Technical Flathead Dual Carb Questions

Always wanted to eventually install a dual carb on my 59A, and about a month ago I found an Edmunds one at Hershey, since then, been doing a little reading up on the subject. Seems thoughts/setups are as varied as the different types of manifolds out there. I’m more interested in actual physical technical/scientific evidence for the reasons/opinions folks have. I’ll list the questions below:

1. What are the performance differences in the 2 x 2 manifolds in terms of carb location and why?

I chose this particular Edmunds style intake because I wanted to retain the stk generator location. Looking at the openings for each of the carbs, the first carb is almost centred in the middle of length between the first and last intake openings into the block.
I also read up on single vs dual intake manifolds, and then mapped out the p***ages from each carb opening to each intake opening. Seems it is of a dual plane design which makes sense from what I read, but then it made me wonder about my next question.

2. Were there ever any single plane intake designs ever made/tried?

3. Progressive vs straight linkage advantages and disadvantages with this particular Edmunds intake?

Seems from what I’ve read, the overwhelming majority opinion is to use a straight linkage setup, but most who say this don’t seem to make a distinction between the different types on manifolds. I’ve come across posts from @tubman here stating the style of intake I have could work with a progressive type linkage. I’m curious to know more about a progressive style linkage as Charlie from Vintage speed sells them, and had one vid on YT where he talks about a progressive style linkage that he sells. In the vid it appears to be a “super” style intake, and IIRC, he states the rearmost carb should be the primary with the front one acting as the secondary, with it opening when the primary is at 65% of full throttle.
Looking at the style of intake I have, it would seem to my pea brain that it would make the most sense to have the first carb act as a primary with the rear acting as a secondary due to the geometric symmetry of the intake openings. My gut though tells me a straight linkage would make more sense, but again interested in actual facts/theory as to why one style would be better than the other.

5. Use of power valves vs no power valves and size selection?

Again, opinion seems varied, with folks stating they run dual carbs with the power valve ports blocked off, while others maintain they’re required. Really didn’t find much of any evidence/theory to support either opinion.

6. What are the steps to “tune/synchronize” a dual carb setup?

Didn’t really find much info on this at all. Even when I did find one post on here in a thread, it didn’t detail the steps to “balance” the vacuum readings. Seen posts/threads where folks say install one carb at a time, to “balance/tune” them individually, then repeat when both are installed. That seems a bit weird to me, but then again I know nothing about tubing multi-carb setups.

7. What is or should be the process for selecting the correct size jets?

I’ll be using rebuilt Holley 94s. I realize the selection is likely very engine dependant, but have read that it seems to be mostly a trial and error type thing, which makes some sense, but there should be a “baseline” starting point (I ***ume).

 

Syncing the carbs will only be important if you don't have progressive linkage. There is a tool that checks the suction of each carb to sync them. But first you must make sure the engine will run with one carb working by putting you hand over the other carb and then doing the same with the other carb, the engine should run. Since you want to use 94's, you will definitely need to get the right power valves as suction is what keeps the power valve closed. If you used 97's, they have a mechanical power valve. This is some information but not everything you will need to know.

 

I would advise straight linkage.

 

With two carburetors you want straight linkage. Otherwise you may have a lean condition in the cylinders that are farthest from the primary carb.

If you had a triple carburetor intake, you could run progressive linkage. With this setup the middle carburetor is most likely to be the primary and the front & rear secondaries.

Tuning suggestions will only get you started. It will be up to you to figure out what the engine/car needs. This can only be accomplished with trial and error, and more trial and error.

 

Typically, there are three reasons to use multiple carburetors: (A) more fuel air capacity, (B) better average cylinder fill density, and (C) eye candy.

(1) I have never attempted to set up the manifold in question; so no personal answer. It would seem not to be as efficient as a manifold with evenly spaced carburetors, and would seem to violate (B) above.
(2) I don't know.
(3) With a dual carb manifold with evenly spaced carburetors, then straight linkage, progressive linkage violates (B) above. With the manifold in question, don't know.
(4) There is no 4 ???
(5) With evenly spaced carbs on a dual manifold, then power valves. Selection, especially with the Holleys which are vacuum actuated, is trial and error. If you were using Strombergs, the stock power valves for the engine as a starting point. If you must use Holleys, no comment. EXTRA: on a triple manifold AND progressive linkage, power valve on primary, no power valves on secondary carbs.
(6) Synchronization

(A) Make certain that you have two IDENTICAL carburetors (check that the tag numbers are exactly the same, except for the production date), and that the carburetors being used are the approximate proper size for your application. If this condition is not true, stop reading and start looking!

(B) Screw the idle mixture control screws in on both carbs until they lightly "bottom". Check the carburetor manufacturer's spec for idle adjustment, and set both screws in the middle of the range. Example: if the spec is 1 to 2 turns, then use 1 1/2 turns.

