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Discussion Starter · #1 · (Edited)
. . . in a downdraft, progressive multi-throat carburetor apparently baffles many on this site, irrespective of their general mechanical ability and/or understanding. The biggest source of confusion can likely be attributed to an incomplete understanding of "idle circuit" nomenclature. This is a HUGE misnomer as it names only half of its function and completely ignores the other half, the transition phase/circuit, by leaving it unnamed and, therefore, functionally unidentified.

Any wonder that many get completely befuddled when trying to understand what "idle" jetting changes actually accomplish during tuning of their 32/36? It does much more than just change the idle mixture! Hopefully, the following dialogue and illustrations will help to clarify this little-understood area of operation somewhat.

I'll be using basic Weber 32/36 diagrams for illustration, but they are functionally the same as the Solex 32DIDTA's idle/transition operation.

First, the primary idle operation showing fuel, air and mixture flow paths . . . red arrow shows the transition ports - a set of stacked, metered orifices - just above primary throttle blade. You will note that these ports share the "idle passage" mixture which is controlled by the primary "idle jet"!



Second, when the carburetor primary throttle begins to open from idle, the transition ports are exposed to primary venturi vacuum and begin to flow the shared "idle passage" fuel mixture . . . which is controlled by the "idle jet"!



Third, with the primary throttle opened between 2/3 and 3/4 and with the primary auxiliary venturi flowing "main-jet/emulsion-tube (fuel aerator) passage" mixture nearly fully, the secondary throttle begins to open exposing the secondary transition ports to the secondary venturi vacuum which causes the secondary "idle passage" mixture to flow.

Though the secondary mixture is controlled by the 'commonly-referred-to' secondary "idle jet", they should more appropriately be called the secondary transition jet/passage, as you can plainly see, no true secondary idle circuit exists!



The interdependence of idle and transition functions would certainly be a great deal easier to understand if these passages and jets would be named for their complete function - idle/transition - not just their traditional monikers, idle jet and passage!
 

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Thanks, that is the clearest explanation I've seen. Not to be greedy... but would you have something similar for the 38 series downdraft?
 

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Great job, Professor Otto. Now can you explain what "emulsion tubes" do, and why the they make such a difference when you swap them with different ones.

Thx,
James
 

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Discussion Starter · #5 · (Edited)
38DG*S carburetor . . .

Thanks, that is the clearest explanation I've seen. Not to be greedy... but would you have something similar for the 38 series downdraft?
. . . is a synchronous carb, which means that both barrels are a mirror image of each other, operate at the same time - i.e. interconnected to each other by geared sectors - and use identical jetting, E-tubes and idle/transition passages and orifices, unlike the non-synchronous 32/36 "progressive" carb.

The best way to visualize its operation is to take the 32/36 primary barrel illustration of the first post and mirror/duplicate it exactly for the second barrel . . . including duplicating the idle/transition passage and "idle mixture adjustment" needle for the idle orifice.

Synchronous 38DG*S idle operation:



Synchronous 38DG*S transition operation:




The synchronous, mirror-image throttles, "both-exactly-alike" operational nature of the 38DG*S is the primary reason why this carb is so much more difficult to "tune" than the 32/36 . . . tiny changes, which must be done alike to both sides and at the same time, have twice the impact as those done separately! to only the 32/36 primary or secondary!
 
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Thanks Otto. I thought maybe the transition passage on the 38 might be larger and on one side only. Didn't realize it had two idle mixture adjustments.
 

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Discussion Starter · #7 ·
Synchronous 2-barrel carbs . . .

. . . is a synchronous carb, . . . , unlike the 32/36 'progressive throttle' carb.

