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Discussion Starter #1
Hello All-

Trying to cure a tip-in stumble on my 2L equipped with 40 DCOE 149 carbs. The engine has 10.4 pistons, 11mm cams, Shankle headers, MSD ignition and Supersprint horizontal air filters with curved ram pipes. New NGK BP6ES spark plugs. The ignition timing has been checked and is correct.

Jetting as follows:

F9 Emulsion tubes
50F9 Idle jets
Main 130
Air 180


Curiously the previous owner said the car did not stumble with velocity stacks and air socks, but does with the Supersprint air filter setup.

I dropped the idle jet from a 55 to a 50F9. The stumble got a bit better after adjusting the idle screws, but still there. It's driving me nuts as it's difficult to drive the car at small throttle openings....

Do I need to drop down to a 50F8 idle jet?

Thanks!
 

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Discussion Starter #2
For grins I removed the Supersprint filters and air horns tonight. The stumble is 100% cured. Has anyone experienced this before??? Seems odd that an air filter can cause the car to stumble so badly. Am I missing something?

Guess I'm in the market for a Euro OEM airbox setup for an Alfetta now. Anyone have any leads where I can source this?

Thanks!

Scott
 

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What you may be overlooking is that by removing the filter, what you've done, in effect, is to increase the proportion of idle air. In other words, your instincts are right--your idle jet is flowing too much fuel (or the air bleed is too small). A 50F8 will probably work better than the 50F9, but you may want to try a 45F9.

And if you haven't already done so, you'll want to make sure the carbs are properly synchronized, as this makes a huge difference.

BTW, what choke are you running? (your jetting is theoretically right for a 32 choke). And according to the guides, at 500cc/cylinder, you're marginal for an F9 emulsion tube; you ought to try an F2 tube.
 

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For grins I removed the Supersprint filters and air horns tonight. The stumble is 100% cured. Has anyone experienced this before??? Seems odd that an air filter can cause the car to stumble so badly. Am I missing something?

Guess I'm in the market for a Euro OEM airbox setup for an Alfetta now. Anyone have any leads where I can source this?

You may have created a "pulse plate" by having the air horns too close to a solid surface, i.e., the hood in this case. Max Banks has an interesting post on this in the "Aluminum Inlet Manifold Extender. . ." thread.
 

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You may have created a "pulse plate" by having the air horns too close to a solid surface, i.e., the hood in this case. Max Banks has an interesting post on this in the "Aluminum Inlet Manifold Extender. . ." thread.
Jim,

That's certainly possible, particularly with straight air horns inside a filter air box. In this case, 1150ss' curved velocity stacks make this a bit less likely, but clearly there's some restriction inside the filter box. It sounds like the best answer, based on the previous owner's experience, is to run the velocity stacks with filter socks.

But 1150ss' comment "The stumble got a bit better after adjusting the idle screws" leads me to believe that the carbs aren't properly synch'd. The idle mixture screws on DCOEs are easy to mess up by blind trial and error (and dead simple to get right with a synchrometer). And I'm still willing to bet that those idle jets are too rich.

On a 1.6L, I'm running DCOEs with exactly the same jets & tubes as 1150ss, and curved velocity stacks (although without the filter box), and I had the same issues. The stumbling was instantly cured by synch'ing the carbs, and the light throttle running was further improved by dropping from 50F9 to 45F9 idle jets (although 50F8s may also work).
 

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The idle mixture screws on DCOEs are easy to mess up by blind trial and error (and dead simple to get right with a synchrometer). And I'm still willing to bet that those idle jets are too rich.
You are so right about this. With my first Alfa, a 1600GTV, I'd watched experts tune the Webers by ear. It didn't seem that difficult, so I decided I could do it myself. I drove around ----for months---with a very long skinny screwdriver and tweaked the Webers everywhere I went until I finally learned how to get the car to run right. It's really easy to create the kind of stumble we're talking about. I had it for months.:) While you can learn to tune Webers by ear, a tool like a Synchrometer makes the job much easier and more accurate.

IMHO every Weber owner should have two tools: a Synchrometer and a Colortune.

Oh, yeah. The Passini book has a good discussion of how the transition from the idle circuit to the main circuit is influenced by jets and emulsion tube choice. I think a true measure of being an Italian car enthusiast is being able to read, and understand, the Passini Weber book. Enlightment does come, but only after much pain. I think Passini just dictated his ideas into a tape recorder, transcribed, and then published 'em. There's not much evidence of editing, but the ideas are actually pretty good.
 

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IMHO every Weber owner should have two tools: a Synchrometer and a Colortune.

