factory 2X4 vs homemade set up - flow?

I'm setting up a 2X4 for my '55 Mercury using a vintage Edlebrock Y-block manifold (257) and two '56 Mercury Holley teapots. I know the factory '57 E-code 2X4 carburetors had restrictors on the boost venturies to reduce the flow throught the primaries. Basically, thick rings on the outside of the boost venturies. I've only seen these in drawings - not real life. Supposedly this was to help low speed driveability. From what I've read, the factory carburetors would flow about 270 cfm each in a 2X4 set up. The normal, single 4 bbl would flow about 350 cfm.

So, could I (or should I?) reduce the flow of my two '56 carbs by making a pair of restictor plates with smaller primaries? Or should I just wait until I have the whole thing on the car and see what it does (or doesn't do!)?

 

you're the Rocket Scientist....

you had to know that was coming....


but I would say go ahead and run them first, you can always go back and restrict them if that's what you think needs to be done...

 

Atleast I don't have to worry about the whole thing disintegrating in a ball of fire if it doesn't work! I wish I had access to a fowbench. Maybe I should build one of those instead of my Doomsday Machine! Tough decision, though - flowbench or Doomsday Machine?

 

Two 350cfm carbs sounds a little small on a 2x4 set up. Most 2x4 setups can run nearly twice as much cfm as a single carbset up. Meaning that if the car runs nearly perfect with a single 500cfm, two 450 cfms will usually get you a near perfect set up.
I don't know exactly why this is, but a factory 401 Buick ran twin 625-650cfms with only a very slight cam change and distributer curve change from the single 4bbl cars.
Similar set ups with the factory sbc setups, dual Rochesters etc...
If you do a little work to the motor, recurve the dist, and add headers, I don't think the twin 350cfm carbs will be any trouble (once you get them tuned). If you are concerned about low speed 'tip in' performance, run them on progressive linkage.
I am sure a factory set up for Mr Average back in 1955-1956 was probably tuned down a little for drivability and fuel economy. Not many people were ready for 6-8 mpg etc...

 

Don't confuse a Nailhead dual quad setup with others. Nailheads like a lot of carb, more than most other engines. A lot of engines their size like a 500-650 carb, where the hot setup for the Nailhead is the 850cfm Quadrajet for a single carb. When I had dual quads on a SBC it really was a chore to get the 2 500's to act right. If you look at a bunch of older setups they're running 450's and less...

 

Excerpted from a book: The engine is in a yellow 54 Ford convertible.

(An excellent combo that works from what a couple of Y-block runners have told me.)


"What she was doing, was pulling the air cleaner on the front carb of a dual four barrel, teapot carb setup and tapping on the float bowls with the screwdriver handle. After a bit of that, she replaced the air cleaner, got in the car and after a short bit of cranking, it fired right up.
Leaving the hood up and the engine idling, she put the tools back in the trunk and walked up to the front of the car where she was going to close the hood.

