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 passages 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.
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 passenger 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 for 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. Assume max rpm is 4,000 Assume 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?
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