Webber Transition Ports

[Bill Cuthbertso]

Gary,
When I turn a carb upside down and operate the butterflies, it appears that once the butterflies leave their seat all of the transition ports are exposed. Give it a try. It seems to me that in order for the ports to be exposed one at a time the carb would have to equipped with a slider of some sort instead of butterflies.
Plugged holes - pull the plugs and tap the holes.

[Gaz64]

Bill,

Can you post a pic of your carbs progression ports?

I've yet to see any Weber carb where the progression holes aren't exposed "progressively".

Pardon the pun.

[Bill Cuthbertso]

Gary,
The transition ports in my Webers are in the stock position except for the third port which is at 12 o'clock, one millimeter above center. After watching the way the butterflies work, it looks to me like all of the ports are subject to lthe same air flow once the butterflies are off seat. I think the problem with 48's performance in the mid range is a lean condition which is at least partially remedied by a third transition port. As I'm sure you know 48's were developed for racing not curising. Other changes that might help are: reducing the size of the venturies which should increase air velocity ( at the expense of top end), increasing the size of the idle jets, and/or decreasing the size of the air correction jets which should bring the mains in a little sooner. The potential success of these changes would depend on how the carbs are set up before the changes are made. The last alternative is that we may just have to agree to disagree however I do appreciate your comments.
Best regards,
Bill

[Gaz64]

Bill,

I can't comment on IDAs, I have never owned a set, and only worked on a few.

Are you saying the ports at 6 and 9 o'clock are exposed at the same time?

Can you post some pics for my and others curiosity?

[Rick Parker]

I am of the opinion that the emulsion tubes, and accelerator pump jets with their various diameters, holes sizes and patterns are intended to remidy the very condition you are describing. There is more than one Main jet/air corrector pattern that will get an engine running properly. For instance a Roller cam engine because of faster valve timing and action can generally utilize a smaller main jet than a flat tappet engine, because the roller cam engine creates a stronger signal (depression/vacuum) sooner that acts upon the fuel within the main well. In the idle system the 3rd hole receives a signal "later" with progresive opening of the throttle blade, the notch in it passing the hole after the OEM hole is exposed. Then fuel is drawn from it rather than the other because of a stronger signal that it is receiving. Then when the blade is open far enough, the siganal becomes strong enough to pull fuel and air from the auxillary venturi. However, once the 3rd hole is drilled It may work for one application or cam in a particular engine but not another?? Just my opinion, not in favor of drilling holes. The volume of fuel mixture rendered is determined by how far the mixture screw is opened combined with the accelerator pump shot.

Here is a link to a great article of how to completely rebuild the 48 IDA, included are several pictures including one of the aformentioned 3rd hole:

http://www.aircooled.net/gnrlsite/re...idarebuild.htm

[Gaz64]

It is also worth noting that adding more progression holes adds more "airbleeds" to the idle/transition circuit.

The idea is to lengthen the transition, ideally 4 or 5 0.75mm - 0.8mm holes would offer smoother progression.

But one has to be willing to block all existing holes and start again.

There are 5 hole DCOEs, something to think about.

[tjyak50]

Forgive me for rehashing an older thread.
I have Triple DCOE 40-151 on a Datsun and they only have 3 progression and a massive lean-spot with the throttle cracked.

I have researched endlessly and think that an additional progression hole would fix the problem. But then others say NO way, never touch the progression (yet they modify every other part of the jets and chokes and stuff).

This picture really highlights my issue. At 1000 rpm with everything dialed in and calibrated the butterfly is still way too far towards the engine to immediately expose the 1st progression hole when the throttle first moves.



Did you all have good experience adding additional progression holes?

[Rick Parker]

DCOE's are prone to fuel foaming because the hang on the side of the engine. But for all intents they are very similar in function to an IDA, consequently many of the parts interchange.
Do you have rubber isolaters between the carbs and manifold? If not get them. I used 45 DCOE's very successfully on a L-18 Datsun 510 built for autox . It was an 8200 rpm engine withe the biggest cam that Isky had. I think we ran 36 mm chokes an a 145 mains with 180 air correctors. I don't recall the idle circuit but that can be calculated effectively.
I suggest you do NOT drill more holes.
This is on what displacement Z engine, what's been done to it.

