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CARB CFM

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BADJAV2 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BADJAV2 Quote  Post ReplyReply Direct Link To This Post Topic: CARB CFM
    Posted: Oct/31/2012 at 7:26am
i have a 1969 amx 390, with 4 speed, i'm going to put the OFFY DUAL QUAD MANIFOLD on i have a COMP CAM 280-H, what CFM carbs works best with this set up.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote tsanchez Quote  Post ReplyReply Direct Link To This Post Posted: Oct/31/2012 at 9:45am
#1803/#1804 edelbrock are 500cfm avs carbs calibrated for dual carbs, it would be your best choice without doing it yourself with other carters or eddys.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote KLW Quote  Post ReplyReply Direct Link To This Post Posted: Oct/31/2012 at 8:52pm
I personally do not like carters (or edelbrocks) so I would opt for a pair of 465 holleys or 600 holleys. On my Fairlane, I have a 331 stroker with dual 450's which I am going to replace with 600's this next summer. I had them on last year, but one or both had a shot power valve and/or needle valve so they were running rich at times. When they were running good, that thing went like a scalded dog!

Anyway, I would go with Holleys - either 465 or 600's. The other nice thing about going holley is that there is help out there if you run into problems. With carters, it's hard to find an expert.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO3904SP Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2012 at 3:14pm

I've tried two 500cfm AVS's with progressive throttle linkage and my car was way too anemic. Think about it you are initially running half or less of a 500cfm until you pass 20 degrees of throttle travel when the dump carb’s primaries finally kick in. That is way too small for a stock 390 motor let alone a motor like yours

To make it work satisfactory you’d have to run them with a non-progressive linkage. The trouble with that is you will foul up your plugs more often because of too much fuel, that’s almost all of a 500cfm carburetor (two sets of primaries) from off idle on.

The best setup for me was to use two 650cfm AVS’s, one with electric choke (rear of manifold as your primary carb) one w/o choke (front of manifold called “dump” carb).

By using a progressive linkage and two 650cfm carbs your car will react completely normal until you reach that 20 degrees of throttle travel and your motor will come alive once your dump carb kicks in. Your secondary’s are fully adjustable so don’t worry about too much carburetion. As a matter of fact I had to loosen my secondary’s to come on sooner for best performance.

It took me three years and $2000.00 worth of carburetors/jets/metering rods and since using this setup I’ve had zero issues, crisp throttle response and smooth transition throughout the entire range.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote KLW Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2012 at 3:52pm
I never run dual carbs with a progressive linkage setup, especially smaller carbs. Right now, I run 2 450's in a 1:1 setup and get good response. I think I mentioned I am going back to the 600's I have once I change the power valves. Again, they will be 1:1.

I just feel that with a progressive setup, some cylinders starve for gas, while other may even be rich. When I checked out the plugs with my dual 450's, they looked perfect. I would leave this setup alone, but it seems like it blubbers a bit over 6000 rpms. When the 600's I had on there ran good, that wasn't there and the engine zoomed right up to 7,000 with no flutter. Also, these are vacuum carbs.
Unless you are all-out drag racing, don't run dual mechanical linkage carbs on the street. Then you will most probably get a big bog.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO3904SP Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2012 at 4:25pm


"I just feel that with a progressive setup, some cylinders starve for gas, while other may even be rich."  
 
 
Is this from experience? And since you are running the Holleys sideways isn't one side of the motor getting more fuel then the other side?
 
 BTY the primaries are close to the center of the manifold so there's no problem with uneven fuel distribution using the progressive setup with two 650 AVS's.
 
 
 
 
 


Edited by BBO3904SP - Nov/05/2012 at 4:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fast401 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2012 at 5:21pm
Two 500 edelbrocks are to much for the 390/401with that cam.  I have two 500's on my 401 and I am using the ISKY 280HL with 10.1 compression.  They have been tuned on the dyno but it is still to much cfm.  I have stuck with it mainly because of the COOL Factor.  I think it would run much better with 400 cfm or less.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2012 at 7:13pm

Whatever is decided for carbs, think I'd avoid the Holley list 9776 450 cfm carbs.  They're mechanical secondary carbs but do NOT have a secondary accelerator pump..... have yet to figure out what Holley was thinking when they made those carbs like that.  Maybe that has something to do with there being no info available on the Holley website about the list 9776 450 cfm carbs, at least none that I can find.....  Yet they are still available brand new from Jegs and Summit for like $260 each.  And I'm pretty sure the original Holley list #4584 carbs called out originaly for the AMC Crossram intakes were the same screwy mechanical seconday with no secondary accelerator pump style of carbs....  Where during the same time period what Holley developed for the Chevy TransAm Camaro 302 CrossRam was a dual feed mechanical secondary List #4295 carb with dual accelerator pumps like all mechanical secondary carbs should be, aka a "double pumper".  


( 2019 Edit / Revision )
New learnings, Holley List 9776 450 CFM Tunnel Ram Carbs
with a Single Primary Squirter and Mechanical Secondaries
compensate with very small Secondary Main Air Bleeds
which Quicken Secondary Booster onset of Delivery
with a trade-off in cruise speed fuel control...



Edited by PHAT69AMX - Jun/30/2019 at 9:00pm
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BBO3904SP View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO3904SP Quote  Post ReplyReply Direct Link To This Post Posted: Nov/06/2012 at 10:09am
Originally posted by fast401 fast401 wrote:

Two 500 edelbrocks are to much for the 390/401with that cam.  I have two 500's on my 401 and I am using the ISKY 280HL with 10.1 compression.  They have been tuned on the dyno but it is still to much cfm.  I have stuck with it mainly because of the COOL Factor.  I think it would run much better with 400 cfm or less.
 
 
You are right two 500cfm performers you are using and as seen on my motor are too big regardless whether using progressive or non-progressive. But two 650 AVS's in progressive mode is a different story; the secondary's a fully adjustable.
 
 
 
 

I used this setup but modified/created rod length in order to accommodate center to center carburetor spacing. With this setup the dump carburetor's primaries kicks in when the "primary" carburetor reaches 20 degrees of travel, see below.

 

- http://www.speedwaymotors.com/search-progressive-linkage.html

 
 


Edited by BBO3904SP - Nov/06/2012 at 10:21am
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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Nov/16/2012 at 10:29pm
A Holley Vacuum Secondary 450 cfm carb that is like the "screwy" Holley List 9776 450 mech sec carb with no secondary accelerator pump is the Holley Truck Avenger 470 cfm carb List 90470.  It comes with an Electric Choke on it and has the same size 1-3/32" venturi diameters and 1-1/2" butterfly diameters as the List 9776 450 cfm carb.  But the Truck Avenger is longer than the 9776 since it has a rear Metering Block instead of a rear Metering Plate, so mounting (2) in-line may not work unless the secondary metering block is removed and replaced with a metering plate, which is doable. 
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