[1930u]

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

[F-BIRD'88]

You have the right carbs for the job but the stock jetting will not be right and the edelbrock recomendations will not be right for the 1407 carbs.

And the metering rods power piston vacuum control source needs to be modified so the rods are controlled by intake manifod vacuum under the blower. Otherwise it will go lean at WOT under boost. Can you say BOOM?

You will need the larger hiflow needles and seats. You will need a fuel system that can flow enough fuel to keep the bowls full at WOT.

You will need different pri and sec jets and metering rods.
You will need to start richer on both and find the tune using a Wide band AFR gauge.

DO NOT attempt to run this until you have done the metering rod/power pistons boost reference modification.

The sec air door opening rate may need to be tweeked (counter weights)

The distributor will need to be recurved to work with the blower. The stock advance curve will not work.


You need to start with rich jetting. Rich is safe. Lean is bad.

The worst you can do with over rich jetting is foul a set of spark plugs.

Start rich and tune leaner using the Wide band AFR gauge.

I suggest .116" pri and .116" sec jets. I suggest .068x .042 pri metering rods.
This is rich and safe . This is your base line. It will need to be tuned from this point.

Start with the distributor advance locked out at 28deg BTDC fixed.
Blower motors like lots of initial idle timing but not much timing @WOT under boost. A normal distributor timing curve will not work well.

What are the cam specs or part nuimber?

[1930u]

This all makes sense...how do I modify the power piston for lower manifold vacuum source? Thank you!

[cobalt327]

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:

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

[F-BIRD'88]

Originally Posted by 1930u

This all makes sense...how do I modify the power piston for lower manifold vacuum source? Thank you!

Good question. There are two power pistons in each carb. with two vacuum passages going to the base of the carb. You can look on the base of the carb and find them.

You can epoxy up the base at the vacuum spot and create a new path over to the base flange to connecting to the ported vac fitting and block that off.
The ported vac spot on the base becomes the spot for the power piston vac reference.
Or you can make your own exits with pressed in tubes on the carb flange edge.

2nd method:

Or you can get a 4 hole carb spacer and drill a spot for a (pressed in) pass thu tube to extend up into the power piston vac passage in the base of the carb and then drill a intersecting passages over to the side of the spacer for a vaccuum tube for external vac source. The carb spacer becomes a extension of the carb base and the vac passage passes thru into the adapter. A press fit tube connects the carb base to the adapter passage.

As long as the power pistons are controlled off the intake manifold vacuum below the blower,,, it doesn;t really matter how you do that..

I doubt a external passage in the side of the carb body can ge drilled to intersect with the power piston vac passages as is done on a holley.
Have a look. I don;t have a edelbrock carb here on hand to look.

You can get small inline one way air check valves to put inline on the vacuum lines to stop the power piston and transmission modulator from actually seeing blower boost @WOT.

[1930u]

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????

[F-BIRD'88]

I also like to upgrade these carbs for dual fuel inlets on each side of the carb.
for hi power applications. its a straight forward drill the hole and tap for the new fuel inlet fitting. The newer thunder series AVS carbs come like this now.

The sec air door may need weight added or less weight on the counter weights. Nice to start with two spares and dial it in.

If you end up right back at or near the stock #1407 default jetting, after tuning it so be it , but starting rich and tuning down leaner, is safe.
The wideband and a few plug checks will show you the way.

If/when you increase the blower drive ratio for more boost you may have to fatten up the jetting a bit more.

When you jump to the larger .110" needle and seats you will have to play with the float height settings and fuel pressure to find the happy spot. Start with default float height and drop specs and 5.5PSi fuel pressure at idle.

You want around 12.5 to 14:1 at idle (best idle vacuum) Mid 14.5:1 at part throttle easy cruise no boost. And a 10.5 to 11.5:1 afr under boost at WOT. WOT Timing under boost 26 to 32deg on pump gas.

electronic Boost spark retard that takes timing out as boost rises, allows a higher base timing setting.

[F-BIRD'88]

How in the heck can Edelbrock say they will work out of the box in a blower appilcation????

You have to take these things with a grain of salt.

On the little 144 blower that don;t make much boost or pull that hard on the carb @WOT, suckingthe power piston down lean @ WOT is usualy not a problem with the stiff 10' vac springs. and low boost drive ratio.


Your blower will pull very hard on the carbs @ WOT, you must do the power piston signal source mod or remove the rods and rejet. (this makes for a on-off always fat race only blower carb tuneup that does not cruise well and eats spark plugs.)

Trust me it is well worth the bother to boost reference the metering rods/power pistons so the motor will idle and cruise clean and go power rich and stay rich at WOT under boost.

[1930u]

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.

[1930u]

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!

[cobalt327]

Originally Posted by 1930u

His only recommendation was to up grade to the high flow needle and seat.

That pretty well sums it up.

[F-BIRD'88]

Originally Posted by 1930u

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!

He is ignorant as to how a supercharger effects the carbs power piston at WOT. When the vacuum created by the blower pulling on the carbs @WOT equals or exceeds the springs pressure holding the metering rods up ( power position), it will pull the rods down in the jet and it will go lean and damage your engine. The edelbrock carb needs this modification for the same reason a holley does.

The only way it will run safe is very low boost/limited added power (therefor the blower will never pull that hard on the carbs @WOT) or disable the power piston altogether ( remove the primary metering rods and jet to correct) . The carb will always be in fat WOT power rich mode at PT cruise when set up like this and continuously foul the plugs on the street.
Ok for 1/4 mile blasts, but not good for the street.

You got bad advice. Think about how the power piston works. And why its there.

Take any carb and disable the power piston/power valve (block the vaccuum source so its always in power mode) and drive it around for while and see how long your plugs last until the motor won;t run any more.

Take any carb on any motor and while running at WOT, suddenly force the power valve closed or power piston/metering rods down (cruise mode) and watch what happens to the motor. Can you say boom? On a supercharged motor it will only take seconds to burn the motor down. Thats why you boost referenced the powervalve/power piston on a supercharged motor with a carb.

[F-BIRD'88]

If you are going to follow their bad advice KEEP THE BOOST LOW!!!

Phone BDS or Dyers or Oddy's and find out the truth.

[1930u]

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!

[F-BIRD'88]

Take note of the price in the picture For $1200, I tell you anything you want to hear too.

"its all in the jetting and tuning" Fine as long as you never run serious blower boost or power.. Other wise they "retune it" by yanking out the metering rods. And call it a full race tune up. Cause its WOT rich full time.

I know better. But they well 1000's of carbs for $1200/pr ... They must be right.

When ever, if ever you run it with serious boost and melt the motor, you will know what to do. $SS after you rebuild the motor $$$