Alfa Romeo Forums banner
1 - 14 of 14 Posts

·
Registered
Joined
·
277 Posts
Discussion Starter · #1 ·
Hi,

If you have a weber cross motor duct (the kind with the three little tubes going into it from the carb float vents) can you please take a couple photos of INSIDE?

I assume the little tubes bend round and point towards the air cleaner, but I'd like to see exactly where the Alfa engineers decided to place the ends inside the duct.

Thanks in advance.

John

[email protected]
 

·
Premium Member
Joined
·
22 Posts
Hi,

If you have a weber cross motor duct (the kind with the three little tubes going into it from the carb float vents) can you please take a couple photos of INSIDE?

I assume the little tubes bend round and point towards the air cleaner, but I'd like to see exactly where the Alfa engineers decided to place the ends inside the duct.

Thanks in advance.

John

[email protected]
Hi John,
I've been wondering the same thing. It's now 11 years later. Did you discover anything about that? Yesterday, I called a gentleman up in Canada who used to have a TZ. They also had the Ram Air induction. I asked about what the tubes did on the inside of the pipe, but he seems to remember that they didn't protrude inside at all. But in my opinion, that doesn't make sense. I would think that the Bernoulli effect would actually cause a lower pressure in the float chambers and that would raise the float levels. This would lean out the fuel flow.
What did you finally settle on and how was your experience?
Bob
1698480
 

·
Push hard and live
Joined
·
11,491 Posts
Hi John,
I've been wondering the same thing. It's now 11 years later. Did you discover anything about that? Yesterday, I called a gentleman up in Canada who used to have a TZ. They also had the Ram Air induction. I asked about what the tubes did on the inside of the pipe, but he seems to remember that they didn't protrude inside at all. But in my opinion, that doesn't make sense. I would think that the Bernoulli effect would actually cause a lower pressure in the float chambers and that would raise the float levels. This would lean out the fuel flow.
What did you finally settle on and how was your experience?
Bob
View attachment 1698480
I've been dealing with the peculiarities of Weber conversions, particularly from Solex, since the early 70s. Mysteries remain. However, the biggest hurdles I have found are owner/modifier assumptions as to what is causing the various flat spots, fuel economy, off-power behavior. The above post is typical.

We can see decades of Alfa Weber applications where float chamber vents are displaced well up the intake tract. As we move into the 60s, the displacement came gradually closer to the carb air inlet. We see in GTA inlets no displacement.

I have my own conjecture as to what Alfa was accomplishing with picking a remote float vent pickup. I seriously doubt they were seeking to increase the bowl overhead pressure to create a richer high-end mixture.
 
  • Like
Reactions: Robert Piacentini

·
Push hard and live
Joined
·
11,491 Posts
Bob,

What makes you think raising the float bowl level would lean the mixture? What makes you think creating a vacuum in the vent port would raise the bowl level?
 

·
Premium Member
Joined
·
22 Posts
The more I look at it, the more I think that as long as the float levels are within the prescribed range and it is ensured that all of the floats valves are operating normally I can't see how the vent pressure would make much difference at all. It's baffling why Alfa went to all the extra trouble of running these rubber tubes up to a remote location. Much has been said about it on these forums, but I didn't see anything about it in my Weber tuning manuals.
 

·
Registered
Joined
·
4,370 Posts
Here is a photo Jay Nuxoll posted years ago. It shows how the ends of those tubes must enter the airstream to prevent issues.

Motor vehicle Font Auto part Illustration Machine


Mark
 

·
Push hard and live
Joined
·
11,491 Posts
Mark,

I disagree that the pictures show how one must arrange the vent tubes to prevent issues. The pictures show examples of what Alfa tried in the 50s, and maybe 60s, before 95% of the emulsion tube variations had been invented. It's like claiming that a picture of a piece of flint is what one must use to create fire.
 

·
Registered
Joined
·
4,370 Posts
That is a nice analogy with the flint and all. I am just trying to help John out by providing the pictures that Jay located long ago and John requested. The float chambers on my car are just vented to the atmosphere and it runs fine. When Robert Piacentini (above poster) de-siamesed my intake manifold twenty years ago, he said that he tried venting the float chambers into the intake manifold on his car and it ran like [email protected] He told me not to bother running lines for the vents. I am not sure how the ends of Robert's vent tubes entered the air stream but I suspect it makes a big difference in how the engine runs -based on all the comments I have read and heard over the years.

Once we make the change to Webers, very little of the intake system remains original but John wanted to see exactly how the engineers at Alfa placed the ends of the tubes inside the duct, so he appears to want to retain originality or at least know how it was originally done by the factory.

Mark
 

·
Premium Member
Joined
·
22 Posts
That is a nice analogy with the flint and all. I am just trying to help John out by providing the pictures that Jay located long ago and John requested. The float chambers on my car are just vented to the atmosphere and it runs fine. When Robert Piacentini (above poster) de-siamesed my intake manifold twenty years ago, he said that he tried venting the float chambers into the intake manifold on his car and it ran like [email protected] He told me not to bother running lines for the vents. I am not sure how the ends of Robert's vent tubes entered the air stream but I suspect it makes a big difference in how the engine runs -based on all the comments I have read and heard over the years.

