Dual Quad tuning tips?

[F-BIRD'88]

If you are now only getting a "blip of lean" as you add throttle, but it does stablize, it may just need a squirter shot tweek. (linkage slack)
Might also need to fine tune the pri/ sec idle throttle position bias a bit . Close both secondaries a hair each even if that reduces the curb idle speed a hair.

[TheGreatPumpkin]

ya .052" IAB was a big big jump.
I would have tried .062" (1/32")....

Whats happening with the pcv valves flow when throttle is off idle in that pesky transition zone?
Might need a pcv air flow metering tweek.
What happens when if you tweek the pcv flow +/- with your finger on the end? Does doing that effect the AFR?

You will only get it so good without plenum heat to aid fuel vapourization.. A cold intake is always more pesky than you'd like.
As soon as you add a bit of throttle rpm the heat in the intake gets absorbed, without having a active heat replacement source... This also makes the afr seem to read lean(er). a bit.

not sure about the PCV. Should I delete it for now and just run a breather?

[F-BIRD'88]

ya .052" IAB was a big big jump.
I would have tried .062" (1/32")....

Whats happening with the pcv valves flow when throttle is off idle in that pesky transition zone?
Might need a pcv air flow metering tweek.
What happens when if you tweek the pcv flow +/- with your finger on the end? Does doing that effect the AFR?

You will only get it so good without plenum heat to aid fuel vapourization.. A cold intake is always more pesky than you'd like.
As soon as you add a bit of throttle rpm the heat in the intake gets absorbed, without having a active heat replacement source... This also makes the afr seem to read lean(er). a bit.

not sure about the PCV. Should I delete it for now and just run a breather?

No but you can manually manipulate its airfliw with a finger over the end a bit and note that effect when in that pesky throttle transition zone.
Its essentually a lil pesky air leak..... but you need it.
Make it work for you by fine tuning it +/-.

[F-BIRD'88]

Also try just giving the idle mixture screws a hair more fuel mix adjustment.. How does that then effect the transition zone.. Give it what it needs to run , not what seems right , on them idle mix screws.. Its a cold intake manifold. It will always be a bit fuel hungry to be happy.

[TheGreatPumpkin]

ya .052" IAB was a big big jump.
I would have tried .062" (1/32")....

Whats happening with the pcv valves flow when throttle is off idle in that pesky transition zone?
Might need a pcv air flow metering tweek.
What happens when if you tweek the pcv flow +/- with your finger on the end? Does doing that effect the AFR?

You will only get it so good without plenum heat to aid fuel vapourization.. A cold intake is always more pesky than you'd like.
As soon as you add a bit of throttle rpm the heat in the intake gets absorbed, without having a active heat replacement source... This also makes the afr seem to read lean(er). a bit.

not sure about the PCV. Should I delete it for now and just run a breather?

No but you can manually manipulate its airfliw with a finger over the end a bit and note that effect when in that pesky throttle transition zone.
Its essentually a lil pesky air leak..... but you need it.
Make it work for you by fine tuning it +/-.

I'll check it out. It's not an adjustable one

[TheGreatPumpkin]

If you are now only getting a "blip of lean" as you add throttle, but it does stablize, it may just need a squirter shot tweek. (linkage slack)
Might also need to fine tune the pri/ sec idle throttle position bias a bit . Close both secondaries a hair each even if that reduces the curb idle speed a hair.

Lemme ask you this, right now i have my primary blades at 0.020" of slot and and secondary blades near the minimum amount of opening, idles at 800. I noticed that at idle of 700, it's much quieter, sounds more like an idle than a droning "whomp-whomp-whomp" like it is at 800. Should I start to adjust both blades down to try to get a lower idle? It might help richen up the part throttle cruise maybe? At 800 the idle just seems too loud, like it's just "off idle." And if my low rpm part throttle cruise is lean, does that mean I'm too far into the transfer slot at cruise and also getting that exhaust reversion lean condition?