The slight lope the engine gave off at idle got me curious enough to get out and look under the hood. I saw the dual Teapot carb installation and the cast aluminum Thunderbird rocker covers along with one of the weirdest headers I ever saw. What I was looking at was what would later be called a Tri-Y header. She didn’t say anything and stood there while I looked. When she figured I’d seen enough, she shut the hood and without a word got back in the car.

~~~~~~~~~~~~~~~~~~

. . . we were flat amazed she remembered all the details - Earl and Johnny, found a good, rebuildable engine, a 292 out of a wrecked 55 Ford police car. It was bored .060 over which knocked it out to 301 cubic inches, had a pair of ECZ-G heads that were ported and fit with 1.64" 368 Lincoln exhaust valves along with a pair of hand built Tri-Y style headers.
The cam was an Isky E2, the very same cam as the Ford EDB cam, which was supplied by Isky anyway. Intake was a factory dual four barrel manifold fitted with a pair of the #4000 teapot four barrels that came on the Lincoln’s. Preferred, as they were bigger than the Ford Teapot carbs.

The Teapot carbs used to drive the car guys nutso as the common belief that they didn’t work was shown by the good running, overbored 292 to be a fallacy. Even with the fairly big cam, the car ran well and idled reasonably smooth, all things considered. It was the ideal combo for the automatic trans pulled out of a wrecked 57 Ford and bolted up behind the overbored 292.

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I agree with the guys, try the carbs as they are.

A small fwiw, little brother and I built an overbored 312 for his 54 Ford two door and he ran a single Holley on it at first.

Later on we got a dual quad intake, it may have been factory, but I'm not sure there.
The carbs were small 4 bbls off a 56 Plymouth, the Plymouth ran them as singles.
I want to say they were AFBs, but they had four metering rods per carb . . . Quadrajet?
Ran good fwiw.

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I think the dual quad setups run well with bigger carbs because the carbs are designed to run well at low and mid-rpm levels.
The carb is seeing airflow equal to these rpm levels even if you're at full throttle and not near top rpm.

Also the reason why multi-carb setups use stock or close to stock jets.

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Most here know it, but I keep seeing references on other boards and in person where a guy jets a carb down to make it a smaller carb.

All they've done - if they've actually done it, a lot of 'tuners' seem to have done most of their tuning in their imagination - is to set the carb up terribly lean in most cases and then wonder why the engine runs badly.

Little engines run fairly well with big carbs, but that's usually a temporary expedient until the owner can get the right carb.

Same deal with those who disconnect accellerator pumps.
The bog they now experience is simply a large mass of air without the extra fuel to get it over the lean area until the main metering system can start to draw fuel.

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Run a choke.
They work fine on the rear carb only and that's were the factory usually put them.

Chokes on both carbs work ok, I did that one with my dual quad setup, but it's not necessary.
An easy hookup though.

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A bit of rambling along this morning and I'll just add, run straight linkage and see how you like it.

If you do go to a progressive, get an Edelbrock.
They make an excellent one and it's easy to copy at home for a gal with a mill.
Get a lathe yet?

There are a lot of progressives out there that are junk, pure and simple and are in fact, dangerous.

 

Thanks for the input Jay!

One of my biggest concerns is a decrease of vacuum signal through both carbs causing the power valve to open too early. I thought maybe some restrictor plates would allow more vacuum to build. Faulty thinking?

I have both chokes installed but I'll set them to remain open. It's plently warm here! On my present single teapot, one or two stomps on the accelerator and a hold open will get the motor started.

I now have a little lathe to go with my little mill. So, I made my own straight linkage. I even added an extra return spring for added safety.

 

Good for you.
A lathe of any size is a great help in building a car.

Double springs for throttle return are a good idea.
Be careful you don't get them too strong and you wear your ankle out simply driving the car.

The commercially available one spring inside the other double springs are an excellent way to go and throttle pedal return pressures aren't bad at all.

 

"a decrease of vacuum signal through both carbs causing the power valve to open too early"

Yes, at a specific throttle blade angle there will be less vacuum, but it just means the throttle position for cruising will be different. Much more important to see where the valve actually opens in use. Attempting to modify a booster opens a huge can-o-worms, and changes many things at once. Last resort only.

"I have both chokes installed but I'll set them to remain open. It's plently warm here!"

Really? It's 180 degrees? That's how warm it has to be before no choke is helpful. If it "starts just fine" with no choke", your primary jetting is way too rich.

There is no relationship between engine brand or type and "how much carburetor they like" - another myth that just won't die.

 

