#1  
Old 05-09-2021, 12:13 PM
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67gtospud 67gtospud is offline
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Default Carb tuning help

I've been fighting stumbling issues for awhile now and could use some help.

432ci
Edelbrock 72cc round port heads
10.5:1 cr
14 degrees initial, 34 all in by 3k
Lunati 296/303 hr
Offenhauser 5499 2-4 intake
2- Edelbrock avs2 650s
Straight linkage, electric choke on rear carb
Idles at 800rpm at 6-6.5 inhg

Carbs are setup as follows:
.095 primary jets
.070 secondary jets
73-42 metering rods
Orange step-up springs
Accelerator pumps in closest hole

Idle afr: 14-14.5
Cruise afr: around 14
Mid throttle afr: goes lean around 16-18
Wot afr: 11.5-12

I just changed from the yellow to orange step-up spring and it helped a little but didn't solve the lean stumble under light throttle.

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1967 GTO, 432 (428+.030), 4-bolt mains, factory Nodular crank, scat rods, icon dished pistons, Lunati HR 243/251@.050, .618/.622 lift, Edelbrock 72cc round port heads, 10.5:1, offy 2-4 intake, Edelbrock 650cfm carbs, Super T10 trans (2.64 first), BOP 10 bolt w/ Eaton posi and 3.36 gears
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Old 05-09-2021, 12:24 PM
62posbonny 62posbonny is offline
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You may want to jump all the way up to the silver springs to get the rods to lift sooner under partial throttle situations.

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Old 05-09-2021, 01:03 PM
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How many turns out on the idle screws?
If more than 2 turns out on all 4 and no improvement youŽll need to open the idle tubes and economizer restrictions a couple of thousands.

HTH

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Old 05-09-2021, 03:15 PM
PontiacJim1959 PontiacJim1959 is offline
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Not enough info.

Transmission type? Converter stall if automatic.

Gear ratio?

When does it stumble" At idle and you nail it? Cruising and you nail the throttle? Lower RPM's, mid RPM's, upper RPM's?

Vacuum advance on the distributor? Direct or ported? How many degrees does the vacuum advance add?

14 degrees Initial with the vacuum advance disconnected? What is it with the vacuum advance hooked up?

Have you tried it with just one carb hooked up rather than both? Same stumble?

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Old 05-09-2021, 03:24 PM
tom s tom s is offline
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JMHO,having driven pontiac 2-4s since the early 60s I have ALWAYS found progressive linkage better for street driving.I like them solid for racing but not on the street.Tom

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Old 05-09-2021, 04:39 PM
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Quote:
Originally Posted by 62posbonny View Post
You may want to jump all the way up to the silver springs to get the rods to lift sooner under partial throttle situations.
Will this give me issues at slower speeds and idle since the pistons won't be able to hold the rods down with my 6.5" if Idle vacuum?

Quote:
Originally Posted by Kenth View Post
How many turns out on the idle screws?
If more than 2 turns out on all 4 and no improvement youŽll need to open the idle tubes and economizer restrictions a couple of thousands.

HTH
1.25 turns out on all 4 screws



Quote:
Originally Posted by PontiacJim1959 View Post
Not enough info.

Transmission type? Converter stall if automatic.

Gear ratio?

When does it stumble" At idle and you nail it? Cruising and you nail the throttle? Lower RPM's, mid RPM's, upper RPM's?

Vacuum advance on the distributor? Direct or ported? How many degrees does the vacuum advance add?

14 degrees Initial with the vacuum advance disconnected? What is it with the vacuum advance hooked up?

Have you tried it with just one carb hooked up rather than both? Same stumble?
Manual trans, 2.64 1st gear, 3.36 rear gears. Stumble is upon lighter acceleration. For example: if I'm going 25-30 and lightly accelerate to 60, it goes very lean. If I accelerate moderately hard it goes lean for a split second then is fine. If I accelerate hard there's no lean stumble at all. No vacuum advance connected.
I was running progressive and had the same issue

Quote:
Originally Posted by tom s View Post
JMHO,having driven pontiac 2-4s since the early 60s I have ALWAYS found progressive linkage better for street driving.I like them solid for racing but not on the street.Tom
I had progressive linkage initially and found the throttle response to be soggy. With the straight linkage I'm much happier with the throttle response and low speed drivability is not near as bad as I thought it'd be.

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Old 05-09-2021, 04:47 PM
tom s tom s is offline
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Have you tried it with VA hooked up?Tom

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Old 05-09-2021, 05:47 PM
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I don’t understand why your idle vacuum is so low!

If you do a hot cranking compression test what results do you get?

It’s good though that you experimented and found out that you can not run a progressive linkage set up on a manifold like that with it’s nature to act more like it has two separate Plenums even though it’s pretty open on the inside!

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Last edited by steve25; 05-09-2021 at 05:54 PM.
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Old 05-09-2021, 06:17 PM
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Quote:
Originally Posted by tom s View Post
Have you tried it with VA hooked up?Tom
I disconnected it until I get my tuning sorted out.



