Greetings All, Can anyone on the forum running with a single Weber 40 DCOE on their engine please let me know what size Auxiliary Venturi they have fitted in the carb, the basic state of tune and the capacity of the engine too, thanks.
Ian.
Ian.
Technical Weber 40 DCOE venturi sizes.
- Thread starter Bleeding Knuckles
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Technical Weber 40 DCOE venturi sizes.
Nowuries
New member
Hi Fiat Gurus::wave:
I have a tuned 700cc 126 engine for my Fiat, and it came with a 40 DCOE. I am hoping someone can provide their carb tune specs for the venturi and various jets for their engine so I can use it as a baseline before I go take it to a "rolling road".
Anyone?
Thanks!:worship:
I have a tuned 700cc 126 engine for my Fiat, and it came with a 40 DCOE. I am hoping someone can provide their carb tune specs for the venturi and various jets for their engine so I can use it as a baseline before I go take it to a "rolling road".
Anyone?
Thanks!:worship:
OP
OP
Hello NW, Can you tell me what the venturi's you have fitted in the 40mm choke?? Thanks.
Ian.
At the rear of this book 'Weber Carburettors' by Haynes Publishing (ISBN 0 85696 393 3), there is an appendix of Weber conversion kits fitted to various cars. Included is the Fiat 126 Gruppo 2. I assume this is some kind of racing? (Group 2?)126, doesn't give any details of engine type/size/level of tune but someone on here probably knows of this car. Anyway this appendix give the settings for a single Weber 40 DCOE as fitted to what is probably a race car.
Fiat 126 Gruppo 2
1 x Weber 40 DCOE 102
Choke size = 28
Auxiliary venturi = 4.5
Main jet = 1.10
Emulsion tube = F11
Air corrector jet = 2.50
Idle jet = 0.45F8
Accelerator pump jet = 0.45
Accelerator pump inlet valve with exhaust orifice = 1.00
Needle valve = 1.75
Hope this helps someone
AL.
Fiat 126 Gruppo 2
1 x Weber 40 DCOE 102
Choke size = 28
Auxiliary venturi = 4.5
Main jet = 1.10
Emulsion tube = F11
Air corrector jet = 2.50
Idle jet = 0.45F8
Accelerator pump jet = 0.45
Accelerator pump inlet valve with exhaust orifice = 1.00
Needle valve = 1.75
Hope this helps someone
AL.
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Nowuries
New member
Hello NW, Can you tell me what the venturi's you have fitted in the 40mm choke?? Thanks.
Ian.
The carb has a 30mm venturi choke tube. There is no.marking on the auxiliary venturi.
Nowuries
New member
Thanks for the good info. Right now the carb (not a real Weber, an FJAS imposter) is set up with the following:
Choke size / primary venturi = 30mm
Auxiliary venturi = Unknown, no markings
Main jet = 1.20
Emulsion tube = F11
Air corrector jet = 1.80
Idle jet = 45F8
Accelerator pump jet = 0.35
Accelerator pump inlet valve with exhaust orifice = Have no idea what this is
Needle valve = 1.75
Any recommendations on changes for high altitude and/or normal altitude. I read you should decrease your main jet by .05 for every 5000 ft in density altitude, but other than that, I don't know what to change, if anything (and can't get to a rolling road/dyno easily).
Choke size / primary venturi = 30mm
Auxiliary venturi = Unknown, no markings
Main jet = 1.20
Emulsion tube = F11
Air corrector jet = 1.80
Idle jet = 45F8
Accelerator pump jet = 0.35
Accelerator pump inlet valve with exhaust orifice = Have no idea what this is
Needle valve = 1.75
Any recommendations on changes for high altitude and/or normal altitude. I read you should decrease your main jet by .05 for every 5000 ft in density altitude, but other than that, I don't know what to change, if anything (and can't get to a rolling road/dyno easily).
The following info comes from the book I mentioned (Weber Carburettors by Haynes) :-
The auxiliary venturi is located on the inlet side of the choke (aka the main venturi) and contains the mixture delivery nozzle which operates in conjunction with the main circuit.
The size marked in mm indicates the cross-sectional area of the narrowest part of the nozzle channel, which is equal to the area of a circle having the same diameter. For instance, the cross-sectional area of a nozzle in a 4.00 mm auxiliary nozzle will be Pi x 2.0 mmsquared since Pi x rsquared is the area of a circle.
By fitting a larger size auxiliary venturi, the main mixture supply will be delayed and vice versa. The most common size of aux. venturi ranges from 3.00 mm to 4.5 mm, although on some carburettors there is no choice. The eventual size will depend on the results obtained by carrying out the procedure given in the following Sections (sorry, too much to type out!) and therefore as an initial chice, a medium size should be fitted.
Also, in this book chapter it shows two choke diameter selection graphs. For the DCOE carbs, it recommends that for normal road usage, where low speed flexibility is desirable, a reduction in choke AREA of approximately 10% where one carb barrel feeds one cylinder and slightly more than 10% where one carb. barrel feeds more than one cylinder may be required.
AL.
The auxiliary venturi is located on the inlet side of the choke (aka the main venturi) and contains the mixture delivery nozzle which operates in conjunction with the main circuit.
The size marked in mm indicates the cross-sectional area of the narrowest part of the nozzle channel, which is equal to the area of a circle having the same diameter. For instance, the cross-sectional area of a nozzle in a 4.00 mm auxiliary nozzle will be Pi x 2.0 mmsquared since Pi x rsquared is the area of a circle.
By fitting a larger size auxiliary venturi, the main mixture supply will be delayed and vice versa. The most common size of aux. venturi ranges from 3.00 mm to 4.5 mm, although on some carburettors there is no choice. The eventual size will depend on the results obtained by carrying out the procedure given in the following Sections (sorry, too much to type out!) and therefore as an initial chice, a medium size should be fitted.
Also, in this book chapter it shows two choke diameter selection graphs. For the DCOE carbs, it recommends that for normal road usage, where low speed flexibility is desirable, a reduction in choke AREA of approximately 10% where one carb barrel feeds one cylinder and slightly more than 10% where one carb. barrel feeds more than one cylinder may be required.
AL.
Nowuries,
Sorry, I copied directly from the book.
'Accelerator pump inlet valve with exhaust orifice' is perhaps better known as an 'Accelerator pump back bleed'.
I've no info to hand re. changes to jetting etc. to take account of changes in altitude but I'll check further and post if I find anything possibly useful.
AL.
Sorry, I copied directly from the book.
'Accelerator pump inlet valve with exhaust orifice' is perhaps better known as an 'Accelerator pump back bleed'.
I've no info to hand re. changes to jetting etc. to take account of changes in altitude but I'll check further and post if I find anything possibly useful.
AL.
Last edited:
Nowuries
New member
I did remove the aux venturi and there were no markings. Similarly, there was no marking on the pump exhaust valve. That is likely what you get when you get a knock-off carb that the engine builder tried to pass as a real Weber 40 DCOE. Don't deal with Mr. Fiat... that is all I have to say at this point. PM me for more info if you intend to get an engine from them.
