750 Mech Secondary IAB sizing question

[SP70Nova]

I have a question on the Secondary Idle Air bleed sizing on a Mech Secondary carb. This is more of theory that the actual size of the IAB.

Based on my notes on what I've read over the years, there seems to be different schools of thought on this:
1. match the primary side IAB
2. size similar to a vacuum secondary IAB since the time spent in or transitioning through the transfer slot will be very short.
I understand the need for a smaller IAB on a vacuum sec carb since there is no accel pump shot available for the secondary's.

My questions has to do with the the fuel curve of the secondary's:
Since the accel pump is available and is tied to the throttle movement, except for when the throttle is held in the secondary transfer slot. 1. Would the extra richness/small size of the IAB cause issues/effect during the opening of the secondary's?
2. Could this be enough to mask issues with the accel shot (duration/amount)?
3. When would you match the primary IAB besides when the carb linkage ratio is 1:1?

I had some time on my hands recently and started thinking is the main reason for this. I haven't had time to go play/test to see if I could feel a noticeable difference between the two IAB setups. I'm just looking for thoughts/opinions.



Here's my original topic for more details on the car/use from a couple years ago:
http://racingfuelsystems.com/viewtopic.php?f=5&t=282

Q-750 mech sec w/Downleg Boosters (bought new in 2010)
Setup out of the box (factory)
MABs: 30’s pri & 30’s sec
Main Jets: 74’s & 65 PV - prim and 84’s & no PV – sec
IABs: 72’s pri & 72’s sec
Idle Mixture Screws: 1-1/4 turn out
Floats set to bottom of bowl window

IFRs: 33’s pri & 33’s sec - Located in upper Idle well
Emulsion Stack - both primary & secondary metering blocks
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - open
E3 .028” - plugged
E4 .028” - open


This is what has been adjusted/done to the carb so far (I'm using a 4779-2 750DP as a base line setup):
MABs: 26’s pri & 26’s sec
Main Jets: 70’s & 85 PV - prim and 80’s & no PV – sec
IABs: 74’s pri & 27’s sec
Idle Mixture Screws: 1-1/2 turn out
Floats set to middle of bowl window

I lowered the IFR's and modified the emulsion stack to more of a typical/original Holley emulsion setup:
added TSRs to main body - .081" both primaries & secondary's
IFRs: 31’s pri & 31’s sec
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - plugged
E3 .028” - open
E4 .028” - plugged

HEI Dist w/ MSD 6AL (Analog) & external E-core coil
Timing:
Initial 15
Total 37 all in by 3000
Vacuum Advance - Manifold sourced 10° at Idle - Vac pot advance restricted to 10°.

Thanks,

Scott

[1969Camaro]

Is it a 4 corneridle carb? If so, front and back should be equal on air bleeds

[SP70Nova]

Yes - my Q750 is a 4 corner idle setup - which I did not think about.

How about on a carb with just a 2 corner/primary only idle setup?

[1969Camaro]

2 corner screws use a baseline setting of 1 1/2 turns, 4 corner is 1 turn. IMS and IAB should match front to back

[rgalajda]

4779 carb is a 2 corner idle, front metering block mixture screws only.
2 corner idle baseline mixture screws 1.5 turns. 4 corner idle baseline mixture screws .75 turns.

If you look at the baseplate of most 2 corner idle holley carbs , there is no idle idle fuel outlet drilled. But there is a dribble ( tiny ) outlet drilled below the transfer slot. This tiny outlet was said to enable a small amount of fuel to be used from the secondary bowl in case grandpa never used the secondaries.
The secondary slot was usually shorter than the primary transfer slot.

If you adjust the mixture screws richer ( opened more ) you will also richen the transfer slot . This will richen the AFR even at cruise speed. If you have an AFR gauge try it, and you will see.

You say you changed your secondary IAB from .072 to .027". And set your idle mixture screws to 1.5 turns. Would be interesting to see how this works.
Do you have an AFR gauge?

For this carb , street use , I think I would have started with a #70 primary main jet. .75 turn idle mixture screws, Then add a TFR .064" leaving all the bleeds the same. I'm not saying the old emulsion stack is bad idea along with lower IFR.

[1969Camaro]

Do you Put in a TSR in the rear of a 2 corner idle carb?

[SP70Nova]

With the emulsion stack reworked to a typical/original Holley setup, I was using the 4779-2 as a reference since it and my Q-750 have the same style boosters, venturi & bore sizes, and similar PVCRs. This was basically a guide to help in sizing the main jets and the HSABs along with the primary IABs. The secondary IABs on my Q-750 should've been listed as .74s...

I know what the original configuration ran like and outside of the emulsion stack changes & the lowering of the IFR, the rest of the current carb setup was run last year. Now I'm changing the emulsion stack & IFR position to see what the effects are. Basically an A to B test. The engine and drivetrain hasn't changed so this should be pretty straight forward as how the changes affects things. I've been working on the car since last fall - suspension & interior work. Currently the car isn't road worthy - that's why I have had time to think about this.....

So on a 2 corner/primary only idle setup except for when the throttle is held in the secondary transfer slot, about the only thing a small sized IAB would effect during the opening of the secondary's is it would add to the accel shot as the throttle transitioned through the transfer slot.

[rgalajda]

[SP70Nova]

[rgalajda]