I'll go along with you on the brand bit, but type indicates to me whether and how much the engine is modified.
As well as the engine CID has a lot to do with carb size choices . . . although I don't believe in the sizing chart thats been kicking around forever.

An ok place to start, but you can run quite a bit larger carb than the chart calls for with no problems....

 

Oh, yeah it's not the brand, but the design, the heads, the intake manifold, etc. etc. How much velocity through the ports. Compression. Vacuum signal.

bingo...


Nailheads like a lot more carb than other engines of similar CID... is that better?

 

Reading...non-fiction...some stuff from Dave Vizard. Read especially his carb and manifold book.
The more you can increase flow through a carb WITHOUT enlarging venturi, the better...
he was able to bump stock venturi size carbs way past their original flow numbers by removing minor restrictions, sometimes enlarging throttle bore. I think the rings should go, along with all minor divots and joints in castings. Whatever you do, his writings will likely help shape your thinking on carbs and flow. I think those rings as you describe then restrict without actually reducing the critical dimensions of the actual venturi walls...or the inner booster venturi.
I think Holley introduced the big boosters on the last developments of the 94 type. Vizard uses them to increase signal on otherrwise too big Holley 4150 types, giving massively over-carbed drag engines better driveability and response without hurting the top end.

 

Power valve is almost a separate issue, a secondary tuning point. You have to see what vac you have and at what point, then you choose your best guess at a power valve number based on the vac curve you are seeing.
I think the '56 vintage carb will still have provision for Lodamatic, handing you a neat tool: The vac port connects to both venturi signal and ported vac. Block the branch leading to ported vac, hook up a sensitive gauge, and you can directly read venturi depression/signal and so one of the carb's most important mysteries!

 

Funny thing is, the big Buicks seem to like lots of carburetion.

Kenne-Bell recommended I run two 750 Carters when I built the 32's engine, but since I was aiming for a strong mid-range I chose 500 Carters.
I have to agree with K-B, it would have done ok with a pair of 750's.
Got a couple I'm going to try when I get the 31 up and running.

I think the oversquare aspects of both the bigger Nailheads - 401 & 425 - and the later 430 & 455 have a lot to do with the little fact that both series of engines run great with lots of carb.

 

So?
Solve?

Not trying to second guess you here, but I got confused.

 

Since we're getting into other - and interesting - areas, perhaps Chris would like to read this.


Spark Advance Hop-up Trouble Spot By Barney Navarro
Among the mistakes made in hopping up engines, few exceed in number the misapplication of spark advancing principles. The chief source of error is the limited information available on the subject of spark lead. That which is distributed, unfortunately, fails to cover some essential factors and very often is no more than a comment to the effect that fuel charges take a certain amount of time to burn so spark must be advanced enough to compensate for the time lapse.
Well informed engineers wish that the problem really was that simple. Most ignition system purchasers overlook every factor except the amount of spark produced. The wrong system can cause plenty of trouble: plug fouling, poor gas mileage (even though the engine has no tendency to misfire), overheating in slow traffic, and other maladies. Basically, engines require some means of advancing spark timing as rpm increases since the pistons, in effect, try to get ahead of the burning speed of fuel charges. Combustion, witch takes a definite length of time, must occur when pistons are at the top dead center before the start of the downward power stroke. If burning finishes too early, energy is wasted because the resultant pressure rise produces a force opposition to rotation. This is readily apparent when starting an engine that has too much spark lead; it will actually kick back against the starter’s efforts. Modern high compression engines, while under full load, audibly indicate spark that is too far advanced by pinging. So the popular method of setting spark timing for maximum horsepower is to set it just below the ping point under full throttle operation. Distributors that employ flyweight governor advance mechanisms use a spark advance curve that conforms to the engine’s requirements under full throttle at any point within the rpm range. At low rpm a lesser spark lead is required so the governor advances a small amount. As speed picks up it advances more and more, always conforming to the full throttle full load requirements. On a drag machine, where full throttle and full load conditions are maintained, the flyweight governor is required. But for ordinary driving, which consists mainly of partial throttle operations with Very light loads, it is not enough. Some other means of compensating for varying loads must be provided. The load compensator is necessary because a light fuel mixture burns more slowly than a heavy charge since the concentration is less and flame takes longer to travel from one fuel particle to the other. If the utmost energy is to be obtained from light charges, their burning should be completed at the same point that the heavy charges finish. So if they take longer, the only way to make them finish at the same