Dual Quad tuning tips?

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TheGreatPumpkin
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Re: Dual Quad tuning tips?

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I'm very close with the above tune. IMS ended up 1 1/8 out for an idle average in the mid-14s (I switched my gauge to lambda, way easier to read, so that's 0.98 or so). Cruise up to 2200rpm or so is about the same as idle, over that it gets a bit leaner, nothing over 15.5 afr. Timing is high at that rpm with vacuum advance so I'm confident I'm safe at steady state low load.

I did switch the timing advance springs to the next heaviest, brings the full timing in at about 3300 instead of 2800. Same bushing, the blue one. I don't think it solved the hanging idle issue - I'm guess that's a combination of factors like residual fuel, temperature, PCV open/close, whatever. I'll keep it for now though since it didn't seem to upset the AFRs very much after I retuned the idle and throttle plates.

Mains are ok, maybe a little lean. Held it at steady state around 4000 rpm until the pump shot ran out and got around 15 AFR. Under 10.5 vac, PV opens, got 13.5ish AFR.

Pump seems to want both more and less fuel at the same time. No load rev it goes a bit too rich, driving down the street and snapping it open at 3000rpm it goes lean... that's why in the previous post I said maybe less nozzle but more aggressive pump cam? Does that sound right?
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Richen up the mains a bit more before messing with the pump shot.

Set the curb idle to best idle quality, not a AFR on the guage..
All carbed engines idle at richer than 14.7:1.
12.5-13.8 is typical. Best idle is the priority, not AFR.

Because its a non heated intake its going to need a bit more fuel all the way around.
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

This is not a bad time to take the car to the track and drag test for best ET and MPH to dial in the main jets..
Give it all the WOT fuel it wants for WOT..
The AFR guage is just a "you've jetted it too lean" warning device when WOT jetting tuning.
It wants what it wants to make best WOT power at the track.. (often NOT exactly the same as Dyno tuned best jetting.). The AFR guage keeps you out of trouble.
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Re: Dual Quad tuning tips?

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F-BIRD'88 wrote: Mon Aug 15, 2022 1:58 pm Richen up the mains a bit more before messing with the pump shot.

Set the curb idle to best idle quality, not a AFR on the guage..
All carbed engines idle at richer than 14.7:1.
12.5-13.8 is typical. Best idle is the priority, not AFR.

Because its a non heated intake its going to need a bit more fuel all the way around.
Will do. Good point about the non-heated intake
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Re: Dual Quad tuning tips?

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Here's the current tune, I'm fairly happy with it:

TB P: 0.020
TB S: ¼ turn
IMS: 1 turn
IFR P: 0.025
IFR S: 0.031
Jet P: #66
Jet S: #72
PVCR: 0.043
IAB P: 0.070
IAB S: 0.045
MAB P/S: 0.027
PV: 10.5
Nozzle: 0.031
Cam: Orange #2

I'm going to start tuning the vacuum secondary spring (I had put the Black one in there up until now). Going to start with the Yellow. Any tips on tuning the secondary IFRs and IABs? The ones I have in there now are basically guesses.
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Re: Dual Quad tuning tips?

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Car is the happiest it's been so far with this tune. Also found and solved a surprising issue:

TB P: ~0.020
TB S: ~⅛ turn (found the sweet spot)
IMS: 1 turn
IFR P: 0.025
IFR S: 0.031
Jet P: #66
Jet S: #70
PVCR: 0.031
IAB P: 0.070
IAB S: 0.045 (go to 0.041? Goes lean on secondary opening)
MAB P/S: 0.027 (could fine-tune this)
PV: 10.5
Nozzle: 0.033 (reduce this once secondary lean spot is removed)
Cam: Orange #1 (this seems to be the only cam that works without bending the arm a lot)
Spring: Orange

I'm impressed with how much secondary opening rate this thing can tolerate right now. Not sure if that will change. Idle is very stable at 13-13.5 AFR and 800rpm. That's where it likes it. No longer chasing a super lean idle.

So, I had checked the timing to enter it into my notes and found that the MSD vacuum advance was adding 15 degrees, not 10. Apparently this is a known issue. Measured, drilled, and tapped the base of the vac advance unit and screwed in a 10-32 bleed to limit the advance to about 10, which was successful. Not sure if it had early ignition anywhere but it seems happier. Now running this timing:

~15 initial
~10 vacuum
~25 initial w/ vacuum
~21 mechanical all in at 3300rpm (blue bushing and heavy-silver and light-silver springs)
~36 total WOT
~46 max cruise possible

I think it could take more timing (more aggressive ramp) but I've reached a point where I'm happy. Will work on the secondary opening point AFR. Thanks for all your help guys
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Re: Dual Quad tuning tips?

