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Well I could never find what was causing my Dellorto DHLA carb to suddenly cause cylinder #4 run so rich. After much cleaning and inspection I finally decided there must be a crack somewhere in the body causing fuel to leak into the idle circuit. So I ditched the Dellortos and purchased a (modern) pair of Weber DCOE 40 carbs. I set up the Webers using the values on the Centerline website (https://www.centerlinealfa.com/sites/centerlinealfa.com/assets/files/default/weber_jet.pdf)

My 1972 Giulia is a 2 liter with a Eurocam and I live at 6,500’ elevation. I decided to start with Centerline’s sea level jet suggestions because I found in the past that sea level values still work pretty well in my other cars. It’s likely all the ethanol in our fuel here affects the traditional altitude adjustments anyway.

The Webers provided noticeable better performance than my fifty year old Dellortos. I’m very happy… except for one thing. I have a VERY noticeable flat spot when I open the throttles. Because it occurs at all speeds and rpms I think it points to an issue with the acceleration circuit.

Centerline recommended a numerically smaller pump jet (35 vs 40) and a numerically larger (60 vs 50) pump exhaust valve than what came with the standard Weber setup. My first question is does “numerically” translate into physically larger. I would think so but just want to make sure. If this is the case then the Centerline values mean considerably less fuel going in than the manufacturers shipping configuration. Second how do the pump jet and pump exhaust valve interact to determine how much fuel is being provided to the engine? Finally, if there is anyone out there using Weber DCOEs on their 2 liter would you let me know what jetting values you are using?
Thanks, Phil
 

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Pump jet - the larger the number the higher the flow rate.
Exhaust valve - the larger the number the less total volume, therefore the shorter the fuel pulse.
35 is the "normal" pump jet for Alfa Nord engines. I doubt that you would notice much difference between 50 and 60 exhaust valves AKA bleed backs.
What is the complete model number of your Webers? The later models typically have smaller progression holes which keeps the mixture pretty lean until you are on the main jets.
 

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Discussion Starter #3
Thanks Ed

The number stamped on the carbs is 40 DCOE 151. I got them from Redline which sells then under the number 19550.174.

Phil
 

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Discussion Starter #4
I'm also running 50F8 idle jets and and 130 mains with F9 emulsion tubes.

Phil
 

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I THINK that the 151's have smaller progression holes than pre - emissions Webers. They retain some features of the full emissions Webers (40DCOE72/73 for instance) but not all of them. I am using 45DCOE152 which I believe are similar to yours and I have spent a long time (years) trying to get them to run without lean spots. I have opened up the progression holes a little at a time and tried much richer idle jets and I have made good progress but they are still not as good as the earlier Italian Webers with bigger holes that are in a different pattern.
The challenge with modifying the progression holes is knowing which ones to enlarge. In my case enlarging the first hole has had the best results. As an experiment you might try 50F9 or 55F8 idle jets and see if there is an improvement.

The above is assuming that you have a lean flat spot. Maybe at your elevation you are going too rich in which case the Centerline recommendations may be better.

130 mains and F9 tubes should be fine. I prefer F11 or F16 but F9 should not cause your problems.
 

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Thanks Ed. After reading your first message and doing a bit more research I think you are on the right track. I noticed my idle screws had to be out much further than I thought appropriate (2+ turns) when I balanced the cylinders with the airflow meter at 800 rpm. So it seems to be on the lean side how. I ordered a set of both idle jet sizes you recommended and I'll post what kind of success I have. I might put the larger pump jet in later and see if that also helps.
 

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2+ turns is not a lot for those carbs which have the long taper needles. Mine are at 3.25 turns with 55F8 idle jets and my idle AFR is about 14. The guys at Centerline know about carb setup for higher elevation and you may get the advice that you need from them. I am starting to think that you may be running too rich. The discussion here may be helpful

 

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I would say the OP's suggestion that Ethanol has changed a lot of the convention wisdom on carb tuning is spot on.

In general, we aren't leaning things out as much as we used to for high-altitude running, and I think this is because the Ethanol causes a somewhat leaner mixture in general.
 

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A wideband AFR meter would tell you if your flat spot is due to rich or lean running and that is the ideal instrument for tuning carbs. However, if you just want to know rich or lean without an exact AFR then a narrow band gauge will work.
You can have this one for free if you want to give it a try. Just pay shipping. A new sensor is included.
 
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