(C) Set the throttle positioner screws higher than normal (so the engine will start and run at a high idle).

(D) Install both carburetors but NO linkage.

(E) Acquire a manometer. Uni-syn is a brand name that is readily available. Try the local motorcycle shop.

(F) Start the engine, and run at a high idle until the engine is at normal operating temperature, and that the chokes (if used) are completely off on both carburetors.

(G) Reduce the setting on the throttle positioner screws approximately an 1/8 of a turn at a time on each carb until the idle approximates desired RPM.

(D) Open the center adjustment on the Uni-Syn to the wide open position, and set the Uni-syn on the carburetor of your choice (it makes no difference). Adjust the control knob such that the plastic bobber is directly in the center of the column.

(E) Remove and replace the Uni-syn and verify that the engine does NOT change RPM with the addition/subtraction of the Uni-syn.

(F) Move the Uni-syn to the other carburetor. Adjust the throttle positioner screw of the second carburetor such that the plastic bobber is directly in the center of the column. When the carbs are synchronized, one should be able to move the Uni-syn from carb to carb with no change in RPM and no deflection in the position of the plastic bobber.

(G) If the idle is too high, again adjust the throttle positioner screws on both carbs to the desired RPM, and repeat the synchronization process.

(H) Install the linkage between the two carburetors. Make certain there is NO movement of the throttles.

(I) Install the linkage from the footfeed. Again make certain there is NO movement of the throttles.

(J) With the throttle linkage installed, verify the synchronization. If adjustment is needed, remove the linkage, and start over.

(K) Once synchronization is complete with throttle linkage in place, have an ***ociate move the footfeed inside the p***enger compartment to WOT, and visually ensure that the carburetor throttles also move to WOT.

(L) Done!

(7) Unless one is duplicating a KNOWN working setup, on an otherwise basically stock engine, ALWAYS start with stock jets for the carbs, and tune from there.

Jon

 

You’ve literally asked questions that have been answered here for the past twenty years.
I’d guess you will find volumes of information on the technical aspects of carburation that includes jet and power valve sizes, tests on the best intakes and the advantages of using dual intakes.
Here’s some starters.
https://www.google.com/search?q=fla...MwgcGMC4yLjEyyAc5&sclient=mobile-gws-wiz-serp

 

On my 250 inline six, I use twin Carter W-1 single barrel carbs. For metering, I started with factory settings. I use straight linkage, mine are set apart so the front three cylinders use one carb, the rear three use the other one. Everything about the carbs is identical right down too using a dial indicator to set metering rod depth.

Using a Uny-syn balance tool, you can fine tune the air flow through each carb. Its not 100% due to the design of the tool and the little red float thingy, but you can get real close. What you want is both acting as one big carburetor. I don't bother with a vacuum gauge, I didn't see enough difference to tell if they were close or not. The balancer tool is way more accurate for me. It's a lot of back and forth trying to get the idle speed set and still have equal air flow. I also use both throttle stops, I've seen some where they use the linkage to control the second carb idle. Linkage needs to be tight so when one moves, the other does with no delay. My Uny-syn tool wasn't working to good till I pulled it apart and cleaned all the ports, they had casting flash in them, I also opened up the holes. Before I had to have the baffle almost closed to lift the float, now it will lift the float with way less restriction making for more accurate readings. I set the baffle so the float is about 3/4 of the way up the tube, level with one of the lines on the tube. Then move it to the other carburetor and adjust the idle speed till the red float is in the same spot. If the idle is too fast or too slow, you must turn each carb idle screw, then recheck the air flow. Motorcycle balance tools use four separate vacuum gauges, but each carburetor is on its own intake port. The vacuum on car engines is the same at each carb due to the single plenum, that's why the vacuum gauge setting doesn't work very good, at least mine didn't.

My metering is still close to the factory setting, I believe the jet is still the one they came with. The Carters use mechanical enrichment by way of metering rod and linkage, the rod is a 3 step design. I change the rods to fix mid range and high speed. I do have a A/F gauge mounted on the steering column to watch the ratio, it was great time saver when I was started tuning them. 1/16 of a turn of an idle screw will change the A/F ratio 1/2 or more of a point, if I turn both a 1/16, the ratio will change about a full point. It makes for easy setup and adjustment.

Just be sure to start out with two identical carburetors and take your time setting them up, it makes a difference. I tried, running one, then the other for tuning, but found just using both worked the best. If they are set identical, it should take right off and go.

With less air flow through each carburetor, your power valve ratings will be lower. The vacuum will still be high, just as a single carb would be, but with two carbs, it drops much quicker and reacts different. Not sure how much lower you need to start with, but 1/2 might be about right.