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update for those interested, and questions about tuning the transition slot:

I finally got some decent micro drill bits and was able to tune the secondary IAB down to 0.020, which is what it is supposed to be on these carbs from factory. Here's the tune right now:

TB P: 0.020" of slot
TB S: ~0.3890" (high side of blade from base, ~1/8 turn from fully closed)
Idle Speed: 800
IMS: 1 turn exactly!
IFR P: 0.0225"
IFR S: 0.0225"
IAB P: 0.064
IAB S: 0.020
Jet P: #65
Jet S: #70
PVCR: 0.035
MAB P: 0.025
MAB S: 0.024
PV: 10.5
Nozzle: 0.028
Cam: Orange #1
Vac. Spring: Plain (will go to Purple soon, since they are not opening all the way at 6000rpm redline)
Total Timing: 35-36
Dist. Bushing: Black
Dist. Springs: Blue/Blue
~17-18 initial
~15 vacuum
~32-33 initial w/ vacuum
~18 mechanical
~35-36 total WOT
~50-51 max cruise

Idles nice and steady at 0.88-0.92 lambda, 800rpm, about 17inhg vacuum (best vac)
So - had a lot of frustration with the transition slot tuning, had a big lean spot. Made my way down to 64 from 72 for the primary IABs. All modes are good and respond to tuning except light acceleration and idle cruise, which are both lean (up to 1.10 lambda or so). Should I continue to reduce the primary IABs or increase the primary IFRs, or both? I don't want to lose my great idle setup I have now but light accel should be a whole point lower in AFR. All questions welcome. Thanks
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

dropped the secondary IFRs to 0.021, and increased the primary IFRs back to 0.024. Now low speed cruise is a point low (down to .80) in AFR instead of a point lean. Less of a lean spot in the transition though. I'm going to drop the primary IABs to 0.060 and go back to 0.0225 for all IFRs, and see if the IABs richen the cruise AND takes care of the lean spot. Anyone have any thoughts?

EDIT: with the 24/21 IFRs the IMS are 3/4 out instead of 1 turn. I'd like them 1-1.25 turns. To me, this means the IFR of 24 is too large for all modes, but .0225 is too lean based on the previous tune. I have 0.6mm bits coming (.0236) and I'll try those. IAB of 60 seems low, should I be concerned about that?

It's possible that it's a secondary opening lean spot. I'm using the Purple spring. Thing is, it was there when using the Plain and Black springs also. Car is MUCH happier on the highway and at heavy accel with the Purple springs, after I established that the Plain ones were not opening all the way. What should I try with these IABs? Keep reducing?
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

The only way to know is to try it.
I think you're right....
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Re: Dual Quad tuning tips?

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F-BIRD'88 wrote: Thu Sep 15, 2022 6:26 pm The only way to know is to try it.
I think you're right....
Very true. But what do you think I'm right about?
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Further adjustment if the IAB s.
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Re: Dual Quad tuning tips?

Post by F-BIRD'88 »

Remember that even the smallest air leak in the exhaust system within 3 ft. of the O2 sensor will make it read a bit leaner than true.
Also in the critical off idle transition throttle driving zone, the cam overlap which creates exhaust re mix and thus somewhat incomplete combustion, this will also shiw a leaner than true afr as the O2 only sees left over or presence of oxygen in the exhaust.
Could be just 1 or 2 cylinders. VS all 8.
So at some point you got to take the afr guage with a grain of salt....
Plug appearance... , drivability.....
But you are folliwing a logical path.
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Re: Dual Quad tuning tips?

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Absolutely agree those could be factors. I'll check all my exhaust joints and stuff. I can't wait until I can take the gauge off, believe me, but you can still feel the hesitation. I'm confident I'll know when it's right!
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Re: Dual Quad tuning tips?

Post by TheGreatPumpkin »

F-BIRD'88 wrote: Thu Sep 15, 2022 8:05 pm Remember that even the smallest air leak in the exhaust system within 3 ft. of the O2 sensor will make it read a bit leaner than true.
Also in the critical off idle transition throttle driving zone, the cam overlap which creates exhaust re mix and thus somewhat incomplete combustion, this will also shiw a leaner than true afr as the O2 only sees left over or presence of oxygen in the exhaust.
Could be just 1 or 2 cylinders. VS all 8.
So at some point you got to take the afr guage with a grain of salt....
Plug appearance... , drivability.....
But you are folliwing a logical path.
I thought about this a lot and the overlap during transition makes sense. I went to 0.052 on the pri IABs, naturally it ruined the idle settings and didn't even richen up the part throttle cruise. Still got the blip of lean. Going back to where it felt happy.
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Re: Dual Quad tuning tips?

Post by 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.
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