point is to start them earlier. Consequently, partial throttle partial load operation requires more spark lead at any given speed than is required at full throttle full load. Load compensation is the most commonly achieved by using intake manifold vacuum to actuate a diaphragm. This diaphragm advances and retards the distributor breaker plate and in some cases the whole distributor case. When the engine is operated with Very light throttle pressure, the manifold vacuum is high, so the diaphragm advances the spark timing to produce the most efficient combustion possible. As the throttle is depressed, the vacuum drops of and the diaphragm produces less advance until it reaches a point of being completely ineffective at wide open throttle. Thus the ideal load compensation is always maintained and results in more power from every drop of fuel.
The second most popular method of obtaining load compensation, though further from perfection is that employed in Ford V-8 distributors from 1932 trough 1948. Instead of a diaphragm, there is piston brake actuated by manifold vacuum, The flyweight governor mechanism is equipped with a breaking disk which cancels five degrees of the governor’s advance when pressure is brought to bear on its edge. At this edge a spring-loaded piston is located in a small cylinder. The spring is on the side of the piston opposite the disc so it causes the piston to be pushed against the disc. Vacuum is introduced on the spring side to oppose its action and lift the piston off the disc. In action, the high vacuum produced by operation with small throttle openings lifts the piston of the disk, allows the full action of governor weights to take effect and gives the Ford engine five degrees more spark advance. By depressing the throttle further, the manifold vacuum drops off and the spring again pushes the piston against the disc to retard the spark. The flaw in the operation of this mechanism lies in the fact that it is either "full on or full off" and permits no gradual compensation like the diaphragm.
Ford’s latest method of controlling spark advance employees an ingenious system utilizing manifold vacuum and venturi vacuum. With this system the flyweight governor is eliminated and in its place is nothing but a diaphragm. This diaphragm not only advances the spark to conform to rpm changes but is also makes load compensation adjustments. All ´49 through´54 Ford and Mercury carburetors have in addition to the conventional manifold vacuum takeoff, such as is found in the throttle body of most passenger car carburetors, a connecting venturi vacuum passage. The manifold vacuum, as usual, is obtained from a small port in the throttle body located slightly above the butterfly’s closed, position, on the side where the butterfly swings upward to open. When the throttle is closed at idling, the vacuum port does not receive vacuum because it is on the opposite side of the butterfly. As the throttle is opened slightly, this port is uncovered and a vacuum is applied to the distributor diaphragm to advance the spark. If the throttle is fully depressed, the manifold vacuum is destroyed and no advance takes place. As speed increases, however, the venturi vacuum increases gradually and advances the spark to conform to the rpm. Letting up on the throttle increases the manifold vacuum (Provided it isn’t let up all the way) and the spark receives load compensation. A balance is always maintained so that the correct amount of spark advance is supplied for all speed and load conditions.
The greatest installation errors center around the misunderstanding of the late Ford distributors. A distressingly large number of mechanics are unaware of the difference between manifold vacuum and venturi vacuum. In fact many attempt to operate Ford and Mercury distributors by connecting the vacuum line to the windshield wiper connection on dual intake manifolds. This sometimes results from a desire to use the old Stromberg carburetors, which are not equipped with vacuum takeoff. So the simple solution seems to connecting the distributor vacuum line to the handiest apparent source of vacuum. Such practice is worse than having no spark control at all for when the engines idles the spark advances fully and retards as throttle is depressed. There is no venturi vacuum available to advance the spark as the speed picks up and it remains retarded until the throttle is let up. So if the old style carburetors are preferred, the stock Ford distributors must be discarded on the late models. However, Stromberg has resumed production of the old "97" and is now fitting it with a venturi vacuum takeoff to make its use feasible.
Four throat carburetion installations also have had their share of improper distributors. Early articles in certain publications gave the impression that no vacuum control whatsoever could be tolerated. It wasn’t pointed out that the only forbidden type is that of the stock `49 through `54 Ford and Mercury distributor. This caused many to purchase distributors and magnetos that were equipped with flyweight governors only. Such installations get very poor gas mileage, so the car owners blame the four-throat carburetor. Even more irritating, is the tendency for spark plugs to foul. Having no load compensation, the spark is never far enough advanced under partial throttle to fire the fuel mixture charges at the most opportune time. In effect, the engine is being operated with a lower effective compression ratio because burning is completed as the pistons travel down the cylinder bores. And since the plugs never receive a hot flame, soot collects on them. Furthermore, the condition cannot be remedied by using hotter plugs because they will burn up under full throttle operation of flyweight governor distributor with vacuum-operated load compensation device.