 

I have been experimenting with a progressive setup on my Ford the last year or so. The Offy manifold has the front carb pretty much where the stock one was and the carbs are very close together. These are Stromberg 81s and a stock 221 with headers. It runs better than it did with straight linkage and spark plugs don’t show any sign of difference in cylinders to me. On a wider spaced manifold like an Offy Super progressive would not be a good idea.

 

So, me trying to be the engineer that I am, went to do some digging.

An engine is essentially an air pump, and CFM can be calculated using the following formula:

CFM = (Engine displacement (cu. in.) x RPM x Volumetric Efficiency)/3456

In my case, IIRC displacement is 244 cu. in.

***ume max rpm is 4,000

***ume volumetric efficiency = 50% (see AI summary pic)

CFM = (244 x 4000 x 0.5)/3456
= 141

Seems CFM rating for a Holley 94 is around 150 to 185.

So, in theory a single Holley should be sufficient for the “average” flathead. I think we probably already know that, and IIRC, I saw something stating a 4% increase with a dual carb intake. That is probably within the margin of error of any dyno.

So again, ***uming a single Holley is sufficient, my approach would be to use the front carb as the primary considering it’s centrally located on the intake, and then use the rear carb running as the secondary using a progressive linkage. This makes sense to me as it would only open when throttle position of primary was around 70%. A momentary boost of fuel for full throttle bursts. Being rear-biased, when both carbs are at full throttle, it would likely cause a lean condition for the cylinders at the front of the engine, but this would be a momentary situation. Does this make any sense or am I missing something?

 

As I see it,
All or most dual carb intakes are designed to evenly distribute fuel and air as the engine vacuum increases.
The differences are in the runner size and carburetor mounting location.
The varied carburetor location is and was used and changed to compensate for spacing issues of whatever extra components that were added or to improve a better mix of fuel and air.
IE… generators, fans, pulleys and blowers.
Installing progressive linkage with intent to run one at a lower rpm and bring both in at a higher rpm will lean the engine at low rpm.
There may be an intake with a runner that’s totally open that would allow even distribution. But why bother? A flathead and normal intake will handle two carbs in perfect sync with no issues on a straight linkage setup.

 

Thanks for all the replies. Will go over them in detail later tonight and tomorrow. As mentioned, I’m most interesting in facts and the science/physics behind operation and improved performance with a multi-carb setup. Lots of anecdotal info out there like PV installed vs PV not installed, etc which doesn’t help one in a making an informed decision.

 

Very nice setup. Really like the vintage vibe. I think I used the same Ford engine paint colour.

 

This guy was recommended by Uncle Tony when it comes to dual quad setups. If you skip to the 10 min mark, he talks about progressive vs 1:1 (straight) linkages. He recommends the progressive setup just as it was from the factory on some cars in the 60s that had dual plane intakes with dual quad carburetors. Advantages are drivability, fuel economy, and top end rush. Although he’s taking about dual quad setups, he mentions the key is the dual plane intake manifold design with each side of the carb(s) feeding each side of the engine. You can see this in my sketch of the Edmunds intake. So, according to him and the engineers of the day, the progressive linkage setup has the advantage with a dual 4-bbl setup. So, I would have to ***ume, having a dual 2-bbl setup on a dual plane intake would have the same advantages ***uming same/similar geometry in terms of carb location. He says he gets 16 mpg with a 600 hp motor in his 3,500 lb truck. That’s really impressive.

 

My point is that with a manifold that has one carburetor in almost the stock location and one behind it, you have rotten fuel distribution, no matter what you do. Might as well run it as a single (progressive linkage with he front as primary with the rear carb coming in very late) and have a car that's driveable 95% of the time. Jon (@carbking) said in his post that "there are three reasons to use multiple carburetors: (A) more fuel air capacity, (B) better average cylinder fill density, and (C) eye candy". This setup fits reason C perfectly.

Hell, you might even get a "whooshing" sound or a little "seat of the pants" push when you mash the throttle.

 

Definitely “C” for me, but at the same time interested in any possible perf gains and maximizing drivability and economy,. I should mention that I have an Isky 1007B grind on the orig cam to the engine. Going through more info, Ol Ron mentioned it was a good cam for up to 5K. Not sure exactly what he implied by that, but if it helped the engine breath better and thus obtain a higher max rpm, that’s interesting for sure.

 

"Pete" was a great resource. It was a bad day when he retired. I sure miss "Ol' Ron" too.

I put the MAX-1 I bought for the Merc in my '51 into my 258" "Hot Rod" motor. It sure runs nice, and I'm absolutely sure it's not too much cam.

