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By scottlboyd - 6 Years Ago
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Hi all. I have a stock 57 Ford 312 engine bore .060 over when it was rebuilt and have been running a Holley list 1849 with mixed results.
I believe this is a 550 cfm carb. I experience idle issues, poor low end performance which leads me to believe that I might have
too much carb. Before I rebuild it, I am considering a brand new Holley 4160/ 0-8007 which is 390 cfm. Research shows that
list 1273 Holley came stock with the engine and was rated at 410 cfm. Engine is 322 cubic inches bored out and is stock in
every other way (auto trans). The Holley carb selector points to the 390 carb to be the one to use. Any help that you can give
me to make a decision would be most helpful. I don't want to choose a carb that is too small either.
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By charliemccraney - 6 Years Ago
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Is the carburetor known to be in good condition or is the history unknown and it possibly needs a rebuild?
Have you spent time tuning or are you running it in the "out-of-the-box" configuration?
In general, something around 500cfm is a good compromise for a stock engine. 550 is more than it needs but possibly is not too big since it has vacuum secondaries.
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By scottlboyd - 6 Years Ago
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thanks for the reply! The carb I have in there now is of questionable origin-in other words, a friend who had supposedly rebuilt it told me it would work well on my engine. If I keep it, I need to have someone rebuild it properly and tune it properly-neither of which I am capable of doing. That is why I was looking at getting a brand new carb as a good starting place. Holley also makes a list 1848 brand new which is 465 cfm but I believe it has scrubrolet linkage and would have to be adapted for the Ford linkage. My car is almost always on the lower rpm scale-never raced or revved high. Do you think the 390 cfm carb is too small?
thanks for the help
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By NoShortcuts - 6 Years Ago
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scottlboyd. I would not rush to purchase a replacement carburetor. The vacuum secondaries on the Holley carb you have will adjust for the cfm you need for how you drive your 'Bird.
I would not expect to be able to take any new or used carburetor out-of-the-box or off the workbench, install it, and not need to do some sorting out of how it is set-up to assure optimum performance.
I would recommend that you connect with someone who knows Holley carbs and is GooD at pursuing vehicle drive-ability issues.
Hope this helps. 
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By scottlboyd - 6 Years Ago
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Thanks for the reply! I have located such a person near me and will consult with him before I make any rash decisions-good advice.
Scott
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By Talkwrench - 6 Years Ago
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Auto ? Sounds like you just have a crap carb , Y's are pretty forgiving, Ive tried a 600 Holley , 450 Holley and a 1406 600 Eddie [on there now] and Vac secondaries, lots of adjustments made and I have to say there's not much between any of them ...
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By skipLYB - 6 Years Ago
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Pick up a copy of HP books Holley Carburetors, Intake Manifolds, & FI and or Haynes Holley Carburetor Handbook, I have both great info on setting up carbs especially idle, off idle & cruise which sounds like your carb. I've never seen a bad carb but have seen a lot of miss-tuned carbs by folks who don't know how a carb works! I've spent my time in that line! All carbs are an amazing engineering feat, they aren't simple and everything does something to contribute to drivability, power, and economy. The trick is get it all working together!! If you don't understand the details you never will.
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By NoShortcuts - 6 Years Ago
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Scott. Don't be surprised if the technician who works on your car indicates that he needs to do a full check-up of your engine other calibrations or operating condition before putting your carburetor under scrutiny.
While your engine's operating problem may be related to the carburetor you're running, there are many other 'things' that may instead be the root OR otherwise contribute to the problem. Generally, the carburetor is the last 'thing' to adjust after virtually everything else has been verified as being properly calibrated or operating. Ruling these other potential contributors out first, is the usual order before suspecting the carburetor or setting out to 'fix it'.
Your technician may begin with an engine vacuum gauge test done at normal engine operating temperature. Depending upon what that testing shows, the tech. may desire to do a compression leak-down test. He may also do an ignition system operating test with an oscilloscope. This may lead to work related to spark plug and / or spark plug wire replacement. After checking the ignition point dwell setting and the existing ignition timing setting, he may, IF equipped to do so, pull your ignition distributor to check the centrifugal advance curve and to assure proper vacuum advance operation on a distributor machine. The checking of the distributor advance curves can be accomplished with the distributor installed on the vehicle's engine, but a distributor machine is preferred if he still has one. If previously removed, after reinstalling the distributor, he may check the engine valve lash setting. Subsequently, he'll check your ignition timing setting at idle (with the vacuum line to the distributor disconnected and taped closed) and then will check the idle adjustment of the carburetor. After doing all of this other 'stuff', he may have you take him for a road test involving low speed operation, stop-and-go operation, engine operation in going up a long hill, acceleration to attain highway driving speed, performance when doing the equivalent of a vehicle pass at highway speed, and performance at consistent highway speed for a reasonable distance.