The best way to visualize its operation is to take the 32/36 primary barrel illustration of the first post and mirror/duplicate it exactly for the second barrel . . . including duplicating the idle/transition passage and "idle mixture adjustment" needle for the idle orifice.
. . .
Thanks Otto. I thought maybe the transition passage on the 38 might be larger and on one side only. Didn't realize it had two idle mixture adjustments.
. . . are likely easier to visualize for comparison by using a much more familiar American 'common throttle shaft' 2-barrel carburetor like GM's Rochester 2GC or Ford's Holley 2300 derivative. Everyone inherently understands that every change must be done alike to both 'barrels' because they 'open/close at the same time' . . . synchronously!
 

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Awesome! It helped me out some. I knew some basics but the cutaway view makes things better! Now I see why changing the "idle jets" doesn't really have as much to do with idle but more with the transition from idle.
 

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Discussion Starter · #9 · (Edited)
Bingo!

Awesome! It helped me out some. I knew some basics but the cutaway view makes things better! Now I see why changing the "idle jets" doesn't really have as much to do with idle but more with the transition from idle.
You hit the nail on the head . . . transition is much more important for performance tuning the carburetor overall!

Idle is essentially a 'static' setting - affects and is tuned for a single throttle position only, whereas 'transition' is a 'dynamic' setting - affects and must be tuned for a range of throttle positions from 'just off-idle' all the way to where the auxiliary-venturi begins to control the complete air/fuel mixture [emulsion tube & main/air jets] requirements of the engine.
 

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Discussion Starter · #10 · (Edited)
Think about it . . .

Thanks Otto. I thought maybe the transition passage on the 38 might be larger and on one side only. Didn't realize it had two idle mixture adjustments.
. . . THAT would mean you couldn't use this carb on "split (180°) manifold" V-engines, no idle/transition on one bank!
 

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Otto, thank you for taking the time to explain this!

Otto,
Thanks in taking the time to post this. Very valuable information and so important to digest to understand how our carbs work on our car.......This is a must-save!
Take Care,
Mike
 

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Discussion Starter · #12 · (Edited)
Meant to answer . . .

Great job, Professor Otto. Now can you explain what "emulsion tubes" do, and why the they make such a difference when you swap them with different ones.

Thx,
James
. . . this question earlier, somehow just got overlooked at the time I guess. :pat:

I'll be using a ~45%-throttle 38DG*S skeleton diagram I designed for this purpose in Jasc's PSP 8.1 from mid-2003. OK, here goes . . .

The primary function of the emulsion tube in a carb is to emulsify/aerate the fuel flow metered by the main jets in the fuel bowl based on engine demand at the primary venturi and feed that aerated mixture to the auxiliary venturi in the center of the carb throat (primary venturi). OK, quite a mouthful . . . look at the diagram and I'll try to elaborate a bit more for you, using only a single barrel of the carb to keep things simple.



The raw fuel flows into the fuel bowl and through the main jet into the emulsion tube chamber and is connected to the idle/transition passage through the idle jet (shown) and directly to the auxiliary venturi in the center of the throat (not shown). Fuel remains in raw state (unaerated) throughout the carb until the engine is running. The reason for aerating fuel is to provide combustibility . . . fuel needs air to burn!

OK, the engine is turned over and vacuum (suction) is created beneath the throttle plate at the idle port. Engine begins to initially draw raw fuel past the idle needle from the idle/transition passage. As engine fires and flow demand increases, aerated fuel mix is now metered by the idle fuel and air jets, essentially a mini emulsion tube. Step on the gas, and now the stacked transition ports become exposed to engine vacuum adding more aerated fuel mix from the idle/transition passage.

As engine speed increases, flow demand increases and air/fuel mixture is now drawn from the main emulsion tube chamber, the mixture regulated by the carb main jet, the emulsion tube air jet and the number and size of holes in the side of the emulsion tube. This determines the resulting air/fuel ratio of the auxiliary venturi fuel mist.

Care must be taken in determining the emulsion tube main jet size and number of holes in the body . . . if they are too large and too many, main jet fuel ratio could be affected . . . too lean! Hopefully, this post clears things up a bit . . . admittedly, it's not that easily understood.
 
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