[PontiacJim1959]

Throw away the AFR gauge for now.

Back idle mixture screws out 2 1/2 turns.

With your cam, I would think Idle speed should be around 800-950 RPM.

Initial idle advance WITH the vacuum advance connected to direct port vacuum should be 20-22 degrees.

However, if you only have 6.5 Hg (which is low), you need to source the matching vacuum can that will work with that low of vacuum. You may want to go with a "B28" vaccum can - which you can get locally or at Rock Auto.

Read this article:

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

Have you ever synchronized your carbs? Watch this video and make this very inexpensive tool to do so. I got all the parts from Summit to make mine for my dual quads once I am at that point.

https://www.youtube.com/watch?v=SRwZPWxC3Sg

[67gtospud]

Originally Posted by carbking

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.

I'm pretty confident there are no vacuum leaks but ill verify that as well. Would having 2 carbs mean 1/2 the signal at each carb? I wouldn't think so but it doesn't hurt to ask.

While I was assembling the engine, I dry fit the intake and it sat flat on each head. I will verify no vacuum leak tomorrow though.

[Cliff R]

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

Grabbed the wrong picture from the files. It's a 4028S.

+2 on throwing the A/F gauge away, or at least putting a piece of tape over it until you get things well dialed in. I know some seasoned and experienced tuners who use them and don't really know what they are doing as they do NOT understand vacuum advance, how it works, and why we need it for these engines.......Cliff

[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]

I was just comparing vacuum levels Tom which in his case I see as part of his problem, not looking to get graded by you or anyone else for being 100% accurate as for what my intent was !

I appreciate your setting me straight on the details I danced around for the sake of making a comparison thought!

[61-63]

The step up pistons and springs in afb and edelbrock knockoffs including the avs are part of the main circuits and not the idle circuits. Part of his low vacuum at idle issue may be his idle rpm. Pontiac's instructions for idle speed with the #10 SD cam was 1,000rpm and not the 800rpm he is running. Also the cam he is running is considerably larger with more overlap than a McKeller #10; .243/.251 @ .050 and 110lsa or something like that (post #1) vs. the #10s .236/.247 @ .050 and 114lsa I think it is

[Formulajones]

You didn't mention lobe sep on that cam but 6 inches of vacuum at 800 rpm in a 432 engine with a 243/251 @ .050 cam, even with plenty of compression as you have, isn't going to be a stellar vacuum producer anyway. Especially if it's on a 110.

I run a 242/248 on a 110 in a larger 454 with the same compression you have and mine makes around 9-10 inches at 950-ish rpm. In gear it drops to around 6-7 inches.

There are a lot of things you can do to help that situation. One very easy one since you have a stick is bump the idle up some. No need for 800 rpm idle. I'd be at 1000 with a setup like that.

Vacuum advance is going to make a big difference as already mentioned. That really needs to be dialed in. I prefer to have that functional and dialed in before I try to attempt carb tuning.

The other is the idle AFR. An engine like that, with a large cam, really isn't going to be super happy trying to idle around with 14-14.5 AFR. I'm betting you'll find the vacuum pick up just by fattening the idle circuit. Most of the time engines like that with a bunch of overlap I find are much happier idling in the 13:1 range. Best to find what the engine likes without the gauge, and then see what the gauge says once good drivability is found. Might be surprised.

[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."

[67gtospud]

To clarify: yes lobe separation on this cam is 110 degrees.

I checked and verified there are no vacuum leaks. I fattened the idle circuit up to 13.5:1 and that bumped vacuum up to 7"; fattening it up more did not increase vacuum. I raised the idle rpm up to 900-950 and that increased vacuum to 7.5-8". I connected the vacuum canister to manifold vacuum but it made no changes in vacuum, rpm or timing. I verified it is operational by giving light throttle and the advance is adding about 10 degrees under light load. There must not be enough vacuum at idle to pull any additional timing.

I did change the step-up springs to the pink (7") ones and that seems to help some more but the lean surge continues. Apparently this avs2 carb comes with .032" nozzles and the largest edelbrock offers is .042; is there a place to order just that size nozzle or do I have to purchase the 3 pack?

[Formulajones]

You're going to need a more sensitive vacuum can for that deal.

Many numbers have been discontinued but a popular one you might try looking for that works well on engines like this is the B28 can.

Or get an adjustable unit and modify them as I've explained before as well as others on this subject.

[67gtospud]

For the time being, I connected the vacuum advance to the ported vacuum source on my rear carb. I test drove the car and 90% of my hesitation is gone. I still have a slight hesitation on very light throttle. For example; if I'm cruising at 50 and very lightly bring the car to 60 it'll stumble for a split second. I'm wondering if the larger pump nozzles will cure this??