Oh, yeah. The Passini book has a good discussion of how the transition from the idle circuit to the main circuit is influenced by jets and emulsion tube choice. I think a true measure of being an Italian car enthusiast is being able to read, and understand, the Passini Weber book. Enlightment does come, but only after much pain. I think Passini just dictated his ideas into a tape recorder, transcribed, and then published 'em. There's not much evidence of editing, but the ideas are actually pretty good.
Couldn't possibly agree more. In addition to the Passini, the Weber Technical Manual (occasionally available on eBay) has a good--if translated from the Italian--discussion of carb operation, as well all listings of all jets & correctors and technical drawings with the dimensions and drillings for every emulsion tube.

Voice of experience (and it sounds like Jim's experience is the same): don't mess with the setting screws or the jetting and tubes unless you've got a road map. Once you understand the principles and have the right tools, it's still informed trial and error. But if you don't understand what you're doing, you're much more likely to make things worse--not better--by fooling around.
 

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I think that you have the completely wrong idle jets and emulsion tubes. 40DCOE149's are emissions type carbs. The idle circuit is fed from the well and not the float chamber, as on "classic" carbs. They require different emulsion tubes and idle jets. I posted a list of suitable idle jets in rich to lean order a year or so ago. I am at work and I don't have the data in front of me but I think that you need F34 emulsion tubes and 55F17 idle jets.

Ed Prytherch
79 Spider
2 x 88 Verde's
 

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I think that you have the completely wrong idle jets and emulsion tubes. 40DCOE149's are emissions type carbs. The idle circuit is fed from the well and not the float chamber, as on "classic" carbs. They require different emulsion tubes and idle jets. I posted a list of suitable idle jets in rich to lean order a year or so ago. I am at work and I don't have the data in front of me but I think that you need F34 emulsion tubes and 55F17 idle jets.
Well, that’s an excellent point; I’m running much older DCOE 15s; I certainly can’t vouch for the jetting and emulsion tubes of the later emission-series carbs.

But the principles remain the same: 1. make sure that the carbs are synchronized (use a synchrometer), and 2. make sure that the idle jets aren’t too rich (Colortune is the best tool).
 

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Discussion Starter #10
Thanks for all the advise guys!

The carbs are properly synchronized using a Synchrometer. Tried both 50F8 and 45F9 idle jets and the stumbling was MUCH worse. Can't get my hands on 55F17 idle jets to try.

At this point I am wondering if I should go back to the velocity stacks and socks ???

Cheers!

Scott
 

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Thanks for all the advise guys!

The carbs are properly synchronized using a Synchrometer. Tried both 50F8 and 45F9 idle jets and the stumbling was MUCH worse. Can't get my hands on 55F17 idle jets to try.

At this point I am wondering if I should go back to the velocity stacks and socks ???

Cheers!

Scott

As Alfaparticle correctly points out, the F8 and F9 jets are correct for earlier series DCOEs.

I run open velocity stacks (mesh screens only), but I carry oiled socks that I’ll use if conditions are at all dusty. If it runs well with stacks and socks, why not run it that way?
 

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Discussion Starter #12
Well- I threw a set of K&N filters on the car and it runs perfecto with the 55 idle jets.

The Supersprint setup sure looks great, but certainly didn't run great in this application....

Thanks again for the assistance.

Scott
 

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There is not a lot of published information on the late type Webers. The only source that I kow of is the Pierce Manifolds Weber Tuning Manual.

Ed Prytherch
 

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dcoe 27 idle jets size?

Hi all

New to the blog, new to weber side drafts

Trying to cure a flat spot at 1500rpm to 2000rpm

I have 2 dcoe 40 27 weber side drafts
just need to know what is 1 size richer than 50 F8

Thanks
 

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I size richer is 50F9. One size leaner is 50F11
 

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Same issue here. Off idle bog then cleans up nicely 1500 rpm to up top .
specs....
2L 11mm cams, DCOE 32's(classics), light port work. euro manifold and airbox.

32 chokes
130 mains
currently running 50 f9's(have 50f8's)
195 bleed
F9 emulsion

mixture turned out about 1 turn plus or minus on 50f9's( its very touchy... a 1/16 of a turn makes a difference in idle quality). 50f8's have the same bog at off idle but definitely have slight hesitations transitioning to mains ( 2500-3000 rpm) under cruise. Too lean.


Main question is: best method to determine if I'm lean or rich on idle jetting? go 50f11 or 55f8?

Thanks
 

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Make sure that you have a good spark before you mess too much with the Webers. Expect difficulty if you have a points distributor and a stock coil.
 

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axis_powers said:
32 chokes
130 mains
currently running 50 f9's(have 50f8's)
195 bleed
F9 emulsion
I´m very much missing pump jet and pump back bleed valve size used in your application.....130mains are standard in DCOE32´s for a 1750, so may be to small for a slightly hotter 2l.....just my thoughts.
 
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