Apologize up front for the long post, but I want to give full background: 56 TBird with 312 Yblock, ported ECZC heads, ECZ 9425 B intake, mild cam (275/.423") dual-point Mallory YH with dwell at 32, stock exhaust, initial ignition timed at 14 BTDC with mechanical advance at 24, all advance in by about 2500 RPM. I burn only 93 octane with 4 oz. of Marvel added to every 10 gals.

A couple years ago I installed a BG Speed Demon 575 cfm carb. The carb is seated properly. The pump shooter is #31 with orange cam set at #1 hole; accelerator pump is set carefully so that it reacts immediately. The idle speed restrictors were originally 31s, I went down to 27s to offset a too-high idle. The primary jets are #60s (down from the #62s it came with) and the secondaries are #72s. Float settings are exactly to the middle of the sight glass. In the past I closed the Idle Eze under the air cleaner stud off completely to simplify tuning variables for myself, and ended up leaving it closed off. I've had the carb out recently to make sure everything was clean, gaskets aligned, etc., and to make sure the primary throttle blades are set so that the exposed transfer slot shows as a little square as suggested. The secondary throttle butterflies are set to the bottom of the transfer slot. A brand new PCV valve is located between the hole on the valley pan to the port on the back of the carb. There is no trace of fuel leaks anywhere. In the past I've checked and re-checked for vacuum leaks and there are none. In line fuel pressure gauge shows steady delivery of gas.

Barry Grant literature  suggests that the four idle mix screws be set between 1 and 2-1/2 turns out. (When screws are bottomed out, engine does quit). The idle mix screws are set at exactly 1 turn out only; yet my exhaust shows sooty black residue and the plugs are black-carbony. Rich, right? Not sure why it's so rich at only 1 turn out...but Demons like it rich. Let's leave that be for right now.

Here's the thing. The engine idles fine at 650-700 rpm and basically runs strong, but under gradual acceleration there is a s-l-i-g-h-t hesitation until the secondaries kick in.

When you slightly choke it manually at normal running temp (giving it more fuel) the hesitation goes away. One might think on the surface that maybe I should go up two thousandths on the primary jets...but they're already at 60, and when I had 62 primaries in the carb there was no appreciable difference.

And here's the other odd thing...if you remove the air cleaner assemby (giving it more air) the hesitation is gone, and the throttle response smoothes out all the way through the spectrum.

So, it seems the hesitation goes away when either more fuel OR more air are introduced...that seems counter-intuitive (at least to me). I'd appreciate any input from you gentlemen.