Your technician may do some other things, but the above narrative will give you an idea of what was common to do with pre-computer controlled engines that had carburetors, solid lifters, and ignition distributors with ignition points. 
Hope this helps!
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By scottlboyd - 6 Years Ago
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Very good, solid advice and I thank you for it. I will follow your recommendations and get the car into my tech guy and have him narrow the problem down before I go out and spend a lot of bucks on something that really isn't a problem (like I have done before). I appreciate your taking time to help me!
Scott
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By Florida_Phil - 6 Years Ago
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I bought a new Holley 1848-1 465 CFM and couldn't be happier with it. My motor is a .040 over 292 with 1957 G heads, iron intake and distributor. I have no flat spots and it idles smooth as glass. The secondaries open when they are supposed to. Installed it right out of the box with no mods. You'll love it.
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By Ted - 6 Years Ago
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scottlboyd (2/12/2018)
......Holley also makes a list 1848 brand new which is 465 cfm but I believe it has scrubrolet linkage and would have to be adapted for the Ford linkage. My car is almost always on the lower rpm scale-never raced or revved high. Do you think the 390 cfm carb is too small? thanks for the help .
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/
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By PF Arcand - 6 Years Ago
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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..
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By 62bigwindow - 6 Years Ago
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I got a 600 Summit I'd trade for your 500 Ted. It's a little too big for my set up.
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By Ted - 6 Years Ago
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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.
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By 62bigwindow - 6 Years Ago
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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.
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By charliemccraney - 6 Years Ago
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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.
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By 62bigwindow - 6 Years Ago
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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.
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By charliemccraney - 6 Years Ago
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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.
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By 62bigwindow - 6 Years Ago
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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.
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By charliemccraney - 6 Years Ago
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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.
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By 62bigwindow - 6 Years Ago
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Will do Charlie. I never thought to do that. I might be close because that's where my idle mixture screws are. I have the transfer slot where it's supposed to be(no more than .020 exposed) so i know its not that.
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By PF Arcand - 6 Years Ago
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Scott: backin up here, you describe the engine but what vehicle is it in? Are you running a 1957 or later Distributor? If not you need one. And what about possible vacuum leaks? A 57 - B intake can be tricky to get to seal properly due to the raised flanges. Some 4 Bbl gaskets are really to big & verge on not sealing unless alighned perfectly. Do you have a filter in front of the carb? Could there be dirt in it? What about the timing?.etc.
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By scottlboyd - 6 Years Ago
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I want to thank all of you who gave advice and the benefit of your knowledge and your experiences. I especially appreciated the great comments about not forgetting the other issues at hand: i.e., vacuum, timing, valve clearances and other issues. I am happy to report that I did, in fact take the car to a qualified mechanic with 35 years experience with carburetors and ignition systems. Like several of you, he chose a Summit 600 cfm carb to replace the ailing Holley I had. He uncovered valve clearance problems and a few other details I neglected like a fuel filter that was too small and the timing was off a bit. End of story is that the car has never run as good as it does now-great acceleration, smooth idle, good top end speed. The Summit carb seems to be all that he said it would be so I heartily recommend it. Again, many thanks to all of you who put me on the right path. It is greatly appreciated!
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By Loon - 6 Years Ago
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When we got our ’56 T-Bird it came with a 600 CFM Holley. After working through and hand full of carburation issues, the car still had a lean surge at low-speed cruise. Research in Dave Emanuel's Super Tuning and Modifying Holley Carburetors, gave rise to a theory that the engine was not flowing enough to properly “signal” the 600 CFM carburetor. In the absence of equipment to measure the intake flow, I built an Excel model to compute the engine displacement and intake flow required based upon the number of cylinders, bore, stroke, maximum RPM and volumetric efficiency. Just enter the values in the light yellow cells and the CID and CFM are displayed. The attached model has the data for your 312 bore 3.80 plus .060 over, stroke 3.44 at 5000 RPM and 100% volumetric efficiency will flow approximately 466 CFM
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