[tom s]

Did not see if you had tried manifold Vac?Tom

[67gtospud]

Originally Posted by tom s

Did not see if you had tried manifold Vac?Tom

I did but it made zero difference at idle since I don't have enough manifold vacuum to pull the canister so I just put it on ported.

[steve25]

Not to be a pain in the Ass, but it still seems your resisting putting in the effort to do a cranking compression test and get that big Cam installed point question put to bed or not!

[61-63]

Originally Posted by 67gtospud

For the time being, I connected the vacuum advance to the ported vacuum source on my rear carb. I test drove the car and 90% of my hesitation is gone. I still have a slight hesitation on very light throttle. For example; if I'm cruising at 50 and very lightly bring the car to 60 it'll stumble for a split second. I'm wondering if the larger pump nozzles will cure this??

When you "very lightly" add speed there is a good chance you aren't getting any squirt at all thus a nozzle change won't help you.

Get a pair of Pontiac AFB nozzles and drill them out to a larger size. They have brass tubes on the outer edge of the nozzle and the inner aluminum ring has the squirter holes drilled in. Thus you can open them up yourself to whatever diameter you wish. If you open them up too big then fill the holes with epoxy and redrill them smaller. I still have your address I will mail you a couple of them.

[67gtospud]

Originally Posted by steve25

Not to be a pain in the Ass, but it still seems your resisting putting in the effort to do a cranking compression test and get that big Cam installed point question put to bed or not!

I'm not putting it off, haven't had a lot of time this week to putz around on the car and I loaned one of my friends my compression tester. I'll be getting it back tomorrow so I'll try to do that this weekend.

Originally Posted by 61-63

When you "very lightly" add speed there is a good chance you aren't getting any squirt at all thus a nozzle change won't help you.

Get a pair of Pontiac AFB nozzles and drill them out to a larger size. They have brass tubes on the outer edge of the nozzle and the inner aluminum ring has the squirter holes drilled in. Thus you can open them up yourself to whatever diameter you wish. If you open them up too big then fill the holes with epoxy and redrill them smaller. I still have your address I will mail you a couple of them.

This crossed my mind as well. With the air cleaners off and I slowly actuate the throttle no fuel is sprayed. Is there any way to manipulate the linkage rod to put the plunger slightly further in the bore and maybe squirt fuel under these light throttle conditions?

[carbking]

The height of the pump in the pump cylinder is not the problem.

Carter brought out the AFB in 1957, and like any new concept, there were teething issues.

Carter engineers identified and solved issues up until a time when Carter engineers had no control of AFB design, and accountants became the ruling authority.

One of the issues was no pump squirt under light acceleration, and sometimes no pump squirt on the first pump. The cause was percolation in the pump cylinder. Air bubbles would form under the accelerator pump, and being lighter than gasoline rise to the underside of the accelerator pump. The air bubbles occupied more volume than the gasoline, thus gasoline would slowly be forced past the pump discharge valve until all of the gasoline in the pump cylinder had been displaced by air. The solution was to add a check ball in the accelerator pump thimble. The air bubbles could not accumulate on the underside of the accelerator pump, as the check ball would rise, and the bubbles would disperse through he accelerator pump release passage.

Rochester also used this design on some carburetors.

This accelerator pump was more expensive to produce. I do not know if the imitation AFB's use this pump design, as the cost is/was higher.

The size of the discharge nozzles is irrelevant if there is no fuel in the pump cylinder to pump.

Jon.

[67gtospud]

Originally Posted by carbking

The height of the pump in the pump cylinder is not the problem.

Carter brought out the AFB in 1957, and like any new concept, there were teething issues.

Carter engineers identified and solved issues up until a time when Carter engineers had no control of AFB design, and accountants became the ruling authority.

One of the issues was no pump squirt under light acceleration, and sometimes no pump squirt on the first pump. The cause was percolation in the pump cylinder. Air bubbles would form under the accelerator pump, and being lighter than gasoline rise to the underside of the accelerator pump. The air bubbles occupied more volume than the gasoline, thus gasoline would slowly be forced past the pump discharge valve until all of the gasoline in the pump cylinder had been displaced by air. The solution was to add a check ball in the accelerator pump thimble. The air bubbles could not accumulate on the underside of the accelerator pump, as the check ball would rise, and the bubbles would disperse through he accelerator pump release passage.

Rochester also used this design on some carburetors.

This accelerator pump was more expensive to produce. I do not know if the imitation AFB's use this pump design, as the cost is/was higher.

The size of the discharge nozzles is irrelevant if there is no fuel in the pump cylinder to pump.

