emulsion tuning for dual quads? experts needed

[TheGreatPumpkin]

Hi all - I'm a novice carb tuner with an engineering background. I try to read (and understand) as much as possible of what's talked about with regards to emulsion tuning. I'm here to learn stuff and get my car sorted out, and I need some help. Some others on this board have already been very helpful to me (FBIRD88 has been great!), and I think I'm close to a good tune. However, I'm currently having "rich everywhere" issues after relocating the IFRs to the low position and experimenting with emulsion tuning on this mild but fun combo. I have a wideband AFR gauge and vacuum gauge in the car.

1970 Torino, 3900lbs, Toploader, 3.50:1 rear gear
1973 466, 9:1, mild/medium ported D3VE heads
Lunati Voodoo 268/276 hyd flat tappet (227/233@50, .552/.564) (yes, I know this is too much cam for the compression ratio)
Blue Thunder CJ dual quad intake with two 600 cfm Holleys (1850C), which have QF metering blocks and plates. Setup 1:1 with each other with homemade (but I think very nice) adjustable linkage. I am open to a progressive and/or backwards setup but I'm afraid of fuel distribution issues.
FPA headers, 3 inch exhaust with X-pipe
Everything else is pretty basic but the car has coilovers, Cal-Tracs, panhard bar, handles pretty nice, power steer, manual brake, brand new wiring harness, etc. Fun to drive regardless of the tuning troubles.

Here's the tune on the carbs, both are identical:
Idles 800rpm with about 16inhg, IMS ~3/4 out
TB P: 0.016 of slot
TB S: minimum without binding
IFR P: 0.021
IFR S: 0.025
IAB P: 0.068
IAB S: 0.018
Jet P: #65
Jet S: #70
PVCR: 0.035
MAB P: 0.024
MAB S: 0.023
Emulsion: 31/25/25/25
PV: 10.5
Nozzle: 0.025
Cam: Orange #1
Spring: Purple
Total Timing: 36 by 2800rpm (18 initial, 18 mechanical, 15 vacuum at 15inhg)
Bushing: Black
Springs: Blue/Blue

Here's the issue: Before relocating the IFRs, downsizing to them to 0.021, and going to the "old school" emulsion package, I was running 0.0225 IFRs and the QF emulsion package, which was 31/xx/31/31 (I think). All else was the same. With that tune, tip in was lean, low speed cruise was too rich, then got too lean before the mains started, WOT was great, secondary opening was a bit lean but ok. IAB changes were not all that helpful.

Now with the 0.021 relocated IFRs, and the 28/xx/28/xx emulsion, the AFR curve was much more stable and flatter, but very rich (.80-.82 Lambda) everywhere, with a big lean blip at 4000rpm, which I thought was weird. It got worse with a 31/25/25/25 emulsion, down to 0.78 Lambda at low speed cruise, only getting up to .88 at 2500 rpm or so. I've heard this can happen when too much emulsion area is used - the larger orifices just make it easier for lots of fuel to flow instead of actually emulsifying the fuel properly.

My first thought was to try a 25/xx/25/xx emulsion package, but it occurred to me that I'm not an expert and I may have missed something else in the tune, OR I'm making a mistake and misunderstanding what the carbs want, since they are each experiencing less pressure drop than one carb would if I was just running a single carb. Does anything seem off with my tune? What should I do next?

I even thought the threads around the relocated IFRs (McMaster 6-32 1/8 set screws) were leaking and causing the richness, but they were tapped straight and are tight, so I don't think that's it. I'm not sure where to go next. Thanks in advance.

[prairiehotrodder]

I'm not an expert but i've been playing with a 2 x 4 setup for a few years. Were your carbs designed and built specifically for a 2 x 4 application ? Mine were not and they initially were way rich at idle and cruise speeds. From reading your post i wouldn't want to make any suggestions because i wouldn't have a clue what to say. I like to consider the " out of the box" tune as a somewhat trustworthy baseline, and when you get to far away from that it becomes difficult to know what to do when testing a carb on an engine. On my carbs (750 holley track warriors) all i did to fix them was installed smaller idle jets. I went from .035 down to .029 if i remember correctly. I bought blanks and drilled them myself. The one thing that jumps out at me from your numbers is the 10.5 PV. That seems way out in left field but once again i am not an expert.
Good luck with what you are doing.
Brian

[TheGreatPumpkin]

