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

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!

Scott

Redding, California

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. 


Paul

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.

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

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

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

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

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

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