 

Some interesting info in this long lost HAMB thread.

https://www.jalopyjournal.com/forum/threads/holley-94-101.186744/

 

My setup is quite similar to your concept. Same Edmund's intake, running an Isky 1007B cam and bored .60 over. My carbs are 97's so no power valve guess work. I ran straight linkage for the first summer but it seemed to bog down a bit on hard acceleration from dead stop. It seemed a tad too rich but I suspected the offset carbs also. Rather that mess with jets size I tried the progressive linkage and it solved the bog down. The second carb comes in at approx 60% and is wide open at full throttle somewhat like a 4 barrel does. I've had straight linkages on a few other flatheads and they worked great but the carbs were symmetrically centered in the intake...not offset like this Edmond's.

 

I like the "Super" dual configuration, so I have a Navarro "Universal" for the "Hot Rod" engine. It's a "Super" with exhaust heat. Straight linkage, of course.

My ultimate desire is for an Edmunds "Racing" dual manifold. It is the "Super" configuration, but is made for 4 bolt carbs. I p***ed on one abou 8 years ago because it was "too expensive" at $400. I'd put a couple of 2G's on it and run away from the rest of you guys.

 

I had a stock single 97 on my 21 stud, then put on a dual offset Edmund's like yours with 97’s using 43 jets/69 power valves (sea level) with straight linkage and synced the carbs. I changed nothing else in the equation going from single to dual. Definitely noticed a difference and put a lot of miles on that set up. When the 21 stud became “tired” i moved dual setup onto the 8ba I currently have in the car. Don’t over think it, but when you do default to the KISS method.

 

Thanks for positing. Good to know from someone who has same config (other than carbs). When the time comes, I will likely try both types of linkages.
Engine looks fantastic!

 

There is just something about an original patina hot rod that is inexplicably beautiful.
Won’t overthink it, but curious as all engineers are or should be. I have what I have already, so it’s decided other than linkage setup, carb jetting, and PV size/rating.
Not necessarily comparing apples to apples, but being a bit of a Mopar guy, I’m always intrigued by the 440 and 340 6-bbl setups. It makes sense that they are progressive linkage setups, and watching vids of Nick’s Garage, when he hammers on the throttle, it brings a smile to his face, and he says there is nothing like the feeling in the seat of the pants when those outboard carbs kick in.

 

Did you ever think about why the 340 and 440 got six-packs, but the hemis got dual quads ???

Jon

 

Do tell! Do tell!
I've noticed that but never given it much thought.

 

Finally got around to this “project” while in the middle of trying to buy a car for my youngest. Hopefully, the deal is completed next week.
Pulled out all the extra carbs I have. All are “Holley 94s”, but have slight physical differences and scripts/logos.

One carb has an “extra” port that matches the same port on the carb I currently have installed on the engine. I forgot what that port is for, but it is blocked off on the carb installed.

Carb installed.






“Matched” used Holley 94 with same “extra” port.

 

That port on the back is for the "Load-a-Matic" distributor advance on later ('49-'53) flatheads. You're running a 59A, so you don't need it; just plug it like the other one.

 

Of course, the second “matched” carb needs a complete rebuild which will include gl*** bead, paint, and a clear coat finish. The current installed carb, and the second “matched” one will be the ones going on the Edmunds intake, unless there is a need to block off the idle circuit on the secondary carb. I just discovered this factoid this morning, and then saw in the Vintage Speed on-line catalogue, he was selling new aluminum carb bases where the idle circuit is blocked off. He mentions these bases can be used on dual carb setups when using progressive linkage, which is what I want to try.



https://www.parts123.com/parts123/y...~S7C90BGKQR10428122880a~Z5Z5Z5~Z5Z5Z50000106a

Doing some searching on the interweebs about blocking off the idle circuit, I came across this ancient thread on the HAMB.

https://www.jalopyjournal.com/forum/threads/idle-circuits-dual-holley-94-set-up.165413/

So, this is now another rabbit hole I wasn’t expecting. For my non-expert brain, it makes sense to “block off” the idle circuit on my secondary carb if using progressive linkage. Not sure what “blocking off” entails though, but also makes sense that the throttle plates seal 100% against the body. I want to avoid spending $$$ unnecessarily if I can achieve the same effect with what I have vs. buying a new base. Plus, I still have two other spare carbs to play with.

 

As a bit of an aside, but related to this, I’ll mention that while doing research I came across this vid today from Vintage Speed. It made me laugh after the fiasco I went through with all the leaking fuel fittings I posted in this thread on the Barn. Seems Charlie is reproducing the very same fitting I posted in post #53.

https://www.fordbarn.com/forum/showthread.php?t=352302&showall=1



I’ll be ordering a few of those damn fittings for sure.

 

I think I’ll fab up my own hard lines with the remaining fuel line I have left, but still not entirely sure yet. I just don’t want to go down the same road as before with a bunch of leaking joints, so rubber hose might still be a solution. From an aesthetic perspective, I like the look of either if done “right”. lol! The cost of the pre-fab SS stuff is crazy.