Quote:
Originally Posted by steve25 View Post
I don’t understand why your idle vacuum is so low!

If you do a hot cranking compression test what results do you get?

It’s good though that you experimented and found out that you can not run a progressive linkage set up on a manifold like that with it’s nature to act more like it has two separate Plenums even though it’s pretty open on the inside!
I'd imagine the long duration cam and large amount of valve lift would be the cause of the low vacuum. I haven't done a compression test. Engine is a fairly fresh build with about 2k miles on it.
With progressive linkage driveability was fine but throttle response is day and night after switching to straight linkage.

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1967 GTO, 432 (428+.030), 4-bolt mains, factory Nodular crank, scat rods, icon dished pistons, Lunati HR 243/251@.050, .618/.622 lift, Edelbrock 72cc round port heads, 10.5:1, offy 2-4 intake, Edelbrock 650cfm carbs, Super T10 trans (2.64 first), BOP 10 bolt w/ Eaton posi and 3.36 gears
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Old 05-09-2021, 06:26 PM
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Nows the time with a fresh motor to get this type of info noted so you can reference it down the road if need be and not be left scratching your head!

You just flat out can’t feed 8 cylinders with equal amounts of the needed fuel and air when the primary of one Carb is opening sooner then the other

6.5 “ hg is what the largest amount of Holley Carbs come fitted with for a power valve, and that also so happens to be the vacuum level they open at and start providing added enrichiment for heavy throttle, yet this is where your idling at!

Once again something is not right to me about that!

Lift has nothing much to do with levels of vacuum, it’s duration and especially overlap that does.

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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!

Last edited by steve25; 05-09-2021 at 06:36 PM.
  #11  
Old 05-09-2021, 06:37 PM
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Quote:
Originally Posted by steve25 View Post
Nows the time with a fresh motor to get this type of info noted so you can reference it down the road if need be and not be eft scratching your head!

Lift has nothing much to do with levels of vacuum, it’s duration and especially overlap that does.
I'll do a compression test when I've got a chance

Cam is 243/251 @.050, and I believe 27 degrees of overlap

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Old 05-09-2021, 06:47 PM
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Cool, good to hear!

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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
  #13  
Old 05-10-2021, 03:06 AM
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Open all 4 idle screws half a turn and see if that helps.

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Old 05-10-2021, 07:59 AM
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"No vacuum advance connected."

You are missing timing so going to have troubles at light engine load like you are seeing.

In order to effectively burn a lean mixture you need to add some timing with the Vacuum Advance.

Otherwise you'll need to richen it up in that range. I see this ALL the time, and have fixed light throttle issues with more vehicles than I can count by hooking up the VA, and more times than not putting stronger springs in the distributor. For some reason I'll never understand, and last time I checked it was 2021, folks are still trying to get all the mechanical advance in right off idle and not adding timing with the VA.

One does NOT offset the other.

Even with that said no big deal if you don't use VA, just fatten up the carb(s) to compensate and use more fuel, soot up the exhaust more, and carbon up the tops of the pistons. Did I mention emptying your wallet more with heavier fuel consumption?........Cliff

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Old 05-10-2021, 09:44 AM
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1. As post #13 states you probably need to open up the idle mixture screws. I say "probably" because I don't know the angle of the points on the edelbrock mixture screws. Go around all four mixture screws a couple of times so as to achieve maximum idle or vacuum (either one) with them. Readjust idle speed to suit as you do so.

2. hook up the vacuum advance and increase initial advance slightly.

See what it does.

I would also say get rid of the edelbrocks and get a pair of real carter afbs but you've had carbs I've done and couldn't get them to work to your satisfaction either.

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Old 05-10-2021, 12:18 PM
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Quote:
Originally Posted by 61-63 View Post
1. As post #13 states you probably need to open up the idle mixture screws. I say "probably" because I don't know the angle of the points on the edelbrock mixture screws. Go around all four mixture screws a couple of times so as to achieve maximum idle or vacuum (either one) with them. Readjust idle speed to suit as you do so.

2. hook up the vacuum advance and increase initial advance slightly.

See what it does.

I would also say get rid of the edelbrocks and get a pair of real carter afbs but you've had carbs I've done and couldn't get them to work to your satisfaction either.
I wasn't going to post since this thread is about the imitation AFB's, and I have no specifications. However John (61-63) brought up the subject of real Carters.

So, if you had real Carters, the following comes to mind.

(1) the idle screws on the later Carters were the small angle (long profile) used with smog emissions, requiring 1 1/2 ~ 3 1/2 turns. A good starting point is 2 1/2 turns.

(2) The calibration you posted is for a Chevrolet! Pontiac, on the original Super Duty street carbs (625 CFM) used a 0.089 primary jet, and step-up rods that were 0.060 x 0.054. Look at the difference from the Pontiac rods to the rods you posted!

