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Discussion Starter #1
Hello all, I've started a rebuild of a set of 40 dcoe 80,81 carbs which I've never used.
It has obviously been opened before (screwdriver damaged jet).
I noticed the #80 carb had the old style idle mix screws (wide taper) but the #81 carb had the newer narrow taper screws.
Are these idle mix screws interchangeable with the carbs or specific to carb models? They're different length but with the same thread. Any help is greatly appreciated, thank you
20201004_162417.jpg
 

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No they are not interchangeable. There are 4 common types.
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However if your 80/81's are emission type, with air correction feed needles over the throats with lock nuts, like these 45DCOE152's....
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It uses the fine needles, 3rd from the left in my top photo. If they are the specialty 107 variation type pictured below that has the TALL rear idle mixture towers....
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...this type uses the brass needles with "O" rings pictured far right in my photo. Early Italian non-emission Webers use the blunt needles.
Hopefully this note may be helpful.
 

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The narrow taper needle is correct for your carbs
 

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Discussion Starter #4
Gordon, Ed,
Thank you both so much for your replies.
The pictures are certainly worth the 1000 words, and you've really made this rebuild possible Gordon with your previous threads on cleaning, disassembly, suppliers, etc.

Cheers,

John
 

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You have "full emission carbs". They are good but they require some specific jetting. A 1750 or 2L motor should have F34 emulsion tubes and 55F17 or 55F21 idle jets.
 

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Thanks Ed.
I refer a lot of questions to your expertise with the emission Webers. GOOD Webers, but different than non-emission set-up either for emission engine or non-emission applications.
G.
 

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Hi Gordon
I had no option but to understand emissions Webers. My 79 Sider had a Shankle Weber conversion with a pair of 40DCOE138/139 when I bought it in 1999. It did not run well and nobody knew anything about the emissions carbs except for a guy in Scandinavia who explained the idle jet situation. After that I gleaned little bits of information from various sources until I had a clear picture. Your posts have been an important part of my education. I think that the situation is much better now. Centerline list standard jetting for both Classic and emission Webers in their tech notes.
They will make as much peak HP as classic Webers but they are optimized for excellent part throttle operation and classics are a better choice if you like to drive WOT.
 
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Discussion Starter #8
I've gotten the carbs apart and cleaned. The internals are as follows ( I'm not sure what the original gear was);
accelerator pump jet - 40
idle jet - 50F9
fuel jet - 130
emuls tube - F9
start jet 65F5
needle valve 175
air correction jet 200
The carbs are off a non running rusty 81 spider 2L I bought super cheap from the long time owner here in Victoria after it sat for a couple years. I'm hoping to fire it up after going through the various engine systems. I'm sure the carb swap was done ages ago, supposedly with a bit hotter cams. I'll find out soon enough.
 

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Discussion Starter #10
Thanks for the link Ed.
I suspect not much on these are original. I measured the accelerator pump open / closed and it isnt what Weber originally had for it.
I wonder if the carb bodies are in even 80 / 81s and not just the carb covers.
 

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The most significant difference between the "classic" and "emissions" webers is in the plumbing of the idle/progression circuit. In Classic DCOE's the idle circuit is fed from the float bowl and it has a fairly constant head. In emissions Webers it is fed from the emulsion tube well and the head varies with the depression in the venturi. I don't fully understand the dynamics of it but the result is that emissions Webers need different idle jets which have much bigger air orifices. They also need different emulsion tubes. There are a number of emulsion tubes that work with classic Webers on Alfa Nord engines - F9, F11, F16 but 2L Nords require F34 emulsion tubes in emissions Webers. You can get away with F41's if the ngine is stock but not if you tune it for significantly more power. There is nothing wrong with emissions Webers. My Spider engine made 170 HP with 40DCOE116/117's, but the driving experience is a little different. The Emissions carbs are great at part throttle and they give very good MPG but they are not as good in the mid range at wide open throttle.
I no longer have emissions Webers and I cannot investigate the physical side of them for you. I would be surprised if someone put 80/81 top covers on a pair of classic Webers as classics are generally more valuable.
 
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Discussion Starter #12
Thanks for the explanation Ed.
If the current setup is no good I'm going to source the factory emissions tubes, jets, etc for the 80/81 setup from pierce manifolds and go from there. They seem to be in good shape otherwise.

John
 
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