Weber DCOE help?

[eschneider]

I'm trying to set up a SINGLE Weber 40DCOE-2 on a Lotus/Morgan 1500 motor. I don't know what the compression or cam specs are.

I arrived at some initial settings to get started, with the aid of my ex-weber vendor, who isn't answering my emails since I purchased the parts from him. Not gonna throw names around - my point is I'm on my own now.

I'm generally pretty Weber-saavy, but this is the first time I've set up a DCOE from scratch. I know the "idle" (misnomer) jets are the first item to tune, but I'm having a hard time isolating the idle and pump jet function as my first problem.

I attached a photo of the carb along with a chart of the methodology to date. Any help is appreciated!

 

[Dallarax19]

I have twin DCOEs on my car but a single is quite different. Look at the following links:

Paste this one in the URL bar
www.innovatemotorsports.com/forums/attachment.php?attachmentid=732&d=1147100105 -

http://www.innovatemotorsports.com/forums/archive/index.php/t-5368.html

http://www.teglerizer.com/dcoe/rasorcom.htm

http://www.fordcaprilaser.co.uk/weber_dcoe_carbs.htm


email this guy and he should be able to help you
http://stores.ebay.com/ALFA1750S-CARBURETORS-PARTS-STORE

I can tell you that your jetting matches the starting set up for a single cylinder per barrel 1500. My speculation is you need to tune the barrel for two cylinders, ie 750cc effective cylinder vol. With that said I think the DCOE 40 is a too small.

From the Haynes Weber book here are some examples of jetting for a single DCOE
Lotus Europa - 45DCOE13 34,3.5,150,f2,180,45f8,40,50,200

From the HP Books
Midget Spitfire - 40DCOE18 28,4.5, 125,F16,160,45F9,50
Midget Spitfire - 45DCOE13 34,3.5, 150,F2,175,55F8,50
Honda Civic - 40DCOE2 30,4.5,135,F16,180,45F9,35

Your first entry looks like you are leaning out at the top end. I can't really tell if you are trouncing on the throttle or rolling it. If you roll on and it peters out and starts to spit and pop through the carbs you need larger main jets. (On a side note I suspect you need to open your chokes to 34s or probably 36s to get the proper air flow too (again opinion). I am thinking your mains should be like a 180-200 also.)

Lastly I think your emulsion tube is too weak. If you have a F15 or better yet, F16, try that I think that will help. From your other notes it looks like the pumpjet is compensating for a weak top end.

First please read the links, they will help. If it were me I would try larger mains first and then the richer emulsion tubes. My gut feel is that will get you to where you want to be.

I don't know the specs on your engine but if you have performance parts, cam, pistons, high flow head, I am certain one DCOE40 is not enough.

Good luck, please keep us posted.

Brian

 

[eschneider]

thanks

exactly the kind of comments I was hoping for!
I will do some more studying per the links you provided......

FWIW, rolling the throttle was what I was calling "moderate acceleration" in my notes. "Trouncing" = heavy acceleration
Specifically, your comment about a huge pump jet compensating for small mains was helpful. I was thinking that might be the problem.
The comments about tuning for 1 vs 2 cylinders was good. I suspect that is the case.

Yes, a single DCOE is small for the engine -- and yes, larger venturis could be used.... But this car (not mine) is being set up for lower RPM ranges rather than full throttle airflow, so the carb and chokes suit the use of the car.

 

[TonyK]

Carb settings.

Okay, I have not dabbled with the 40 DCOE but have spent 29 years on a single 36 DCNF.

Let us look at the basics of the Carb on a Fiat X1/9. First off it was very small with 28 mm barrel and 32 mm vacuum controled secodary. The point here is that the car ran, not great but it ran. It was strangled. Just putting a 36 DCNF on the engine gave it a 15 HP increase. So, your car will run with a single DCOE, maybe not to it's full potenial, but it will run.

What we are trying to do is obtain 14.7 parts of air to 1 part of fuel that is constant through out the entire rpm range. It the engine can draw more and use more it will produce more power. Put that little DTRA on the car and it will run, not great but it will run.

Nomally the idle jet circuit is by it's self. Meaning that if you have the car running at idle you can remove the main jets and nothing will happen. If it does then the barrel/butter fly/ variable ventiuri is partly open and the car is not running on the idle circuit alone but also on the main jet circuit. If this is the case, then the idle jet is too small to feed the engine. Bump it up. Venturies or chokes change the velocity of the air at the mixing point in a carb. If the Venturi is too large for the engine with the correct jet size, then the engine will bog down, because the air flow slows and the mixture goes lean. To compensate a pump jet or acceleration jet is used for this brief time to keep the mixture rich and stop the engine from hesitating. Normally I start with a smaller choke or venturi and get past this point. If the venturi is too small top end will suffer as not enough air/ volume can pass through the venturi to allow the engine to produce it's rated HP. I wish there was an easy way to do all of this, but it is trial and error and run down the road. Because the car may have different piston compression and camshaft what workes on one car will not be the best set up in another. Change just one thing at a time. The emulsion tubes alow for the mixing of a small amount of air with fuel that enters the main stream of air at the centre of the venturi. These matter as well but in my experience not as much as main jets and air correction jets.

Here is my jetting box, on the lid, venturi, main, air correction and idle jet size.


At times I wanted a lot of top end, so I used a larger venturi and then had to make the other jetting changes. Sometimes I wanted good bottom end and better mileage so I used the smaller venturi set up. Once you have the numbers figured out, a change was less than 10 minutes to do.

This information is just the basics the rest will be up to you.

As for the amount of jets I have on hand, some were purchased new, others from ebay and Italian car mechanics that had a drawer in their tool box of more jets than they would ever use.

TonyK

Grimsby Ontario, Canada.