(3) I have no idea what the secondaries are doing on what you have. When Carter produced the AVS, it was (opinion) the worst 4-barrel they ever made!!!!!!!! Check a little carburetor history. Chevrolet used the AVS on one engine in 1966 ONLY, and then discontinued the AVS. The design was dorment until 1968 when Chrysler was forced to use the AVS to pass the new federal smog emissions. No one else even tried it. We would ONLY build them for numbers-matching show cars that were not driven, and were required to have the AVS or lose points. And offered ZERO performance guarantee. We did offer our 5 year authenticity guarantee. Again, if you had real Carters, the auxiliary air valves in the Super Duty carbs had the smallest attack angle, and the heaviest mass of any Carter AFB air valves of which I am aware; totally different than the Chevrolet valves. I am assuming the imitations you have have the spring-loaded secondary, but since the calibration is Chevrolet, perhaps the secondary spring is also.

(4) And as others have mentioned, the idle vacuum is just wrong. The only time I ever had that low an idle vacuum on a Pontiac street engine was when I installed an aftermarket timing set with three marks. After putting it together, and getting the joy of taking it apart, we found a fourth mark. Using that mark solved the issue. But possibly more idle fuel will help the idle vacuum.

(5) I have never had an issue with the Offy manifolds, but have had at least 2 customers which the e-manifolds that were machined at the wrong angle to correctly fit the cylinder heads, and had an internal vacuum leak. Is it possible (don't know, just asking) that the Offy intake is incompatible with the e-heads?

Sounds like an interesting car, hope you find the issue.

Jon.

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Last edited by carbking; 05-10-2021 at 12:26 PM.
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Old 05-10-2021, 04:10 PM
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Jon, was digging around in the core pile cleaning things up and found a 1966 Chevy AVS. I never even knew that they used them on Chevy engines other than a few Marine 327's. Not overly fond of them either, and hardly even get any calls from anyone looking for one, or parts for one......
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Old 05-10-2021, 05:43 PM
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Cliff - the Chevrolet folks hated them! In 46 years of business, I have yet to sell an AVS for a Chevy, and checking my sales, only TWO rebuilding kits! Chrysler folks threw them away 5 nanoseconds after the new car went out of warranty! The Chrysler folks with more money threw them away 5 nanoseconds after they got home with the car! Holley sold LOTS of carbs to replace the AVS!

Because of this, the 440 high performance carbs are almost as valuable (for a show car) as RA IV Q-Jets! We did sell out of the 440 AVS to show people. In 46 years, have yet to sell one for a 383, and only one for a 340. It was a stop-gap for Chrysler giving the Carter carburetor engineers time to build a carb (TQ) with both performance and emission capability.

The Carter literature in 1972 shows AVS aftermarket carbs. Guessing Carter had some castings left over and tried to get rid of them. I have yet to see one, and my friends at Carter had also never seen one (this in the late 1970's). The AVS wasn't even popular with Carter!

OP - I should have mentioned this in my previous post, but forgot. At least with Carter, long taper idle mixture control screws have a taper of about 5/16 inch in length, while the short taper have a length of approximately 3/16 inch. The longer taper provided a more precise setting for smog emissions. I don't know what the imitations use.

Edit: Cliff that AVS you show is for the standard performance 440 from 1971, not Chevy. Chevy numbers were 4027, 4028.

Jon.

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"Good carburetion is fuelish hot air".

"The most expensive carburetor is the wrong one given to you by your neighbor".

If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!

Owner of The Carburetor Shop, LLC (of Missouri).

Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.

Last edited by carbking; 05-10-2021 at 05:49 PM.
  #19  
Old 05-10-2021, 06:26 PM
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To get the calibrations where they currently are I had started from scratch. Edelbrock doesn't have any literature of 2 650s used on a 2-4 setup, only their 500s. I leaned out the idle afr and that raised it from 5.5 to 6.5 inhg. I can see the calibrations are quite a bit different on my avs2s vs original carter's but this seems to be close to where they need to be for my engine combo.

Being that the idle circuit is separate from the primary circuit; do the step up pistons absolutely have to be down at idle? I wouldn't mind trying a heavier step up spring but I believe the next step up (pink 7") will likely hold the pistons up at idle. Under light throttle/cruising they should stay down though.

I will play with the vacuum advance as well and see what that yields. I will add my transition slots are only partially exposed at idle like they're supposed to be via edelbrock


I'm not against trying a set of 625 carbs again from 61-63, I'll have to get ahold of you. How is the base calibration going to be with my current engine combo?

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Old 05-10-2021, 06:52 PM
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Something is causing that low vacuum at idle. I believe that needs to be addressed before anything else.

Not familiar with how your cam compares to the SD cam. If close, then SD calibrations should be a good starting point, but John (61-63) has set up a lot of these. I would suggest starting with his recommendations, but address that vacuum before doing anything else.

How about the compatability of the Offy manifold to the e-heads?

Jon.

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"Good carburetion is fuelish hot air".

"The most expensive carburetor is the wrong one given to you by your neighbor".

If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!

Owner of The Carburetor Shop, LLC (of Missouri).

Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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