Hello gentlemen,

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.

Joey, I am not familiar with BG carbs so this will be just a guess at best. What happens if you close the idle screws on the rear two carb bores? Sounds like too much fuel at idle. The main jets do not operate at idle so they can not influence the black carbon on your plugs that you have.

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

I agree with 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.

 
 Steve Metzger       Tucson, Arizona

Yes, it is a stick.

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

Joey,

       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.

Frank/Rebop

Bristol, In ( by Elkhart) 

Do you mean if I do have one?

I think what Frank's getting at is that lumpy cams will pulse power valves and drip a little fuel through them. Sometimes a change to a lower numbered valve (takes lower vacuum to open) will cut out the pulsing.

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.

 
 Steve Metzger       Tucson, Arizona

Steve, Actually I was thinking he might be going lean until the PV opened. Never considered the pulsing or cycling. You might be right there.

       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.  

Frank/Rebop

Bristol, In ( by Elkhart) 

 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.  

Frank/Rebop

Bristol, In ( by Elkhart) 

Thank you for your input, gentlemen.

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.