[67gtospud]

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.

[62posbonny]

You may want to jump all the way up to the silver springs to get the rods to lift sooner under partial throttle situations.

[Kenth]

How many turns out on the idle screws?
If more than 2 turns out on all 4 and no improvement you´ll need to open the idle tubes and economizer restrictions a couple of thousands.

HTH

[PontiacJim1959]

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?

[tom s]

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

[67gtospud]

Originally Posted by 62posbonny

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?

Originally Posted by Kenth

How many turns out on the idle screws?
If more than 2 turns out on all 4 and no improvement you´ll need to open the idle tubes and economizer restrictions a couple of thousands.

HTH

1.25 turns out on all 4 screws

Originally Posted by PontiacJim1959

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

Originally Posted by tom s

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.

[tom s]

Have you tried it with VA hooked up?Tom

[steve25]

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!

[67gtospud]

Originally Posted by tom s

Have you tried it with VA hooked up?Tom

I disconnected it until I get my tuning sorted out.

Originally Posted by steve25

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.

[PontiacJim1959]

The AVS2 is not the same as the AVS - it has annular primary boosters improves fuel atomization and Adjustable secondary air valve for smooth part to full throttle transition.

The stumble could easily be due to what many carbs experience with low vacuum and big cam - needs a much larger shot of gas when you crack open the primaries. You guys who use the Holley would change out the pump shot from the 30CC accelerator pump to the 50CC pump for starters - then maybe change nozzles.

Along with the other things I suggested, the AVS2 carbs may need the larger accelerator pump nozzles. They come in different sizes, but you have to buy a "kit" to get them, so 2 carbs will require 2 "kits." The image shows what they look like, but there are 3 to a "kit."

[steve25]

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.

[67gtospud]

Originally Posted by steve25

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

[Tom Vaught]

Originally Posted by steve25

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!

1) That statement is totally incorrect. Holley carbs have been built with 8.5 and 10.5 " Power Valves for many years. The higher the Power Valve number the SOONER the carb gets additional fuel for the engine. Commercial vehicles and marine applications use a lot of the higher number power valves in Holley Carbs.

The power valve does not operate at idle. even though many think that it does.
Vacuum is higher at idle and lower at WOT. You want to apply more fuel before you are at WOT.

That being said, the OP is talking about a couple of E-Carbs with a whole different fuel circuit system vs a Holley Carb. Read the inital post.

Tom V.

[steve25]

Cool, good to hear!

[Kenth]

Open all 4 idle screws half a turn and see if that helps.

[Cliff R]

"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

[61-63]

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.

[carbking]

Originally Posted by 61-63

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.

[Cliff R]

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......

[carbking]

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.