[Alfa Romeo Red]

HI Guys,

Has anyone purchased new weber carburetors that are made in Spain. Model 40 dcoe 151.

The standard set up for these carbs from the supplier.

Standard Jetting:
• Venturi / Choke : 30 mm
• Main jet: 115
• Emulsion tube: F11
• Air corrector jet: 200
• Pump exhaust valve: 50
• Idle jet: 45F9
• Pump jet: 40
• Needle valve: 1.75
• Starter Jet: 85F9
• Starter Air Jet: 150

I noticed on the Centerline website a data sheet that lists the recommended setup for your car model. For the 2000 they have two options. Which is best suited for these 151 carbs? Data sheet attached.

I have only changed the main jets to 130. Not much difference in how the car drives. I would say a bit better.

My car has a rebuilt engine with high compression pistons, CB road cams, euro setup with euro manifold.

Just trying to get a bitter sense on what I should consider buying and switching out.

 

[malcolm1]

The standard set-up looks more suited to a 1600. For a 2 litre, I would say, that the Centerline 1750/200 DCOE32 spec is more appropriate. Given your use of high compression pistons and CB road cams perhaps more sporty 34 venturis would be appropriate with additional knowlegeable/dyno based set-up tweaks.

 

[Gordon Raymond]

Contact "alfaparticle" member, on the BB. He runs emission Webers on his Hi-per 2L with excellent results and has researched the jetting needed for 151's on modified engines.

 

[alfaparticle]

I am now using 45DCOE152's with 36 Venturis and my current jetting is different from 40DCOE151 but I did get very good results with 40DCOE116/117 with 34 mm venturis on a previous motor.
I agree with Malcolm that 30 mm venturis are too small for good top end performance and either 32 or 34 will give better results.

 

[Alfa Romeo Red]

The venturi I have are 32. Do you think I would see any noticeable difference going up to 34?

Anything else I should consider changing. Main jets are now 130 and venturi 32.

Car seems to run good. Seems a bit rich. Plugs look OK but exhaust smell rich at idle.

Thanks for the help.

 

[yvesmontreal]

I'll let others comment on the performance, but should you need them, I have a set of 34 venturis doing nothing.

 

[malcolm1]

Larger venturis will enable peak power at higher rpm, possibly at the expense of some on road driveability Selection and tuning of Weber DCOE Carburettors

 

[Alfa Romeo Red]

Malcolm, thanks for info. Very good read. I will probably stay with the 32 venturi.

Maybe switch emulsion tube to F9 and idlejet to 50F8. Keep air corrector jet at 200.

I didn't see any information on the needle valve. Currently at 1.75 looking to switch to 1.50.

 

[alfaparticle]

Larger venturis will enable peak power at higher rpm, possibly at the expense of some on road driveability

Exactly. When I increased venturi size from 34 to 36 the peak power rpm moved from 6200 to 6500 rpm.

I also did some tests comparing F9, F11 and F16 emulsion tubes in my 45DCOE152's and F11 were best. Jim Steck also did dyno tests on a Ford race motor with the same tubes and he came to the same conclusion as me.
I would keep the F11's.

 

[Alfa Romeo Red]

Ok. Any suggestions on the idle jets and needle valve?

 

[alfaparticle]

50F8 is a good starting point and 1.75 needle jets are good.

 

[FlyingBanana]

I am now using 45DCOE152's with 36 Venturis and my current jetting is different from 40DCOE151 but I did get very good results with 40DCOE116/117 with 34 mm venturis on a previous motor.
I agree with Malcolm that 30 mm venturis are too small for good top end performance and either 32 or 34 will give better results.

Hi Ed,

Have you had a chance to dyno your car with the larger carbs fitted? I would be interested to see your results and also hear your thoughts on how the torque curve of the car has shifted with the larger carbs / venturis.

I'm running RJs cams in my car on a pair of 40 DCOEs (which are using venturis opened out to 33mm on a lathe with a decent venturi profile). I've got a Zeitronix ZT2 so have sorted the AFRs as best I can. I've not had the car on a dyno to check the power & torque. The car runs very well but I feel I may be missing some top end.

thanks

 

[alfaparticle]

Post 19 of this thread but note that there were also changes made to the head, pistons and intake manifold. The cams, cold air intake and exhaust were unchanged. 79 Spider on a dyno - The Sequel
The gains in torque and HP are mainly above 4500 rpm.

