Dual Quad tuning tips?

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TheGreatPumpkin
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

F-BIRD'88 wrote: Fri Aug 05, 2022 10:28 pm If yiu are worried about the 2 carvs bowls touching then once you dial in the metering block on the rear primary carb you can copy that new fiund metering back to the stock holley metering block, that you say is a hair thinner.
As long as its not a press fit between the 2 carbs I don't see a issue with them touching slightly.

Keep your mind open to the staged progressive throttle linkage set up for "Dual Quad" street driving.. on a dual plane type intake.
Use what works best.
Too bad there is not a HP body version /or upgrade kit (VS + down leg style boosters) of these 600 cfm VS carbs with the small fuel bowls.
About the interference, I had to shave the heads of the bowl screws to get them to sit down. The bowls touch, but it's not jammed in there. Just enough that the carbs won't shift at all when sitting on the studs. I'm hoping there's no real bore mismatch. I guess we'll see. I don't really want to have to drill out the stock blocks and plugs to clean them when tuning the orifices
golden_commando
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Re: Dual Quad tuning tips?

Post by golden_commando »

On 1:1 vs staged I've found it varies with the intake, some older intakes are flatter and carbs more apart from each other that when staged won't run good. Yours are dual plane so my advice is to run then as they came as baseline and check each plug that way you know which cylinder needs more fuel. On the idle use a vaccum gage, start with the rear carb first as the primaries are more on the center of the intake and will distribute most of the fuel, as baseline start with the same turns on both.
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Re: Dual Quad tuning tips?

Post by ClassAct »

TheGreatPumpkin wrote: Fri Aug 05, 2022 11:14 pm
ClassAct wrote: Fri Aug 05, 2022 11:06 pm
TheGreatPumpkin wrote: Fri Aug 05, 2022 9:44 pm

Guys just to clarify, the carbs are 1:1 with each other but the carbs themselves are the stock vacuum secondary linkage

Can't go sideways, won't idle on half the manifold then
Who told you that? I don’t run in-line carbs. My shit idles. That’s ridiculous.
Ok, ok, noted. Will keep an open mind

I apologize. I wasn’t thinking about a low profile intake manifold. I never think of those. I was thinking tunnel ram. My bad.
rebelrouser
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Re: Dual Quad tuning tips?

Post by rebelrouser »

To get the idle sweet and better response, you need to synchronizer the carbs at idle. I bought one similar to this from Dan Dvorak before he died. I simply made up some adapters out of plastic sewer pipe to fit different style carbs.


https://www.ebay.com/itm/151717668361?v ... a709747ddf
TheGreatPumpkin
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

rebelrouser wrote: Mon Aug 08, 2022 8:40 am To get the idle sweet and better response, you need to synchronizer the carbs at idle. I bought one similar to this from Dan Dvorak before he died. I simply made up some adapters out of plastic sewer pipe to fit different style carbs.


https://www.ebay.com/itm/151717668361?v ... a709747ddf
Will definitely do this. I know these are essential for IR manifolds/carbs, I wasn't aware that just setting the blades equal wasn't enough for dual quads. What's your procedure to use them?
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

Updated info after idle tuning and first drive. Both carbs are set up identically. Here's the map:

Throttle Blade P: 0.020" of slot (measured with feeler gauge)
Throttle Blade S: ¼ turn from bottomed out
IMS: 7/8 turn out
IFR P: 0.028" (in the QFT blocks)
IFR S: 0.039" (stock restriction for QFT plates)
Jet P: #66
Jet S: #71
PVCR: 0.038"
IAB P: 0.072"
IAB S: 0.063"
MAB P: 0.031"
MAB S: 0.031"
PV: 10.5"
Squrters: 0.031"
Springs: Plain

Ignition curve is 15 initial, 21 mechanical, 36 total in at around 3000rpm, 10 vacuum in at 15inhg

In this configuration it idled at 800rpm, dead on, didn't have to touch the idle speed screws at all. Very happy about that. 16inhg at idle, idle was happiest at average of 13.5 AFR (13-14 during idle). Seemed to surge (switching between 15 and 16inhg) at anything leaner.

Cruise under 2500rpm was very rich, 11.5-12.5. Cruise vacuum is 20-21inhg. Running over 2500rpm on the mains was a bit better, around 13.5, and was at around 12.5 with the power valve open.

I'm thinking my airflow at idle is good with these IABs and blade settings, and since I'm under 1 turn out with the IMS, I can reduce the IFRs to see if the cruise leans out. What else should I do?

EDIT: forgot to add, no-load tip in was good, responsive, didn't really skew lean and in fact ended up rich. I think the squirters are too big. Also, I noticed that the idle sometimes hangs up after revving, as in, it will settle at 850-900rpm and not return to 800rpm unless I drive it around. What's up with that? Are my secondary throttle blades not settling down like they're supposed to? Fuel puddling from squirters too big?
rebelrouser
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Re: Dual Quad tuning tips?