I'm not an expert but i've been playing with a 2 x 4 setup for a few years. Were your carbs designed and built specifically for a 2 x 4 application ? Mine were not and they initially were way rich at idle and cruise speeds. From reading your post i wouldn't want to make any suggestions because i wouldn't have a clue what to say. I like to consider the " out of the box" tune as a somewhat trustworthy baseline, and when you get to far away from that it becomes difficult to know what to do when testing a carb on an engine. On my carbs (750 holley track warriors) all i did to fix them was installed smaller idle jets. I went from .035 down to .029 if i remember correctly. I bought blanks and drilled them myself. The one thing that jumps out at me from your numbers is the 10.5 PV. That seems way out in left field but once again i am not an expert.
Good luck with what you are doing.
Brian

Thanks for the reply. I made some mistakes early on, like insisting i needed the billet blocks (i don't, i just like them and they are easier to work with), but a lot of what I learned was from experimentation. It's a low compression, biggish cam combo so the low speed afr has always been hard to get right. The 10.5 pv is because this thing makes a lot of vacuum at cruise, and it brings power in with good timing on the highway.

I suppose my main question is, do i continue to mess with the emulsion, or leave it at 28/xx/28/xx and go to even smaller IFRs? Should I calculate each fuel jet area as being 50% of what it would be on a single 850cfm carb? And, are my throttle blade openings ok? Is that messing with my transition or is that just because i haven't gotten the IABs right yet?

It's my first dual quad deal so it's tough lol

[rgalajda]

So your at 28/xx/28/xx.
Your gonna need a rich idle with that cam, probably 12.5-? AFR / .85 -? lambda
This means your off idle low speed driving is also gonna be rich, probably up to 2,000 rpm.
If you can get idle quality with a smaller IFR , this will help low speed AFR.
Your chart shows the IMS ( idle mixture screw ) 3/4 turn out. You know this is usually about 1 1/2 turns out ( for a single carb 2 corner idle ) when the IFR is sized correctly. At 3/4 turn out ( for best idle/vacuum ) this would usually mean the IFR is too large.

Since you have two carbs contributing to idle fuel, it only makes sense the IFR will be smaller.
You need to get the idle fuel sorted out before moving on to the next circuit.

[rgalajda]

Leave everything else alone .

When the idle fuel is sorted out, if the steady cruise AFR is rich ( btw 2000-3000 rpm ) , then start lowering the primary jet ( smaller ) . Your steady cruise AFR should be 14.5 /.99 lambda or leaner.

Steady cruise: level road , steady rpm, no throttle movement.

[F-BIRD'88]

The emmulsion set up of the main system has nothing to do with the idle circuit.
Leave the emulsion set up stock for the 1850 carb and work on the idle circuit and off idle transition.

You are jumping around.
If moving the IFR on the metering block caused it to now be rich rich. put it back.
Put it back to stock..
That cam is not too big but low cr BBF engines do like the cam advanced... Later mod)( a very minor fine tuning mod)

Progressive dual quad carb linkage does not cause uneven fuel distribution.. It can actually improve it.

Carb emmulsion fine tuning will require dyno testing with all 8 O2 sensors + 8 exhaust temp sensors.
Full dyno cell sensors and data collection.
You won't improve the calibration of the imellsions setup by seat of the pants.

[F-BIRD'88]

What is the stock default emullsion set up on the origional 1850 carb metering block?

Use that.
Is this your intake manifold?
http://www.survivalmotorsports.com/imag ... _small.jpg

[TheGreatPumpkin]

What is the stock default emullsion set up on the origional 1850 carb metering block?

Use that.
Is this your intake manifold?
http://www.survivalmotorsports.com/imag ... _small.jpg

Thanks guys. Will try to stay focused.

Emulsion will go back to stock 1850 (28/xx/28/xx)
Reduce IFR to 0.020
Open primary blades back to 0.020 slot to get more air and even out the transition?
Leave everything else alone until cruise is established

[F-BIRD'88]

This engine with this cam needs a different ignition timing curve.. I suggest a 10 deg mechanical advance curve.
This allows 26 deg base initial idle timing
and 36 deg @Max advance.
This increased idle base timing really improves the idle quality and off idle response..
It works with a more stable slightly slower advance curve RATE. thus slightly more advance spring tension and the centrifical advance will no longer "hang up"
26 + 10 mech adv = 36 max mech advance. say around 3000-3400 rpm.

18 initial is not really enough base initial idle spark timing.