Once we make the change to Webers, very little of the intake system remains original but John wanted to see exactly how the engineers at Alfa placed the ends of the tubes inside the duct, so he appears to want to retain originality or at least know how it was originally done by the factory.

Mark
Hi Mark,
When I found my car and had it shipped back to USA from England three years ago, it was missing all of the stuff on the inlet side of the carburetors. I have since rounded up all of the parts again and made some new adapter plates to attach the air plenum. This time, I made thinner adapter plates and machined out a place to put some elbows so that I can attach the rubber tubes like in the parts manual. The last time, I had adapter plates that had elbows coming out the top and then I made some copper tubes that connected the vents into the original vent locations in the air plenum (for the Solex carb green vent hoses). I had to bring the car to a chassis dyno and a guy there tried various combinations of jets and tubes. He was finally able to get it to run fairly well with the vents connected to the air plenum. Unfortunately, at that time, I didn't write down what the combination was. Now that I have the car back again, I am making some notes about the jets, tubes and venturi sizes. Some folks in England had messed around with it before I bought it back and they got it to run good with just some K&N air filters. After I get everything connected just like the book through the crossover tube, then I'll bring the car back to the dyno guy and test it with the exhaust gas analyser. My mission will be to get it tuned to run good with all the stock intake parts and vent tubes. After that is done, I'll share my findings here. The consensus seems to be that the carburetors need to be fine tuned to run with whatever combination of manifold, plenum, air cleaner, and venting a person decides to employ. Since you have the exact same intake manifold mods as my car (and this seems to be the most common way to do it), I'm hoping that my upcoming results will provide the most useful starting point for your own fine tuning. I live near sea level, but results may vary depending on your fuel type, elevation, average temperature and humidity. I'm planning on getting it to run right at sea level and then back off the main jets to make the most power at about 2500 feet because that should cover most of my driving over nearby mountain passes with only a slight penalty at sea level. The shop I used before was: Log into Facebook in Portland. Stay tuned in - More to follow...
 

·
Push hard and live
Joined
·
11,491 Posts
Mark,

There is evidence of a few early Alfas with Webers using displaced tubes. Except for the few 2600s, and maybe some TZs, I don't think any others had DCOE carbs, just maybe DCO carbs. There are none that I know of using Webers on de-siamesed Solex manifolds, and displaced vent tubes. The 45DCOEs on those two cars were early designs with rich transition and idle circuitry.

I think the options are A) look "original", or B) run well.
 

·
Premium Member
Joined
·
22 Posts
Mark,

There is evidence of a few early Alfas with Webers using displaced tubes. Except for the few 2600s, and maybe some TZs, I don't think any others had DCOE carbs, just maybe DCO carbs. There are none that I know of using Webers on de-siamesed Solex manifolds, and displaced vent tubes. The 45DCOEs on those two cars were early designs with rich transition and idle circuitry.

I think the options are A) look "original", or B) run well.
Yes, you are right, and if it doesn't run well with those vent tubes, then I'll just make a secret hole under the tube to vent right near the carburetor. It will still look like it supposed to. Carburetors are such old technology anyway, that it's sort of like eating ice cream with a shovel.
Bob
 

·
Registered
Joined
·
4,370 Posts
Robert, it is good to have you aboard again! I am glad that you were able to find and acquire the car that you owned so many years ago. Don and several others(?) used the Two-liter Weber intake manifold that OKP in Germany reproduced, along with a fiberglass reproduction of the GTA air plenum -with good results. I bought one of the GTA reproduction air boxes and intended to install it with velocity stacks and the de-siamesed stock manifold but learned from Don that it would only work with one of the OKP intake manifolds. Not wanting to spend another 700 bucks, it was decided that the intake manifold you made about 20 years ago would suffice. Honestly the original engine (with only the valves 'refreshed') still runs great after over 60 years! I had an issue about 8 years ago related to ethanol fuel 💩 that was overcome by cleaning and rebuilding the Weber's -had the same issue on the Duetto's Weber's as well. The brass jets were plugged up with green corrosion products. I still have 90 degree brass elbows on the thick adaptor plates that you made long ago. I remember them being a royal pain to install, so I have not even considered removing them.

"There are none that I know of using Webers on de-siamesed Solex manifolds, and displaced vent tubes". Clearly a 'bass-ackwards' way of doing things, huh Don.
Mark
 

·
Push hard and live
Joined
·
11,491 Posts
Just to clarify...

The plenum that Alberto Guirola fabricated was modeled on the original 2-liter plenum, not a GTA. The two critical changes from the 2000T plenum were to raise the carb inlet holes to accommodate the tilted Weber carbs, and to make the carb inlet holes match Weber inlets rather than the Solex.

The power, smooth tractability, and impeccable manners proved the whole combo was spot on.
 
1 - 14 of 14 Posts
Top