In practice, the installation of dual intake manifold on Fords and Mercury’s of the ’49 trough ’54 series should be accompanied by a change in distributors such as prescribed in the preceding paragraph. The addition of two carburetors divides the airflow so only half as much airflows through one carburetor as previously at normal operating speeds. Venturi vacuum is dependent upon the air velocity through the venturi so any reduction in velocity will result in less spark advance. And connecting a line to each venturi vacuum takeoff of a dual set up will not increase the vacuum---such a practice is just a waste of copper tubing. The best advice to keep in mind when purchasing a distributor is not to pinch pennies. An inexpensive unit, if it doesn’t do the job correctly, can prove to be the most costly. The best way to avoid mistakes is to study the problems involved and learn enough about them so that you can select a distributor that matches your engine requirements.

 

Oh! So, if I could get my hands on one of those elusive E-code carbs, I could really quantify the difference between standard carbs and true dual quads! Sizing those boost venturi rings would be easier with a target vacuum signal!

 

Thanks Jay! The Load-O-Matic distributor has been the source of alot of confusion and poor running Y-blocks. It may have even contributed to alot of the misunderstanding/doubts of the Y-block's performance potential. The only real reason I changed out my Load-O-Matic for a '57-up dual advance was (at the time) Pertronix didn't support the early distributors.

 

Lose the Load-O, one of Holley's worst ideas ever...but that port to the venturi is an interesting tuning tool. If this carb is the ojne I think it is, the venturi depression also works the vac can motivating secondary throttle, telling it when primaries are working at max! Pretty sophisticated spin-off from the Loadamatic thinking.

 

Here's an addition to the Got Time? article I wrote a while back.

The Got Time? article is in the archives.

It covers basic timing systems and this piece is the result of some curiosity and a couple of vacuum gauges.


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More about vacuum sources and timing.

What we’re dealing with here is in effect a variable venturi. At least it is as far as ported vacuum goes. The variable venturi bit due to throttle blade position.

I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment.

The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm competition carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. It’s a smooth cam and the car when warm idles @ 19" vacuum.
The dash carries a large (2 5/8") S-W vacuum gauge which compares favorably with the vacuum/pressure test gauge I have.

Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV).
The car runs very well on 87 octane in summer and winter and does not overheat in traffic.

Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm.
Ported Vacuum (PV) read 12" on startup.

Once the engine warmed up, MV reads 19" and PV reads zero at about 500-600 rpm.

Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV.
Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration.

Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read
10".

Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero.
The key thing is, at idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero.

The lack of additional timing at idle is what creates an overheating problem in the GM engines.
It takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating.
Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level.

There’s a lot of confusion out there about timing, both centrifugal (mechanical) and vacuum as well as the vacuum sources to use.

The key thing is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load.

To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister.

I’ve been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source.
Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and prefer to throw money at the problem.

As far as spinning up a little experiment, I’m not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with.


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An additional note; for those of you who live at a higher altitude than where these tests took place, you’ll find that your vacuum levels at no-load (idle) rpms will read lower.
To the tune of a 1" vacuum loss for every 1000' of altitude.

The tests took place at 350' altitude and manifold vacuum at idle read 18.5".
After moving to Sunny Arizona and ending up at 3300' altitude the manifold vacuum at idle now reads 15.5".
Highway figures and under load vacuum levels remain the same.

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This combined article was written from research on timing figures and real world experience.

You can learn a lot by taking the time to set up some inexpensive instrumentation and taking the time to run a few simple tests.
What you’ll gain is a better understanding as to what’s going on with your engine and gain a small bit of education about the particular thing you’re researching.

Learning from books is one way to do it and there’s nothing quite like taking advantage of what smart, experienced and educated people have done and written down for your educational pleasure.