MSD never gives you the right advance limit bushings.
But this company does have the 10 deg bushings for the MSD diz.
https://4secondsflat.com/MSD%20Distributor%20Tuning.htm
If you're crafty you can make your own.
Or epoxy up the hole and redrill it to create a 10 degree travel limit bushing from one of yours.
Or grind a flat on the side of a bushing as required.

[TheGreatPumpkin]

This engine with this cam needs a different ignition timing curve.. I suggest a 10 deg mechanical advance curve.
This allows 26 deg base initial idle timing
and 36 deg @Max advance.
This increased idle base timing really improves the idle quality and off idle response..
It works with a more stable slightly slower advance curve RATE. thus slightly more advance spring tension and the centrifical advance will no longer "hang up"
26 + 10 mech adv = 36 max mech advance. say around 3000-3400 rpm.

18 initial is not really enough base initial idle spark timing.

MSD never gives you the right advance limit bushings.
But this company does have the 10 deg bushings for the MSD diz.
https://4secondsflat.com/MSD%20Distributor%20Tuning.htm
If you're crafty you can make your own.
Or epoxy up the hole and redrill it to create a 10 degree travel limit bushing from one of yours.
Or grind a flat on the side of a bushing as required.

I'll consider this but remember I'm running full manifold vac advance so that would give me a ton of initial timing, like 40 degrees. I actually already have those FBO bushings somewhere, just have to find them

[F-BIRD'88]

This is independent of vacuum advance..
Yes use vaccuum advance BUT yiu do not want it all in at idle... You can use ported or full manifold vac advance but if its all in at idle creating 40 deg at idle its wrong and part of your problem

Creates incorrect csrb primsry throttle blade position at idle. especially with 2x4 carbs..
change to this 10 degress advance curve and then use 26 base initial and 36 total mech advance..
Then try ported vac advance..
You will likely then need to adjust the RATE AND THE LIMIT of vacuum advance to get it right..
All in at idle is not correct..

You don't want a ton of advance at idle. You want the correct amount with a stable curve.

[TheGreatPumpkin]

This is independent of vacuum advance..
Yes use vaccuum advance BUT yiu do not want it all in at idle... You can use ported or full manifold vac advance but if its all in at idle creating 40 deg at idle its wrong and part of your problem

Creates incorrect csrb primsry throttle blade position at idle. especially with 2x4 carbs..
change to this 10 degress advance curve and then use 26 base initial and 36 total mech advance..
Then try ported vac advance..
You will likely then need to adjust the RATE AND THE LIMIT of vacuum advance to get it right..
All in at idle is not correct..

You don't want a ton of advance at idle. You want the correct amount with a stable curve.

Currently total at idle is around 33
But yes if it makes it so that I don't have to close the throttle blades down below 0.020 of slot to get it to idle under 1000rpm then I'll try it

[F-BIRD'88]

All carb setup is done with vacuum advance disconnected. .. for the set up.
I wondered why you carbs throttle positions are all messed up..

[TheGreatPumpkin]

All carb setup is done with vacuum advance disconnected. .. for the set up.
I wondered why you carbs throttle positions are all messed up..

Ok cool I'll try that

Hey thanks so much for all the attention and help!

[F-BIRD'88]

Part of that problem is that Voodoo hyd cam is a Baby cam for the dual quad engines.
The cam for the old Shelby dual quad engines was quite a bit bigger duration@.050". Solid lifter. about 244° @.050"
These deep breathin dual quad engines make more power but only if you let it eat... Cam choice and carb cfm choice. A 244° cam in these is just getting warmed up. The big cam likes the ton of idle timing and matches the idle air flow of 8 throttle plates at idle.

Do not be afraid to bias the throttle settings on the front/ back (pri) carbs. for effect.

The throttles definatily must be in its sweet spot at idle..
Too far closed is just as bad as too far open at idle.

try 900 rpm. VS 800 rpm idle speed too.

The diz (with a 10 deg adv curve should stay on the base initial (26 deg at idle) to about 1800 rpm off idle. Then start progressing the mech advance up as rpm rises.

Also remember with ported advance only the base idle throttle position is "ported" controlled. As soon as you step on yhe gas to drive it even a little bit. the vac advance becomes active. just like full manifold vac does.
From that point its the same same.
Most cammed cars work best on ported vac advance and the 10 degree mech advance curve that is more dynamicly stable and smooth.