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Edelbrock FM255 vs. FM257

Posted By Florida_Phil 5 Years Ago
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Ted
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Florida_Phil (11/18/2019)
 
I have some questions for Ted. 

1. In the photos you posted above of the FM255 with two Holley 465s, you are not running progressive linkage. Is connecting both carb throttles directly OK for a street car with a manual transmission?

For the Ford Y, fuel distribution is better with both carbs being synchronized together due to the manifold design.  You can have the linkage progressive but if doing so, then you’ll need the carburetor with the primaries being closest to the center of the engine being the primary carburetor.  If the carbs are mounted forward, then it will be the rear carb being primary and if the carbs are mounted facing the rear, it will be the front carburetor being the primary carb.  In either case, you’ll need progressive carb linkage where the primary carb works all the time and the secondary carb comes in later.  I trust that made sense.  If not, then look at how the carbs and linkage are hooked up on a factory 427 Ford dual quad setup.

2. It looks like you removed the chokes on both carbs.  In Florida, I do not need a choke to start my engine.  Is a choke required with dual quads?

Chokes are required to enrichen the fuel mixture when cold starting.  Gasoline by nature only burns on the surface of the droplets and when cold those droplets are too large to provide sufficient burn area for starting.  This is where the choke comes into play as more fuel is simply ‘dumped’ in the engine to provide more surface area.  Also working in conjunction with the chokes is the fast idle linkage to keep the engine at speed.  Once the engine warms up, the fuel vaporizes into smaller droplets and the extra fuel is no longer required.
 
In answer to your question, chokes are not necessarily required in warm climates.  If you are good at feathering the throttle when first starting the engine, then the engine speed can be manually worked that first couple of minutes of running until the engine can idle on its own.  The 427 Ford dual quad engines had a choke only on the front carburetor (facing backwards).

3. I see you did not connect the vacuum units like the carbs on a 425/427.   Is this OK?

The carbs in that picture I posted are mechanical secondary 450 cfm units and as such, they have no vacuum secondary pots to connect together.  Those particular carbs were left over from a Bonneville Flathead engine that was running dual quads on a Hogans sheet metal intake.  The vacuum secondary 390 carbs ended up working better on that 296” Flattie and so the 450 cfm carbs ended up on the pictured Y engine.  If running the vacuum seconardary carbs, then I do recommend running a shared line between the vacuum secondary diaphragms so that the carb signals are equalized.  The 390 cfm Holley vacuum secondary carbs do work well on dual quad setups and I would recommend those over the 450 mechanical secondary carbs.

4. Should I rejet the carbs or run them the way they are?

Always use the factory supplied jetting as a starting point.


Lorena, Texas (South of Waco)


Ted
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Here are pics of the Flathead V8 with the 390cfm Holley carbs.
http://forums.y-blocksforever.com/uploads/images/baf44c1b-500c-4748-acc1-cd0c.jpg 

http://forums.y-blocksforever.com/uploads/images/ff2efcea-8ba0-48c2-a6ab-fead.jpg 



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Florida_Phil
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Thanks for the reply.   Can I successfully run both carbs on the street without progressive linkage?


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NoShortcuts
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Phil.  Having both carburetors' primary throttle plates open together simultaneously is how Ford designed their dual quad carburetion system to operate on the Ford y-block '56 Power Pack set-up and the '57 E code set-up used on passenger cars and 'Birds.  The '56 and '57 Ford set-ups used similar (but not identical) Holley 4000 carbs with a balance tube utilized to equalize the vacuum signal to the two separate vacuum chambers that regulated the opening of the carburetor secondary throttle plates of each carb.  The balance tube served to insure that the secondaries throttle plates controlled by their individual vacuum chambers, opened uniformly together,  Ford used chokes on the primary side of both carburetors because they worked in tandem for engine start-up, idle, and the operation of their low and high speed carburetor circuits.

Hope this helps.   Smile   


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Florida_Phil
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Yes is does.  Thanks for the reply.  I am waiting for my FM255.  It was shipped yesterday. I may have other questions as I go along.


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Ted
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Florida_Phil (11/18/2019)
Thanks for the reply.   Can I successfully run both carbs on the street without progressive linkage?

That’s a ‘Yes’.  Here’s a picture of the factory linkage for the dual quad setup.

http://forums.y-blocksforever.com/uploads/images/dba6e5d5-0b4b-41b3-8e69-a701.jpg 



Lorena, Texas (South of Waco)


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Will my stock '57 vacuum advance distributor clear the rear Holley carb?  If not, what are my options?


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Oldmics
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Teds linkage picture is for a passenger car.

T Birds have additional linkage because the carburators are mounted backwards.Once you get your setup together be mindful of hood clearance since I believe this setup is going on a T Bird.

As far as distributor clearance - not sure if it will clear the carbs. You may have to do a bit of grinding on the rear carb mounting pad.

Its a trial and error fit.

Good luck !

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Joe-JDC
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Page 2 of this post shows you pictures to answer your question about distributor fitment.  Joe-JDC

JDC
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The bowl of a 57 tach drive strib sits up a little higher in the saddle. That may help or hinder the project. Keep in mind I have a dual advance cast iron tach drive strib which sits lower than the passenger car strib. I'm presently using it in my 64 292 so it's something that could be explored if necessary. If requested, I would be glad to supply dimensions and pics. This is assuming this project is a Thunderbird still using a mechanical tach.


64F100 57FAIRLANE500


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