 

[Alfa Romeo Red]

Just wondering what kind of differences I will notice by switching out the idle jets from 45F9 to 50F8.

 

[Gordon Raymond]

The air "F" 9 hole is 1.0mm diameter. The "F" 8 hole is 1.2mm . A 50F8 is leaner than a 50F9. A 45F9 is richer than a 45F8. In use, at idle, after you adjust the idle mix needles, both will idle similarly, BUT transition A/F mix will change and you may either eliminate a transition bog, or create one. Without using an A/F meter to see what's happening, most use the trial and error method of starting with a chart baseline for your engine application, lets say your 45F9. The go to a 50 or 55 F9 and see what happens to low to high speed circuit transition.
Generally a bog suggests you go richer. Some performance engines do better with F8's again going from a lean fuel hole to a richer one until transition bog disappears.
F hole sizes can be confusing. Here's my work chart.

 

[alfaparticle]

In Des Hammil's Weber book he wrote that the fuel orifice generally follows engine capacity and that 45's are usually right for 1600's and 50's for 2000's. Jim Steck told me to treat idle jets like emulsion tube stacks where the main jet corresponds to the fuel orifice and has greater influence in the lower end of the progression and the air hole is equivalent to the air corrector and has more influence on the high end of the progression range. However if you have a modified motor you definitely need a wideband AFR as Gordon suggests. My hot 2Lneeds 60F9.

 

[Alfa Romeo Red]

Thanks for the insight. Much appreciated.

 

[Gordon Raymond]

To simplify idle jet selection, just think "It's complicated". More is going on there than many suppose at first glance, particularly when you consider EACH jet is an emulsion tube and jet.
Ready for it to get worse? Here we go. First of all, DO NOT FOOL WITH THIS! To further influence idle and progression circuit (transition), one can change pump jet size (jets), pump jet volume (rod length and exhaust jet). Also for the truly obsessed, timing, by custom drilled butterfly bars and pump cam positioning! DO NOT DO THIS! It is beyond the scope of necessity and complexity combined with other DCOE system function. Heres why. Every DCOE system has an effect on every other adjustable system. For example, idle mix adds to high speed mix, but NOT the other way around. The DCOE system was designed for maximum adjustable functions, but you do not need to adjust everything for best function in any application. Some places where odd adjustments might be needed are drilled butterfly plates (holes of variable sizes) and complex emulsion tubes used on the older Lotus engines with the long intake runners cast as a manifold as part of the head. You do not need to do that with an Alfa engine!
Many prefer F-16 emulsion tubes because they work the same with slightly variable or changing fuel levels in float bowls caused by race or street fuel slosh, too low fuel flow (not pressure!) odd float settings, too small or big needle and seat and more. It's a forgiving type tube. Others are not as flexible.
Over time, Alfa and DIY experiment has led Ed and Jim, as mentioned above, to recognize what works. It is still possible to experiment endlessly with the DCOE system, but any big gains will be harder and harder to locate in the system function.
Alfa and Weber figured that #35 pump jets are just fine. You may need 40's for some applications, but seldom bigger or smaller. You do not need LONG pump rods. With Alfa engines, pump volume is adequate, and longer rods just need bigger pump exhaust jets to avoid a too fat mix in early high speed circuit with good vacuum.
Again, normal pump exhaust size will be 45 to 70, most around 50.
This goes on and on because the DCOE system was designed for flexibility. As the DCOE system is vacuum operated, mix does not change quickly, but with engine speed (vacuum) and load. Ed's street test system is excellent, and I defer to his experience using emission Weber set-up's for street or track.
It's not electronic fuel injection, and will not be adjusted to give A/F mix like FI.
It is both interesting and one of the best non FI systems available for the performance enthusiast.
ALL the above is only my opinions based on my own experiences.

 

[Alfa Romeo Red]

Thank you Gordon.

 

[alfaparticle]

Just wondering what kind of differences I will notice by switching out the idle jets from 45F9 to 50F8.

There may not be much difference. If you are happy with idle. low speed and cruising performance then there is not much reason to change them but if you do change them then try to come up with some objective comparison tests and swap back and forth at least a couple of times. You may also find that one jet size works best in summer and the other in winter. Again, a wideband AFR gauge is invaluable for setting up Webers but you may not be able to improve on the factory settings in a stock engine.