[PackardV8]

You will only get it so good without plenum heat to aid fuel vapourization.. A cold intake is always more pesky than you'd like.
As soon as you add a bit of throttle rpm the heat in the intake gets absorbed, without having a active heat replacement source... This also makes the afr seem to read lean(er). a bit.

Agree, but then that same intake heat on a 100-degree day can push the combination into pinging when a cold intake will make the same pull without a ping. Tuning carbs is always tradeoffs unnecessary with EFI.

[TheGreatPumpkin]

You will only get it so good without plenum heat to aid fuel vapourization.. A cold intake is always more pesky than you'd like.
As soon as you add a bit of throttle rpm the heat in the intake gets absorbed, without having a active heat replacement source... This also makes the afr seem to read lean(er). a bit.

Agree, but then that same intake heat on a 100-degree day can push the combination into pinging when a cold intake will make the same pull without a ping. Tuning carbs is always tradeoffs unnecessary with EFI.

Well sure but I'm just trying to optimize the carbs and I'm clearly not done yet. EFI is not what we're talking about here

And note/reminder to all in the convo, I'm running a lot of timing so i can probably get away with being a bit lean in part throttle cruise. 18 initial, 15 full manifold vac, 35-36 total at 2500, low compression, bit too much cam, 91 pump. Has never pinged

[TheGreatPumpkin]

If you are now only getting a "blip of lean" as you add throttle, but it does stablize, it may just need a squirter shot tweek. (linkage slack)
Might also need to fine tune the pri/ sec idle throttle position bias a bit . Close both secondaries a hair each even if that reduces the curb idle speed a hair.

I think I missed this post of yours. I'm trying to figure out is my issue is a lean transition, lean pump shot, or lean secondary opening. it IS just a blip of lean and then goes right back to whatever AFR it was right before. This is accelerating mildly from 1000 to 2000rpm.

I'm sure I'll figure it out eventually, I just don't want to be chasing my tail with the IABs when it's something else. Currently I MUST keep the primary IFRs at 0.0225, because the next step up (5.5mm, or 0.0236) makes the low speed cruise unbearably rich, like 11.9 - 12.1 AFR. At 0.0225, the low speed cruise is around 15 AFR. Very weird. My next move would be drop all the IABs a few steps to both richen up the secondary opening AND primary response and see if that cures the leanness all around. I might be encountering the transition spot AND the secondary opening at the same time/rpm depending on how I drive. I'm at 0.028 squirters because 0.031 really provides too much immediate fuel at all rpms

[TheGreatPumpkin]

Does 0.0225 seem too small for an IFR? Essentially my choices are small IFR and small IAB or bigger IFR and much bigger IAB. How do I know if my IFRs are too small? I need at least one constant in this equation

[F-BIRD'88]

Do not get all hung up on momentary afr guage readings any time.. If it passes thru it and it drives well ignor the guage... Make it run well..


I still believe that overall using 1 to 1 front back carb throttle linkage is not going to work as well as progressive carb linkage where most all the low speed driving is done on the rear carb primaries.
(Front carb if the carbs are on the manifold backwards (Ford dual quad,).

If the idle air bleeds are just too small....
A : the AFR will shift too richer as yiu open the throttle from idle...
B: The idle circuit fuel emullsion MiX quality may suffer.

an emmulsion of fuel air MiX is better than raw liquid fuel...

Avoid getting all reactive on minor and momentary shufts in the AFR.. Its a carbed engine . you will not get it perfect..
Intake manifold plenum heat. ALWAYS works better for idle low speed DRIVING.. Thus is why EVERY OEM uses it on all carbed engines. Without the heat the AFR QUALITY will always be a compronise at idle low speed.
Give it what it wants to DRIVE BEST.

All driving evaluation of the tune must be done with the air cleaner INSTALLED. ya ir matters.

[F-BIRD'88]

The IFR is not too small.. Calc the AREA of the TWO carb's IFR. VS the AREA of the IFR of say a single 4 bbl carb.

