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Weber 40DCOE 138/139 emission carb help


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I need some help with my duel weber setup. I have read multiple books on weber 40dcoe and recently discovered my carbs are actually emission carbs 138/139 and not sure where to start with jet tuning. The primary difference that I see between the standard webers and emission webers  is that I don't have a accelerator pump EX valve that goes inside the float bowls. Right now this is the jets I have 


Main Venturi/Choke - 33
Aux Venturi - 4.5
Main Jet - 120
Emulsion Tube - f16
Air Corrector Jet - 190
Idle Jet - 50f8
Accel Pump Jet - 40 
Pump Ex Valve - doesn't have an opening for it. 


Any help would be appreciated. Thank you 

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I found this with the google-



and the pix there make me think they'll tune like any other DCOE with an air bypass-

you can use the bypass to get the progression holes to open up when you want them to.


I think the place to start is where we all do- make sure you have a good throttle linkage,

get them balanced and fed 2 psi of fuel, then run them and see what happens.

Your numbers look to be in the ballpark, if a bit small on mains, so give it a shot.

Then adjust from there.


Early on in the process, I swear by a wideband oxygen sensor in the tailpipe.  It taught me what what going on...


Go for it.




Edited by TobyB

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Pretty sure that the pump ex valve controls the amount of back leakage of the pump jet and hence the duration of the spurt. That is probably a level of refinement too far for most folks (probably why they removed the option). One less thing to worry about. 


As Toby says, concentrate for now on getting it to idle via the air bypasses and get repeatable throttle action and closure with your linkage. You can then refine through one part of your rev range / throttle opening to the next. Wide band gives most info but seat of pants and other senses good too. 

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

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4 hours ago, LarsAlpina said:

Stock engine (and if so what type exactly) or mods done on it?


Regards, Lars.

The engine has been rebuilt but everything is stock. I have the Ireland engineering stainless exhaust from the down pipe all the way back! Electric fuel pump feeding 3.5psi! I got the idle adjusted pretty well but when I try to drive it, the car doesn't accelerate past 3 thousand Rpms and sputters/backfires 

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50 minutes ago, arthb7 said:

The engine has been rebuilt but everything is stock. I have the Ireland engineering stainless exhaust from the down pipe all the way back! Electric fuel pump feeding 3.5psi! I got the idle adjusted pretty well but when I try to drive it, the car doesn't accelerate past 3 thousand Rpms and sputters/backfires 

Seems like main circuit is stuffed with something to prevent gasoline/air flow...

Best regards


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2 hours ago, downhillwolf said:

Seems like main circuit is stuffed with something to prevent gasoline/air flow...

You can check that easily. You'll need a mirror and a good flashlight. After doing all that is mentioned above (including checking the float level) slowly open the throttle using the idle screw to the point where it stumbles. Somewhere in the 2-3K range you should see fuel start to emit from the venturi. This would signal the transition from idle circuit to main jet. If the carbs have been sitting there is a chance the main jets are plugged but I've rarely seen that on DCOE's. There is a slight chance the venturis could be installed 180 degrees out. It would be hard to do and they may be damaged but its possible. If you pull them you'll see what I mean. Your jetting sounds ballpark close, similar to mine. If the float level is extremely low this could be the cause but unlikely. Plugged E-tubes can also do this but again unlikely.


Does the accel jet squirt fuel when opening the throttle?

Andre B.

1971 BMW 2002

Wanna-be racer, too stiff for the street...

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I agree on what is written above, jetting should not be too bad. Fuel level is a good point.


Ignition and distributor do work fine and are adjusted correctly? What number for the dizzy?


Best regards, Lars.

Ei guude wie? (Spoken as "I gooooda weee" and hessian idiom for "Hi, how are you?")


Já nevím, možná zítra.

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Close both air bleed screws on each carb. 

Set the idle to around 850.

Get a synchronmeter or if you also have manifold vacuum ports, a Carbmate vacuum gauge.  Open the air bleed screw on the barrel that is pulling less air and match it to the other barrel.  Do for both carbs.

Set your mixture screws for best idle (note the AFR).

Bring the idle stop screw in until your desired idle rpm (Weber recommends 1000 rpm for 4 cyl dcoe's)

Check the barrel balance again and adjust the 1 bleed screw on each carb if necessary .

Adjust the idle stop screw again if necessary.

Adjust the mixture screws again, if idle speed changes significantly, repeat the process until it is dialed in to 1000 (or wherever you want it).

Idle mix screws should be final adjustment.

Check off-idle accel for lean spots/popping, see if it pops in 2nd gear at 3k rpm no load cruise, and transition lean/rich spots at WOT.

Report back :-)


I agree with Toby, your mains might be a little small if you go by the 4 x 33 choke formula = 130/135 main jet but you can determine that with a WOT test in 3rd gear and an afr gauge (invaluable).  If it leans out in higher rpms, put in a smaller air corrector until it stays at 12.5 afr to redline, if that doesn't work then go up a size on jets and use the smallest air corrector that prevents it going lean or rich at the high end.  You want the final WOT afr to be dictated more by the jet, not as much the air corrector.  Otherwise it screws up the transition and wastes fuel (compensating for a small idle jet) or not enough fuel (compensating for a big idle jet).  This assumes correct timing (I use a 123 ditributor with 15 BTC at idle and a steep curve to 36 all in about 2700 rpm.  DCOE's like a lot of initial and quick timing.  And note fuel pressure/float level issues to check as noted above also.



Edited by Stevenola
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  • 6 months later...

Update: still don't have the car running well but it's getting better. I disassemble both carbs and cleaned them. Then replaced open bearings that had tons of dirt in them with sealed bearings. Butterflies started to open much smoother. Finally found more information on these emission webers thanks to the Alfa forum. Here is the main difference between the classic webers and emission webers attached file. 

The emulsion tube was giving me all the trouble in the higher rps! The original emulsion tube that came with the carburator was f41 and I replaced it with an f16 and it caused all the issues. Once I put the f41 in with 138main and 160air that came with the carb the car became drivable. 


The idle holders and jets jare different according to attachment but I'm not 100% sure because when I read a Weber carb book it said only the idle jet holders were different and the idle jets were all the same between emission carb and classic. (still trying to figure that out) 


If someone is running these retarded emission carbs please help me out with getting them dialed in for the 2002. Im almost at the point of getting rid of these carbs and just getting new 40dcoe151 since there is so much more information on them. 




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I just ran across this thread.  I recently removed a pair of 136/137 DCOE 40s from my car (I'm installing the original Solex PHHs).  I think they are similar to your 138/9 DCOEs.  I believe my engine is pretty much stock, though I'm not sure, and the car ran well with the DCOEs.  Anyway, the jetting is pretty different than typical jetting recommended for the 151 DCOEs commonly used in the US.  Here's what I have:


Choke 34

Main  130

Air      140

Idle     59f21

Tubes  F11

Pump jet  35


The air and mains being so close seems odd, but again, the car ran well, very smooth progression.  I have an AFR installed, and the mixture is fine throughout the range.


Hope this helps.



1971 2002ti

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