Jon.

This is good to know! I know the previous 500cfm afbs I had on the engine did have a check ball under the pump nozzles however I do not know for sure if the avs2s have the check ball

[PontiacJim1959]

Originally Posted by 67gtospud

For the time being, I connected the vacuum advance to the ported vacuum source on my rear carb. I test drove the car and 90% of my hesitation is gone. I still have a slight hesitation on very light throttle. For example; if I'm cruising at 50 and very lightly bring the car to 60 it'll stumble for a split second. I'm wondering if the larger pump nozzles will cure this??

More Initial timing. Bigger cams like more Initial timing. Get it up near 20-22 degrees at idle. You can use the vacuum can hooked directly to the manifold or manually set it and disregard the vacuum can for now due to it being the incorrect one for your vacuum.

If you can set your Initial around 20 degrees on the crank balancer with the vacuum can disconnected, the next thing to do is shut off the engine and try to restart it. An Initial setting that is too far advanced will cause hard starting - ie the starter will really strain to kick the engine over. If it does this, then the Initial is too far advanced. If it does start OK, then the 20 degrees Initial will work.

However, you want to then make sure that your mechanical advance from the weights in the distributor ONLY provide another 14ish degrees of advance for a Total mechanical advance of around 34 degrees, 20 Initial + 14 Dist. Weights = 34 Total at "X" RPM's.

You may need a limiter bushing to keep distributor advance to around 14 degrees. Your distributor may use these bushings if aftermarket?

Now if the engine does not crank easily at 20 degrees Initial, then to get to the desired 20-22 degrees Initial, you will have to get, and use, a vacuum can that will make up the difference when you drop back from 20-22 degrees. Try 12-14 Initial degrees. What that means is you now need to make up another 6-8 degrees with the vacuum can. BUT, it has to be all pulled in by your engines 7 Hg of vacuum you say you have.

The best may be the B28 vacuum can, it begins to advance the distributor timing between 3-5 Hg and is all in around 8 Hg. The amount of added advance timing should be in the range of 5.57-8 degrees based on the vacuum signal from the engine - which is what you want if you set your Initial timing at the balancer to 12-14 degrees at idle.

Next up would be the B20/B26 vacuum can, begins around 5-7 Hg and is all in around 8 Hg and provides 11-13 degrees of advance - which means you want to set your Initial timing down around 9-10 degrees at idle.

Each one of these different cans, B28/B20/B26, will require you to set your Initial timing (vacuum advance disconnected) at idle to a degree number that will jump up once you reconnect the vacuum can, and change the Initial timing (vacuum advance now working) to the 20-22 degrees, ie 9 Initial + 11 vacuum (B26) = 20 degrees, OR, 12 degrees Initial + 8 vacuum (B28) = 20 degrees.

Then adjust your weights in the distributor (either as is or possibly the need for a limiting bushing) to provide your Total mechanical advance, ie 20 degrees Initial + 14 distrib. weights = 34 Total at "X" RPM's (34 degrees Total is just an example on my part as your engine may respond better with more or less Total - the key being NO PINGING/DETONATION under wide-open-throttle).

So you may want to experiment with your timing and timing curve using the different vacuum cans.

Also, the Edelbrock guide for the AFB says this about flat spots and surge at cruise:

CRUISE MODE - If there are any surge or flat spot conditions in the steady speed cruises or light accelerations, a lean condition probably exists. Consult the Calibration Reference Chart and change to 1 stage rich in the Cruise Mode. If it gets better, but not completely fixed, keep going in the rich direction. The Calibration Reference Chart will give you Rod and Jet combinations that are directionally correct or you may choose your own.

BTW, the accelerator nozzles only seem to come as 3 in a kit.

[Kenth]

Originally Posted by 67gtospud

For the time being, I connected the vacuum advance to the ported vacuum source on my rear carb. I test drove the car and 90% of my hesitation is gone. I still have a slight hesitation on very light throttle. For example; if I'm cruising at 50 and very lightly bring the car to 60 it'll stumble for a split second. I'm wondering if the larger pump nozzles will cure this??

On very light throttle, no.
Trying to ignite a too lean mixture by setting timing far out of the scale will never result a proper running engine, it´s a bandaid and the engine will never reach it´s full potential.
With a properly set up idle/lowspeed circuit for the current intake vacuum it is rarely needed to alter main jetting and factory ignition settings.
Hunting a timing curve to suit the too lean idle/off-idle/lowspeed circuit can be very frustrating, time consuming, give you new gray hair, a bunch of unneeded parts and an enriched vocabulary, anything but a proper running engine.
See posting #3.

FWIW