Post by rebelrouser »

rebelrouser wrote: Mon Aug 08, 2022 8:40 am To get the idle sweet and better response, you need to synchronizer the carbs at idle. I bought one similar to this from Dan Dvorak before he died. I simply made up some adapters out of plastic sewer pipe to fit different style carbs.


https://www.ebay.com/itm/151717668361?v ... a709747ddf
I just set the idle rpm and balance so the same amount of air is going through the carbs. Watch the ball float in the gauge. You can angle the gauge so if you have a high idle you can still get a reading.
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

Setting this up for testing tomorrow:

Throttle Blade P: 0.020" of slot (measured with feeler gauge)
Throttle Blade S: ¼ turn from bottomed out
IMS: 1.25 turns out to start
IFR P: 0.025" (in the QFT blocks)
IFR S: 0.039" ? (in the QFT plates)
Jet P: #66
Jet S: #71
PVCR: 0.038"
IAB P: 0.072"
IAB S: 0.063" ?
MAB P: 0.031"
MAB S: 0.031"
PV: 10.5"
Squirters: 0.025"
Cam: Orange
Springs: Plain

Does anybody have a good idea of how I should tune the secondary IFRs? It's a 2-corner carb with the weep holes for secondary idle... the way it is now is the way it comes stock on a QFT 600cfm 4160 Slayer (was able to look that up). What's a good baseline, if not that? Thanks.
F-BIRD'88
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Looks ok for your next drive/tuning session.
About the idle speed not returning consistantly to the base curb idle RPM.. The light tension, mechanical advance springs that help creat a quick mech advance curve can tend to not pull the mechanical advance weights all the way back to baseline .
A slightly thicker slower advance spring set will be more stable. (a but more spring tension).
The less spring tension the more critical the advance weights pivot travel (low) friction is going to be so it returns to base advance without hanging up above base curb idle.

You seem well equipped to work thru this.
💪
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

I think on that manifold that a 1 to 1 throttle set up scheme where both carbs contribute to the idle air and fuel flow at and near idle, fairlt evenly "50-50" is much more friendly if the cam is a lot more racey/radical, creating low manifold vacuum at idle. As the combined (8) throttles airflow is needed at idle.

On this relative mild duration / overlap street perf cammed engine , with 2x4 carbs, is going to want to favour a progressive throttle set up scheme where most all the idle and just off idle air flow
is handled by the front barrels of the (rear) "primary" carb for this one..
Even the factory BB Ford engines with factory 2x4 holley carbs had a more radical cam. (427 Cobra Jet Shelby Mustang)
In thst "progressive" setup scheme the primary (rear) and secondary carbs' specific idle circuits metering is going to be set up different to each other.
As most all of the idle / off idle and low speed part throttle driving is all done mostly by the (rear) "Primary carb".

On the old 2x4 induction Ford engines where the 4bbl carbs were mounted facing backwards, the front carb on the intake was the "primary carb".
F-BIRD'88
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Further, the idle airflow metering of the PCV valve is going to be critical on this one. (It is essentually an additional small air leak at idle)
A stock PCV valve for single 4 bbl carb induction system may not be right for this one... A adjustable PCV valve may be required.
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

A quick and dirty method of reducing the hole size of a metering jet or air bleed hole is to stick a short piece of
steel wire in the hole. Laying the wire across the hole entrance has a more profound effect than if the end of the wire is poking into the metering hole.
Guitar strings are handy for this.
typical guitar strings diameters are
E- .010". , B - .013" G- .017" D- .026" A - .036". E- .046".
Gives you a selection of sizes to work with.
A steel paper clip wire is about .030"
A typical fine wire brush wire is about .005"

Good if you don't own a complete set of screw in holley metering jets, air bleeds and PVCR's.

Once you get it all dialed in you can calc the effective jet hole area to then install as your final jetting setup.
(or solder the wire into the jet)

With an assistant, you can (with a few hands) work the wire(s), by hand in and out of the air bleeds while working the throttle to see the +/- effect on a AFR guage when evaluating the off idle transition at various throttle positions.
Just wrap the test wire around your finger or something so you don't loose it down the carb.

For sticking wire in the metering holes in the metering block or metrring plate you can create a short V shape wire or Z shape wire or P shape and wedge/ anchor the wire in between the block and gasket or anchor/ wrap the wire(s) to a attaching scew. What ever works.
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

Thanks for those replies. The PCV and advance spring issues are things I hadn't considered. I definitely own all the bleeds, restrictors, and tiny drill bits, so I've been busy dialing this in. Right now here's my notes:

TB P: 0.020, give or take. Transition needs fuel. Reduce IAB and increase slot? Green or Blue cam but less nozzle size?
TB S: ¼ turn
IMS: 1¼
IFR P: 0.024
IFR S: 0.031
Jet P: #66
Jet S: #71
PVCR: 0.038
IAB P: 0.070 (go to 0.068?)
IAB S: 0.045 (go to 0.049?)
MAB P: 0.028
MAB S: 0.027
PV: 10.5
Nozzle: 0.025 (stay for now?)
Cam: Orange (go to Green or Blue)
Spring: Black (temporary to see AFR when opening)

Cruise AFR is pretty good, but the transitions, mains, and secondaries need some work. Idle is very sensitive to temperature. I don't recall or didn't write down all the AFRs, I gotta drive it a bit more in different conditions
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Sensitive to temperature.. This means you are close +/-.
The engine and manifold needs to be fully warmed up to evaluate the tune.. It will be bawky when cold first start up like any carbed engine is. This is the purpose of the choke, fast idle linkage and manifold plenum heat.
Just like every OEM xarbed vehicle.
Heat improves vapourization.
Liquid fuel will not burn.

Good progress.
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

F-BIRD'88 wrote: Sun Aug 14, 2022 6:40 am Sensitive to temperature.. This means you are close +/-.
The engine and manifold needs to be fully warmed up to evaluate the tune.. It will be bawky when cold first start up like any carbed engine is. This is the purpose of the choke, fast idle linkage and manifold plenum heat.
Just like every OEM xarbed vehicle.
Heat improves vapourization.
Liquid fuel will not burn.

Good progress.
Good to know! I'm running 1/4 inch insulator gaskets under the carbs, engine temps are 195-205 with a 180 thermostat and 200 on/185 off fan switch. Kinda hot but still fine. Southern California is very very hot these days unless you're near the ocean
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