[tjyak50]

Why do you say Not to improve the transition circuit?
Did it cause problems in your car?

[Tom Howland]

Have you checked all 3 venturi’s to make sure the holes are in the exact same location. I am having problems because my 3 progression holes are not the same. The extra third hole has been drilled in different locations. I.e., 2 o’clock, 3 o’clock, 4 o’clock. I have 4 IDA’s. My symptoms are different than yours. I have a lot of popping when in the idle/transition circuit. Try a fourth hole. If that doesn’t work, fill it and try something different.

[tjyak50]

What I have is a 2.8L inline six with 10.3:1 CR, ported / polished head, Big Isky Cam, headers, Total Seal, Etc. Just a massaged NA weekend warrior.

All 3 carbs and 6 tubes are very well matched. Using a "snail gauge" they flow the same, all rebuilt by me (lovingly), idle mixture set well. Just the transition from idle to mains is ****ty.

The progression circuit doesn't kick in till too late due to the location the first hole. I thought I read that you guys with the Cobras had the same issue so this is why I am here bothing you V8 guys with my little inline 6.

However I think there is a lot we can teach each other about Webers from different motors.

[Gaz64]

Tom,

Adding a hole as in your pic becomes a new "1st hole" not a "4th hole".

The "3rd progression hole" added to IDAs (which only have 2 progression holes) is exposed last, hence "3rd hole".

With your triples, what size plumbing are you using for intercylinder balancing?

If each intake runner is connected together to gather vacuum for power brakes with hose too large, then each cylinder will breath through more than one barrel and the butterflies will be virtually shut to get a normal idle speed.
Ideally you want the first progression hole ready to be exposed as soon as the throttle comes off idle.

What speed does your engine run at, at this point in the throttle?

[tjyak50]

These are pure Individual Throttle Bodies with no interconnect. No vacuum assist and no brake boost. There is no communication between barrels at all.

The picture you see that I posted with light shined in the hole is all dialed in at about 1000 rpm. If I setup the butterfly so that the 1st hole is immediately exposed with throttle movement I would be at 1800 rpm.

Indeed I am talking about a New 1st Hole. One positioned exactly over the B-fly so that any throttle movement exposes the new first hole to vacuum and fuel can flow.

With the progression ports as they were designed (and not designed for THIS motor I am sure) the b-fly has to open a relatively long way before the 1st hole is exposed.

They do have a "soft mount".

[tjyak50]

Follow up.
Pulled the carbs and they went into the CNC Mill and we drilled a new 1st hole .030" further downstream (towards the engine).

This made a massive difference in the off-idle mixture.
I want to make one more jetting change and I think I will have a usable progression.

Your input was very helpful here, thanks.

Tj

[Rick Parker]

If you are totaly exposing the first hole to gain an idle, your jetting is to lean. Your idle should only slightly uncover the first hole.

[tjyak50]

On a DCOE the progression inspection cover is just upstream of the idle mixture screw and under there are various progression port configurations depending on the model.

Off to the side is another cover but that is for the starting circuit.

[Gaz64]

In John Passini's book on Weber theory, he mentions chamfering the bottom of the throttle plate if progression is late.

[tjyak50]

Feedback.
I put the carbs in a CNC mill and had the boys drill the 4th hole (closer to engine and directly over the butterfly). .030" hole about .030" downstream of the old hole.

Car ran extremely rich.

Then I plugged the old "1st" hole and it runs like a dream now.
No leaning out on part throttle, no spitting or sneezing.

Its about 115f here in Phoenix today and my extra layer of thermal blanket even kept the carbs from boiling. (Z motor carbs sit ON the exhaust - stupid).

So far my experience with adding the 4th hole is good.
FYI>
TJ