I’m not including myself in this group.

What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us.
It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much.
Thing to recognize here is the factory engineers know a helluva lot more than we do and get into sophisticated areas that the great majority of us know nothing about.

Keep in mind too, factory engineers are constrained by the bean counters, the necessity to build a vehicle that is useful to the majority and seldom are let loose to pursue a dream or even an interesting idea.
When they do get the freedom to investigate particularly interesting areas, the results can be astounding.

The name, "Zora-Arkus Duntov" should ring a bell....

 

A question...this high-tech late model equipment is getting off the edge of my research areas...but I've been reading an Army manual that covers a truck (governed) version of what I think is the carb we are discussing. This is the Holley with all the float and metering stuff in a pod over the throats...do yours have vac diaphragm secondaries?? Do your secondaries have very tall secondary/inner venturi tubes that stick way up??

 

Originally Posted by Bruce Lancaster
and so one of the carb's most important mysteries!


So?
Solve?

What I'm getting at here is the issue raised of possible over-carburetion; if you can read venturi depression on a gauge, you know something about how the engine is getting along with a given amount of venturi are/airflow capacity. If at high RPM venturi signal is real weak, engine has more carb than it can use...it can't efficiently draw gas from venturis that aren't properly "excited".
Also, when the venturi signal becomes strong enough to draw fuel is important. Until there is significant drop at venturi, the car is running on its off-idle slots with poor control of mixture. Once it transitions to the main circuit in the venturis, fuel flow can be pretty accurately controlled. A severely over-carbed car spends most of its time in off-idle mode, and has poor response until it starts moving enough air to get the main circuit flowing. The Loadamatic venturi connection provides ready made a source to read pressure that would have to be drilled and improvised on most carbs for an engineering study. Just plug the leg that coes down to throttle area and hook up a suitable range gauge.

Not trying to second guess you here, but I got confused

 

Sounds like your manual covers the Holley 2140 or a very early version of the 4000. The carbs I'm using have the vacuum actuator for the secondaries. But, the secondary supply tubes actually connect to the top of the bowl. Why these carburetors got the "teapot" moniker, I'll never know! They look more like espresso machines!

 

Yes, 2140...a truck version of 2140. So, irrelevant for illustration here, I guess.

 

Can I get the same reading from the vacuum secondary source? The reason I ask is I've already drilled out the plug and installed a brass tube in the backside of each carburetor. I'm going to run a balance tube between the two carburetors like the E-code did. Since I'm using the later dual advance distributor ('57-up), I installed a dummy vacuum valve ( it looks like a modern Holley power valve with no guts) in place of the Load-O-Matic control valve and soldered up the old fitting that connected to the distributor. I can always swap a control valve back in and another fitting to source the vacuum if needed.

 

Possibly...I don't have any carbs like yours or any manuals that recent, so don't know where it sources its reading. Stick the wand from a can of carb cleaner in there and see where it comes out...
Some later Load-O-Matics circuits have a plastic ball in there somewhere that gets yanked around under different loads/sources...probably fell out and rolled under a workbench decades ago, but be sure you don't have this as a mystery factor when reading depressions.

 

I also keep a long piece of very small copper wire handy for tracing passages...just a strand unraveled from an electrical wire.

 

.

Thanks for the info,very good read

 

I just had a little bit of a "eureka!" moment! If I establish what the vacuum signal level to rpm range is on a single carburetor set-up, a dual quad set-up should follow the same plot. Atleast to get the same level of driveability.

It looks like my original idea of a pair of restrictor plates between the manifold and carburetors won't really work. To get the vacuum signal from the venturies to increase, the restriction would have to be near the top, around where the boost venturies are. I guess that's why the E-code carburetors have those little rings around their boost venturies! Atleast by knowing what the vacuum signal should be, by a little trial-and-error I should be able to duplicate it in my dual quad set-up.

A flowbench would be really handy, now!

 

You have a safety valve on the with the indirectly opened secondaries. Get the right balance between whatever spring they have and the signal that opens them and you won't have to restrict airflow. They will open to what the engine wants, at least if optimum adjustment can be attained. different tech, but same possibilities as the Q-jet, which was used happily on engines from 230 CI to 500...
If you can use those passages to study the available vac to the secondaries, you can then artificially apply that drop with a hand vac pump and see how much secondaries open directly...that will give practical results without actual knowledge of CFM.