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.

Is your car a stick or FOM? Seems like a lot of cam for a FOM. Pete 

It may be due to the klutz with the screw driver (who is writing this) but I had pure hell with the first Road Demon (not the Jr) I got - until I quit trying to use the four corner idle. With those two back mixture screws snugged - everything seemed to work out fine.

I think the four corner idle was designed more for use on the open plenum style manifolds than the "cross H" systems we have on our street motors.

So I should just screw in the back mixture screws until they bottom out, and then re-tune the front two?

       If you don't have a lean "flat spot" at cruise, or under very slight acceleration, then I'd drop the powervalve (actually INCREASE) it by a half or one inch rating. also check and make sure your air correction jets are open and clean.

And yes I'm suggesting you shut off the secondary idle and just try tuning with the primary screws. It may mean using the center air valve (you mentioned early in your post) in order to maintain your idle transfer position.

       I don't experience that problem on my ranger (408 Windsor, 850 demon) but I did have to richen the secondary idle circuit to smooth out the idle. Not sure this relates to this problem. But I agree, deleting the secondary idle system will certainly simplify the tuning. You might, possibly, need to increase the secondary spring tension to delay the secondary opening when you close off the idle screws. There will be no fuel flow to help smooth out the transition. When you do this, make sure to close the secondary butterflies completely.

     Joe, you will have a power valve located under the base plate. I phrased that wrong.  

 I tend to dismiss this because when the intake sound changes it's difficult to be sure you are using the same throttle setting. Try checking this with a vacuum gage, with and W/O the air cleaner.  

This morning I closed down the secondary idle mix screws while trying the primaries at a bunch of different settings. The result was that the hesitation was still there, while the new settings also created an off-idle bog off varying proportions depending on the new settings.

My next move will be to re-set everything and then go buy a 5.5 power valve and swap out the 6.5 I am running. I'll see where that brings me.

The Demon carbs seem to have a good fuel air curve adjustment range of "rich/lean" based on float setting - on a hunch I raised the primary bowl slightly above the center and it made quite a change.

No, mine's not a Fordo, it's the 3-speed manual. The hesitation doesn't happen exactly when you pull out, it comes around a little bit later in the range under the load of acceleration, more pronounced when you're accelerating up a hill. So a different power valve will open up at different vacuum...vacuum is highest at idle...decreases as you accelerate...so if looks like a lean condition, wouldn't I want to go one number higher so you get fuel delivery sooner?

I did set the mix screws for best vacuum back when I first installed the Demon, but found that to be a little lean. I know it's a rich idle now but I can live with that if everything else gets ironed out.

I have had the same hunch regarding the float setting. Right now it's right in the middle of the glass. These floats are real easy to adjust so it's worth a try to elevate it slightly.

"...use as an example a car equipped with a camshaft that provides such low manifold vacuum at idle or part throttle that the power valve opens, giving richer mixtures, or perhaps flutters open and closed due to vacuum fluctuations. In this instance, a power valve that opens at a still lower vacuum should be installed...this ensures that the power valve will not open until its added fuel is needed."

In the past I've figured that you take your idle vacuum reading, cut it in half, then deduct 2 more inches for lopey cam that might cause variances in the vacuum reading.

If I recall right, the last time I checked the vacuum, it was 19 or thereabouts at a low idle. That was with a Holley carb. But without taking another reading, I don't think there should be much difference if any, right?

No, it occurs before the secondaries kick in.

I'm running a 6.5 power valve now, and am thinking about changing to a 7.5. Or should I try a 5.5?

I was thinking the same thing. Timing is already at 14* BTDC, which should be enough initial ignition for this engine. So after I put everything back together, I went to a stiffer spring inside the distributor, which would "lessen" the timing in the advance curve. As I thought, this made the hesitation worse, and bigger across the rev band. So the issue is at least partially timing-related...which is not news, seeing as just about everything you can mess with is somehow related.

However, with that, I have now also tried all possible spring matchups that Mallory provides (except for single stage curve combinations which they advise to avoid for street use), and have concluded once again that the best combo is the one I had at the beginning of this process...so in essence I'm kinda back to square one.

I think in general what I'm dealing with here is a combination of a somewhat lumpy cam (although I do have good vacuum) coupled with a non-vacuum advance distributor and a pretty big carb which combine to give me some low-end throttle issues.

About the only thing I can think of to try now is the paper clip test that Charley suggests, and then maybe a different power valve. The 6.5 that's on there is pretty standard; given the circumstances surrounding the hesitation, any suggestions as to whether I should go to a 5.5, or a 7.5 ?

When you describe the issue you want to fix you said"

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."

      My understanding is with a carb with vacuum secondaries that the secondaries will NEVER OPEN with gradual acceleration as the manifold vacuum will always stay too high. So can you attach a manifold gage that can be seen from the drivers seat and take it for a spin and confirm that the hesitation occures prior to the point that the manifold vacuum drops enough to cause the secondaries to open. If that is the case then forget the power valve - not involved ( vacuum was too high) and forget the secondaries, same reason

You seem to have two seperate problems, the rich mixture at idle and a hesitation on mild acceleration.

A distributor with no vacuum advance may be your problem. Try a factory distributor and see what you get.

Pete, I have often thought of doing that. But unless I pay big bucks for an original from a '57 Bird or something like that, I'd lose my cable-drive tach.

Originally I had a Mallory from the early 1960s and you're right, it was well-worn and pretty erratic. I got this unit new from Concours in Vegas, paid the big bucks...they're specially fit for early Birds and they have the tach drive built in. It's only got a few thousand miles on it.

Yeah, when I bought this distributor it was advertised as "tuned" for TBirds, but for my purposes it was still way off. So I bought the advance curve kit from Mallory and, over time, tried all different spring combos and degree advance tabs. I ended up using 24* of mechanical advance, adjusting the spring perches as well in order to get all the advance in pretty early in the curve. Sure, I do burn a ton of gas. The car really gets up and goes at higher rpms but my wife drives the car too, and most of our driving ends up being cruising miles, which obviously is not the strong point of this unit. I think I'm now at an impasse...not much else I can do unless I go to another distributor.