The engine wants the SAME AMOUNT OF FUEL AT IDLE.
It is shared between the TWO carbs VS a single 4 Bbl carb at idle.
i belice it will ultimatly run better DRIVE BETTER if the throttle linkage was Progressive... The idle setup will be a bit different..
No rule says that the front back carb's idle circuit fuel or air metering has to be the same on both carbs on a dual quad setup.

No rule says the main jetting and or PVCR's have to match RIGHT to LEFT or same same on the front/ Back carbs. Make it run.. Give it WHAT IT WANTS.

Dual plane V8 intake manifolds are notorious for likeing staggered carb jetting. The OEM's even play with the SHAPE of the boosters R/L to direct fuel here and there in the manifold for effect.
Look at the origional Ford Shelby Cobra 427 FE Dual Quad 2X4 holley carbs for their unique setup and fuel metering tuning.

Giving away secrets now...

[TheGreatPumpkin]

The IFR is not too small.. Calc the AREA of the TWO carb's IFR. VS the AREA of the IFR of say a single 4 bbl carb.

The engine wants the SAME AMOUNT OF FUEL AT IDLE.
It is shared between the TWO carbs VS a single 4 Bbl carb at idle.
i belice it will ultimatly run better DRIVE BETTER if the throttle linkage was Progressive... The idle setup will be a bit different..
No rule says that the front back carb's idle circuit fuel or air mettering has to be the same on both carbs on a dual quad setup. Make it run.. Give it WHAT IT WANTS.

Giving away secrets now...

Oh don't worry, that's exactly what I've been calculating. I'm getting into some very small adjustments, to be honest. The car does drive pretty well

Not going for perfect, but not done yet. The intake actually has provision for heat, but the intake gaskets block it off. I'm not sure that would help here in socal when it's 105 degrees outside

Anyway, for now I'll try the IFR one step richer (0.0236) and the IABs much larger, like 74. That's 2 steps over factory. Your comment about the emulsion quality and off-idle richness is spot on with my situation right now, even when the IFR is 0.0225. Thanks for the help!

[F-BIRD'88]

At idle and low speed DRIVING the engine wants more than 105°F intake manifold heat.
When ut is running Say 25 mph on the transition. it is ALWAYS absorbing intake manifold HEAT as you drive.
Without an active method of replacing that plenum HEAT the manifold cools off rapidly and fuel VAPOURIZATION quality suffers.. This is WHY all oems use active intake manifold PLENUM HEAT on All the carbed engines.
Yes the Amount of heat needs to be dialed in.
Yes in the Winter it will need more heat than in the hot summer... This is WHY all the OEM's employed intake air
(air cleaner) temp control and exhaust heat riser valve control.
Liquid fuel. Does not burn.
Liquid fuel Wears out your engine FAST.

If you are looking for great DRIVABILITY on any carbed engine.. Intake manifold PLENUM HEAT is CRITICAL.
Dialing in the AMOUNT OF plenum heat is a speed secret on any street car where DRIVABILITY matters.

[TheGreatPumpkin]

At idle and low speed DRIVING the engine wants more than 105°F intake manifold heat.
When ut is running Say 25 mph on the transition. it is ALWAYS absorbing intake manifold HEAT as you drive.
Without an active method of replacing that plenum HEAT the manifold cools off rapidly and fuel VAPOURIZATION quality suffers.. This is WHY all oems use active intake manifold PLENUM HEAT on All the carbed engines.
Yes the Amount of heat needs to be dialed in.
Yes in the Winter it will need more heat than in the hot summer... This is WHY all the OEM's employed intake air
(air cleaner) temp control and exhaust heat riser valve control.
Liquid fuel. Does not burn.
Liquid fuel Wears out your engine FAST.

If you are looking for great DRIVABILITY on any carbed engine.. Intake manifold PLENUM HEAT is CRITICAL.
Dialing in the AMOUNT OF plenum heat is a speed secret on any street car where DRIVABILITY matters.

I'll look into getting some parts for the heat riser system