywhere. I also spoke to Gary Dyer and the guy at BDS yesterday and they also said the Edelbrock will work but they recommend and sell the Holleys and other brands. I can't remember the other brand carb but it was similar to the Barry Grant product.

I really appreciate all the feed back but I think at this point, I'm going to run the AFB's with no reference signal and see what happens. Plus I love the way they look. Lol If the Hemi melts down I'll just drag another motor out of the corner and start over. Lol Thanks again folks.
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Old 03-31-2011, 12:37 AM
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While Edelbrock tech is helpful on many topics, my experience with them on carburetion for blower motors above the Weiand/Holley 144 is not promising. On a 7.5:1 SBC 355, I currently run twin Holley boost referenced 600 cfm carbs atop a Weiand 250 (basically a 4:71 GMC) blower. Previously I ran, and had fun with, a Weiand 144 blower with a single Edelbrock Thunder 800 cfm on top. When I switched to the Weiand 250, I sought advice from Edelbrock and was advised that the single 800 cfm Thunder would be sufficient. It was not, as I wasted time and money on both a center-mounted four barrel, then on a forward-mounted (marine) manifold, to run the single 800 on the 250. Detonation proved to be a big issue. "Yes", a big part of that had to do with the long, low design of the 250 blower case, which starves cylinders 1-2-3 for fuel when a single carb is use. But it also is relevant that the Edelbrock carb staff did not advise that a single 4 simply would not work on a Weiand 250 blower. Frankly, I also was not competently advised by Holley tech on blower carbs. It was not until I tapped into Hot Rodders' Bulletin Board that I got competent, experienced, advise about setting up a blower with carbs. Listen to F-Bird and the others who have advised.
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Old 03-31-2011, 06:15 AM
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1930U - See...I told you...
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Old 03-31-2011, 07:44 AM
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I helped setup a friends 6-71 blower on a 468BBC. Street cruiser with 5PSI boost at 5500RPM WOT. We got two used E-bay 750DP Holleys, rebuilt them and boost referenced them to the intake manifold, below the blower. I researched the boost referencing issue a lot before doing it. It is a must on Holleys.

In my opinion the Holley power valve and the Edelbrock metering rod/step-up piston are not equals. The Holley power valve will flow much more fuel than Edelbrocks metering rod/jet combination. Edelbrock relies on large jetting in the secondary for WOT enrichment. Not saying the metering rod/step-up piston doesn't contribute at WOT, but not nearly as much as the Holley power valve contributes in the Holley design.

We started 4 jet sizes larger than stock and a 10" power valve. We ended up with stock jetting in the front and 2 larger in the rear. We could have went to stock in the rear and maybe a 6" or 8" power valve, but chose to leave it a bit rich. It's a street cruiser that seldom sees WOT.

F-bird is correct about the Edelbrock metering rods probably not rising with the blower pulling such a strong vacuum signal on the carbs. But I think, and it's just my opinion, the Edelbrocks get by with it on the blower applications by just running quite a bit larger secondary jets and strong step-up springs for mid range enrichment.

One thing for sure, get yourself a good wide band O2 controller/sensor and learn to use it. It will tell you the information you need to prevent engine damage. Unless you are a very talented plug reader, you just don't know what the mixture is without a wide band O2 sensor.

It is my opinion the Boost referenced Holley 750DP is better suited as a blower carb than the Edelbrock. But for a NA street cruiser I like the tune-ability of the Edelbrock.

My two cents, hope it helps you. Good luck!
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Old 03-31-2011, 11:34 AM
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Hi Overdriv- I must ask how did you modify the Edelbrocks for boost reference? Do you have instructions, or pictures? By the way all good info. I really appreicate it.

1930u
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Old 03-31-2011, 08:05 PM
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Hey gang- perfect world see info. below I found on another forum. Will this work?

RE: This is how I modified my 2 500 Edelbrock AFB carbs on my blown 354 Hemi.

The modification allows the carbs to get the true vacuum signal under boost. Without boost ref carbs the motor can be making boost and pulling a vacuum on the base of carb (blower sucking) and the metering rods are pulled down and lean out the mixture, when you need the opposite.

Machined up a 5/8" aluminum base plate, cross drill the plate with long 5/32 drill. Drill and tap base for 1/8" NPT thread. JB weld the notch in secondaries so vacuum signal will only pull from blower manifold. get a 4 hole carb gasket and punch new holes to reach power valve ports, ( in middle of throtle holes) Plum with 1/4" tubing. It works very well tested it with 2 vacuum gauges.
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