The art of setting up Carbs in 2016 - Alfa Romeo Bulletin Board & Forums
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post #1 of 64 (permalink) Old 01-17-2016, 12:49 PM Thread Starter
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Post The art of setting up Carbs in 2016

Hey there

For the first time I need to setup the carbs (webers 40ies) on my alfa as I have changed the Carb Flanges. I want to get it right the first and I'm under the assumption that my carbs are jetted correctly.
So, there are many, many threads on this BB going into great detail on what to do and how (Check AFR, Use Synchrometers or use Synchronizers likes www.carbtune.com ) but they mostly target one approach at the time or are already more than 10 years old so I wasn't always sure whether provided information is still valid or has been superseded by newer approaches.

So bear with me I have started off by setting up the idle screws and got the car to a okay-ish startover.

I'd be more than happy on hints on where to go from here!

Thank you very much!
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post #2 of 64 (permalink) Old 01-17-2016, 05:07 PM
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Weber setup procedures probably haven't changed in 50 years so the posted procedures remain valid. You don't need a synchrometer or any other special tools. Feel free to ask specific questions.

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post #3 of 64 (permalink) Old 01-18-2016, 12:26 AM Thread Starter
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From what I read, you need such tools to get the mixture right. Otherwise chances are e.g. that the mixture might be too lean especially with higher revs. What would be the approach without external tooling? Besides getting the engine to run at all with the idle screws just open enough.
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post #4 of 64 (permalink) Old 01-18-2016, 06:38 AM
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If your engine is stock or near stock then the factory jetting will be good. You will be unlikely to improve on it unless you install a wideband AFM meter.

I use the factory procedure for initial setup of Webers. You must get them properly synchronized. My method is to short circuit one spark plug at a time and adjust the synchronization screw until the rpm drop is the same on all cylinders. My special tool is a long screwdriver.
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post #5 of 64 (permalink) Old 01-18-2016, 06:44 AM
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Quote:
Originally Posted by AlessandroM View Post
Hey there
For the first time I need to setup the carbs (webers 40ies) on my alfa as I have changed the Carb Flanges. I want to get it right the first and I'm under the assumption that my carbs are jetted correctly.
Here, you indicate the Webers are jetted correctly or are at least jetted according to a known baseline for your engine but you do not list any details regarding your car, your engine, or your specific Webers. I presume by "Carb Flanges", you are referring to air horns. Is your concern that the air horns will flow enough additional air to cause the mixture to go lean at higher rpm? I doubt that is likely on a presumably stock engine but getting a response from someone who knows with some certainty is my suggestion. Of course you can buy a wideband AFR meter and see for yourself and use it to make small, incremental jetting changes. The initial setup doesn't change. Use the normal, established procedure of setting the mechanical synchronization and then set the idle mixture to get the same rpm drop for each cylinder. No fancy tools required.

Oops, there you go. Ed types faster than I do...

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post #6 of 64 (permalink) Old 01-18-2016, 06:47 AM
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Great minds think alike, and almost in synch :-)

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #7 of 64 (permalink) Old 01-18-2016, 09:31 AM Thread Starter
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Thanks guys for your responses.

I assume the jetting ist correct because the car was running nicely before, not overly powerful (it's a 1.6 in 75' GT Junior) but very calm and with okayish mileage.
Now i installed these https://www.okp.de/t3/uploads/pics/A...l_Alu_40mm.jpg and the Engine still runs, but not as refined as it did before. I was told then when changing the flanges, it may change the angles slightly thus causing the throttle to behave differently and that i need to fix that first. On the other hand it feels like the carbs were setup to work with a slight airgap (rich) to avoid lean mixture on some cylinders (which may explain the not so powerful impression). Which may not be the case anymore.
@alfaparticle
You mentioned your approach to the initial configuration. Where do you go from there (without the need of a dyno and afr)?
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post #8 of 64 (permalink) Old 01-18-2016, 09:50 AM
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You mentioned your approach to the initial configuration. Where do you go from there (without the need of a dyno and afr)?

Drive it hard. If there is a hesitation figure out if it rich or lean (or an ignition problem).
Do plug chops if I suspect the main jet/air corrector should be changed. Only change one thing at a time.

I use those solid mounts now but I leave out the O rings and spring washers and bolt them up tight. There has been plenty of discussion on carb mounts in the past 12 months. You will find a lot of different opinions if you do a search.

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke

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post #9 of 64 (permalink) Old 01-23-2016, 11:40 AM Thread Starter
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Turns out my question (this thread) mostly started with a wrong assumption. I thought that there is much more fiddling possible with carbs (without changing the jets) other then the idle-screws. After reading a book on Webers and Dell'ortos I do feel ashamed. Sorry.

So I haven't changed anything on the car and its running rougher, I suspect either the throttle linkage (as car holds its rev sometimes) or that there is still a leak somewhere. So what I'm wondering now is how do you guys figure whether you have a airleak somewhere, without actually reading out CO emissions? I tried the method with spraying carbcleaner over the seals, but that did not change the way the engine ran in any way.

Last edited by AlessandroM; 01-23-2016 at 11:44 AM.
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post #10 of 64 (permalink) Old 01-23-2016, 04:13 PM
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Quote:
So I haven't changed anything on the car
You said that you changed the carb flanges. Do you want to provide more information about that change?

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #11 of 64 (permalink) Old 01-24-2016, 12:14 AM Thread Starter
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Hi Ed,

I was the author of this thread here:
http://www.alfabb.com/bb/forums/gt-1...-alu-ones.html
So you are right, i changed something - i changed to solid mounts. But as opposed to you still with O-Rings. I would not change to the fixed setup unless i cant get it right (some evidence of lean mixture, hence the question) with the insulation.

Thanks
Alessandro
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post #12 of 64 (permalink) Old 10-05-2016, 06:01 AM Thread Starter
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So... it's been a while and I even sold that car meanwhile to a friend. I rebuilt the carbs meanwhile and while the engine is running, but a little rough. We synchronized the carbs with the use of a synchrometer and now we're stuck at the idle screw. All the threads and online help suggest 3 turn from the fully seated screws. Well the hard thing with the webers is - they have springs so what is fully seated? Screwing them fully in almost bends the springs so that can't be right - or is it?

Thanks
Alessandro

Learning, learning..
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post #13 of 64 (permalink) Old 10-05-2016, 07:21 AM
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There must be a qualified Alfa mechanic in Zurich.....

Months of fooling around when taking the car to someone experienced with webers would fix it for a few euros.

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post #14 of 64 (permalink) Old 10-05-2016, 07:22 AM Thread Starter
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Thanks a lot.

Next.

Learning, learning..
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post #15 of 64 (permalink) Old 10-05-2016, 07:34 AM
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Alright, do you have a repair manual with a section on weber carbs?
Follow those directions. As said, the proceedure is the same for decades.
Oh, and those solid carb mounts....
many experienced mechanics have a hard time keeping them from leaking air.

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