The Holley list #1848 4V carburetor is the recommended replacement for the 1957 Thunderbirds originally equipped with the Holley List #1273 carbs.  As such, the throttle linkage on the #1848 carb matches the original Holley.  Always keep in mind that the four barrel carburetors of the era were undersized on the various Ford applications.  Case in point are the 1957 E code engines which simply had two teapot carbs in lieu of the 400-410 cfm Ford, Carter, or Holley 4V carbs that were available on the single four barrel equipped 312’s.  With the 1957 312 single four and dual quad engines being the same other than the induction setup, the advertised HP rating was 25 HP higher with dual quads. Recent dyno testing has confirmed this to be the case and not just some marketing hype.
  
The old calculations for carb sizing work fine for stock engines that have little or no port work, small valves, and stock low lift camshafts.  Anytime the engine has been modified to improve the air flow in and out of it, then the carburetion should be examined to see if it will benefit by having it larger than what the old calculations suggest.  I’m currently running the Summit model 4010 500 cfm on my 272 which is basically stock other than having the Isky E4 camshaft and that carb is definitely undersized once the rpms are twisted up.  It’s a good driver though but if doing it again, I would opt for the Summit 600 or 750 sized carb on that same engine.  Keep in mind that vacuum secondary carbs are running on the primary side most of the time and if that primary side is sized smaller than a suitable two barrel carb for the same application, the carb is marginally on the small side.  I’ve installed a number of 750 cfm vacuum secondary carbs on hopped up Y engines and that is the carburetor of choice for both driving around and standing on the throttle.
 
Here’s the link to the article of a 303” Y build with unported heads and utilizing the Summit 750 cfm 4V  carb.
http://www.eatonbalancing.com/2015/05/01/unported-iron-heads-can-still-make-over-a-hp-to-the-cubic-inch/

Lorena, Texas (South of Waco)

Scott: If you are presumably using a ECZ-B manifold & still dealing with the carb situation, before you spend more money on it..take the carb off & make sure you have a proper seal to the intake. Due to the raised design of the intake flange, larger 4 Bbl carb gaskets such as a #55 will barely seal if at all, causing a vacuum leak..   


Paul

I got a 600 Summit I'd trade for your 500 Ted. It's a little too big for my set up.

Durham Missouri

Thanks for the offer but I’m going to decline.  With no plans to run the car in the Big Bend Open Road Race any time soon, I’ll live with the 500 cfm carb for awhile.  It drives around good as it is and unless I really twist the engine rpms up, the lack of cfm is not noticeable.
 
In your case, the 600 cfm carb should be a good choice as it’s difficult to be oversized assuming the vacuum secondaries are working correctly and you are running any kind of upgrades such as camshaft and larger valve heads.  You might consider a stiffer spring in the secondary side to slow down the secondary opening.  The factory installed silver spring does seem to be the best choice in most instances but your combination might want the secondaries to open a bit slower.  If the car is sluggish in normal driving, then I would consider looking at the basic tuneup as you may have something going on that’s not carburetor related.

Lorena, Texas (South of Waco)

The problems more at idle and low speed driving. Has a rich condition at idle and low speed cruising. I have a Holley 450 I'm going to put on it and see if it changes. I've looked at everything I can think of with no improvement.

Durham Missouri

The summit carbs have adjustable air bleeds, which make them far more tuneable than most other carbs at a similar price point.  That should allow it to work with a much more broad range of engines.


Lawrenceville, GA

Tried that Charlie. I put the smallest idle idle feed restrictors in with no help. I've checked everything I could think of. I think the biggest culprit is the cam and the compression ratio.

Durham Missouri

Air bleeds have the opposite effect of jets.  Larger = leaner.  Smaller = richer.  If you were running rich and then went to the smallest air bleed, it will run even richer.


Lawrenceville, GA

I went by the kits instructions for idle feed restrictors. I thought the same thing but it said to put smaller jets in for a rich idle.

Durham Missouri

I do see that they say that.  I think their instructions might be incorrect.

Since you have the tuning kit, try going larger.  One way to tell if you are in the ballpark is that the idle screw will only need to be 1.5 - 2 turns out.  Anything else suggests that the idle jet or restrictors are incorrect.  In this case, all you can change is the restrictors.


Lawrenceville, GA