DCO jetting

[gtvpete]

I'm converting my track car, good 'ol #73, from Spica to carbs (and from Class D to Class C), and having no prior Weber experience i need a little help in the jetting department.

The engine in question:
'73 2 Liter GTV
10:1 Motronic pistons
12mm Colombo cams, 254deg. duration at 0.05"
48 DCO/SP carbs, with 38mm main venturi

the goal:
7400 rpm rev limit
6900-7000 rpm max horsepower

I have read or gotten the following parameters at this point:

Main jets: 150 - 165 (depending who you ask/read)
Air corrector jets: 170 - 200 (depending who you ask)
Emulsion tube: F-16 or F9 or
Idle Jets: 50's or 55's, F8 or F9 or F11

Would be happy to receive lots more conflicting information, or the secret magic formula for free HP, or maybe just a good baseline starting point.

Thanks,
Peter Kemos

[italcarguy]

Peter,
To me that seems like too much carburator for the motor. 45's will work just fine. I have 45's on my 1750 full race motor, and the dyno guy told me I was on the border line for 40/45 DCOE's. Are you planning on eventually throwing bigger pistons in the car, if so, then the 48's will make a difference? What we did with my 2 liter and Fabrizio's is set the carb jetting at what the book says for GTA's since they came with 45's. From there, we threw the car on the dyno and made the nesccessary changes. If I were in your shoes, that's how I would approach it. Anyone else out there wanna add to this or have any other suggesstions?

[John M]

I concur with Anthony. I really hesitated about going to 45s with my 2L setup running 10.4:1, cut head, oversized valves, P & P, etc. I think the 48s will be overkill. I feel like the 45s on mine are probably overkill. I also understand where you are wanting to place your power curve and why you are considering 38mm venturis. I can say the 36s I am using really suck the low end out of the car. I would imagine that 38s will probably do the same or worse. Mains, probably 145 would do it. Airs and the rest seem to be on target. Papa is really good with this stuff too so be sure to get his opinion.

Hey Anthony, what was the setup on the GTA and for what size motor?

Best Regards,
John M

[papajam]

When one wants to increase the power output of an engine, the first thought is to increase to carb size i.e. from 40 to 45. Herein lies a basic misunderstanding on the operating principles of the DCOE. It's not the barrel size (40, 45, 48 etc.) that determines the potential power increase but the venturi size.
The first step in selecting the proper "setup" is to arrive at the correct venturi size. This is based on engine size and rpm of max power. There's a chart for this that I'll post here over the weekend. In Peter's case, his nicely warmed over 2l will produce max power in the high 6000rpm range. The 38mm venturis currently installed in his 48s are a little smaller than the 40mm venturis called for in the chart. But it's a good starting place.
Next up is calculating the barrel size. Simply multiply the venturi size by 1.25.
For the main jet size, multiply the venturi size by 4.
The air correctors are the main jet size plus 50 - 60.
The emulsion tubes and idle jets are based on engine size.

So for Peter's engine, I'd start with this;
Mains - 150 or 155
Air correctors - 200 to 215.
Emulsion tubes - F2 (but F16 will work too)
Idle jets - 50F9 or 55F9
Accelerator pump jets - 45

Keep in mind that while the mains affect the mixture thru out the rev range, the airs affect the mixture only toward the top end.
The difference in accelerator jets is not the amount of fuel they deliver but the length of time it takes to deliver it; the larger the accel jet, the shorter the delivery time.

[John M]

Here is a link to that chart Papa was referencing as well as additional info. Great page.

DCOE Tuning

Best Regards,
John M

[geh458]

Great document. I was wondering about this vary subject as well.

Thanks

[John M]

Here is another page with some good definitions of what each part does from the Triumph guys.

Triumph DCOE

Best Regards,
John M

[gtvpete]

Thanks for all the input guys.

Reason i'm going with the DCO's (this is a done deal already) is that yes, i will try to build a 'big' 2 liter next year, with the Cosworth pistons and all of the bells and whistles, and i don't want to have to get another set of carbs in 12 months.

At this point, the motor will be in an intermediate stage of development, thus the reason for choking the carbs down, etc.

Here's another puzzler: Who out there has fitted the widebody, DCO/SP style carbs on a stock Alfa manifold? The reason i ask is that the DCO's are wider than the DCOE's, and when mounted on normal mounting blocks (ignore for a moment that the mounting block inner diameters are too small also) the throttle shafts of the carbs will foul each other. Apparently one must either shave the throttle shafts and nuts down (alot), or fabricate an offset mounting block.

Anyone BTDT??

Peter

[italcarguy]

Factory 1600 GTA Jetting (45 DCOE 14)

Venturi: 30
Auxilary Venturi: 4.5
Main Jet: 135
Air Correction Jet: 220
Emulsion Tube: F-16
Idle Jet: 55F8
Starting Jet: 65F5
Needle Valve: 150
Accel. Pump Jet: 35
Accel. Pump Bypass: 50

[John M]

Ok....I am sitting here scratching my head wondering if I need to consider a really high air corrector. 220 air and 135 main seems like they were really looking to lean her out at higher rpms. I think I will get a set of 190s, 200s, and 210s and see which is best.

Pete....I don't have a clue on that one.

Best Regards,
John M

[papajam]

Wow! So THAT'S where this Weber document originated that I acquired a while ago. Now I can replace the two missing pages.
Thanks John!

[120]

that gta jetting looks strange to me. Why go with 45 dcoe then choke it down to 30 venturies?
and the idle jets are rich enough to choke a horse. so they really have very conservative numbers except for the idle jet.
top power rpm is held down by the venturies, must be very strong on twisty courses. low side of the rpms should feel pretty strong.
is that what you have found, italcarguy?

bruce

[gtvpete]

Can we address just the emulsion tubes for a moment?

I have gotten from various sources the recommendation to go with either:
f-16, f-9 or f-2

i found one resource online:
www.cruzers.com/~twakeman/TR/WeberDCOEinfo.htm

which has a very extensive write up on the functions of the jets, etc.

and it has a sequential, rich to lean, listing of emulsion tubes.

the f-16 and f-2 are listed midpack, while the f-9 is listed as being on the extreme lean side.
this confuses me as it makes little sense that one could get equally good results with either a moderate emulsion tube or a dead lean one....

could anyone comment?

thanks,
Peter

[italcarguy]

i'll look for my dyno sheets as well as fabrizio's to give you something better to work with. our venturies were bumped up to i think 38. i'll try to get it up some time tonight. i didn't think about it but our dyno results might be better for you to use over the standard gta set up since our motors are closer to yours than the 1600 GTA

[John M]

Hey Peter:

Here is a table for emulsions:

Cylinder capacity_________Suggested tube

250-325 ________________ F11
275-400 ________________ F15
350-475 ________________ F9, F16
450-575 ________________ F2

A 2L motor is right at a changing point but best would be F2, but most of these places say if you already got F16, not worth changing. F9 would be for more like a 1600.

Best Regards,
John M