Project DCOE

On to jetting. Engine fires right up, carbs are balanced, idle is smooth. Jumped in the car and power take up from stop is fine but.....beyond initial tip in of the throttle the motor gets the blind staggers and feels like it's running on just two cylinders. Me think the main circuit is way too lean?
Jetting is:
mains 115
air correctors 170
emulsion tubes F11.
Remember, it's a 1500 which is stock other than the header and carbs.
 
I would strongly recommend to install a wideband O2 gauge. Once you have set up carbs with one, you realize that it is the only way.
And it makes it much more fun too!
Just did that with a hotrod X motor today. :)
 
For even more FUN, install an sideband O2 sensor and thermocouple (exhaust temp can be useful) on each exhaust port. This way data from each cylinder can be made available for evaluation.

Running conditions have a very significant effect on set up.


Bernice
 
Hi Carl,
My current set-up is mains 135
air correctors 170
emulsion tubes F15.
Also running 40 DCOE's. (40 DCOE 18)

My engine is not stock. Intensively modified PBS head with bigger valves, a more radical cam, lot of shaving, hi-comp pistons.

Since the car is not a driver yet, I can only say the engine seems to be running strong, with only a tendency of idling too fast... That's also where I am actually.

Transition between idle and acceleration is one of the challenge of these carbs and there is no easy answer. In theory your F11 emulsion tubes are barely enough for a 1300cc. That maybe the problem, but I doubt it. I think it would show only at the limit, but I'm no expert.

Some carb guru can jump in, but in theory, at idle, fuel is entering by the idle mixture screw. Under acceleration , it's coming first by the idle mixture screw, the progression holes and finaly also by the pump jets. Then, when normal speed is reached, the main jets are regulating the quantity of fuel.

The first question would be, is this a transition problem or is this just because there is not enough fuel in the normal running phase?

Timing may also be part of the equation. Have you tried loosening the distributor bolt then turning the distributor while the engine is in the running phase? Also looking into the carbs, notice the fuel delivery related to the different phases. Does it seems there a fuel cut-off at one stage?

In my case, I worked weeks to discover I had a weak acceleraton pump rod spring, causing one carb's throttle to remain barely open, then leading to backfires (or spitting).

Ulix, tell us more about the wideband O2 gauge...

Daniel Forest
 
Turns out the problem was a very loose jet stack in #1 venturi so it now runs tentatively OK through the throttle range....at least as far as I can tell by just driving through the neighborhood. The carbs came with among other things a set of 125 and 135 mains but until I get out of the neighborhood I won't be able to dial in final jetting. Dual 40 IDFs as used on my spiders for years run 130-135 mains, 210 air corrector and F11 emulsion tubes and as I recall, the 40 IDFs I ran on my 1500 128 ran the same jetting. O2 sensors are great, I used a digital volt meter and a standard FI spider oxygen sensor on one of my spiders to set the jetting on my IDFs. I run long tube headers on the X and have no clue where to run a single oxygen sensor unless I just tap into one tube and assume the other cylinders are the same.
 
Unlike a narrow-band O2 sensor, the wideband does not like to be mounted very close to the engine. So on most headers you can mount it in or after the collector.
With a 4-2-1 header, you could mount it into the "2" part, where you are monitoring only two cylinders. I have done both in the past.
A nice guide to where and how to mount it is found here:
https://wbo2.com/lsu/LsuInstal.pdf

I use a AEM gauge, mostly because it can be switched to Lambda rather than AFM. (In Europe, Lambda is more commonly used).
http://www.aemelectronics.com/?q=pr...ies-wideband-uego-afr-sensor-controller-gauge
(I also have an Innovate LM-2 which allows data logging, but I don't use it).

This gauge, combined with some understanding of the different "circuits" of the carbs and a selection of jets etc. makes for a lot of fun.
Ideally, drive around with a like-minded passenger and test various conditions. The passenger monitors the gauge and takes notes. Then you pull over and discuss the symptoms and possible cures.
Then pull out the tools make changes (only one at a time!). Record the changes and corresponding test results in the notebook.
If you are a geek like me, this process is very enjoyable, especially when the engine starts running better and better.
 
I thought about taking it to a dyno that uses a tailpipe sniffer to dial in the jetting but I'm guessing that's just max power (throttle wide open) and not for real world street driving where you need midrange and even low rpm jetting done right.
 
Exactly.
The dyno would be great to determine optimum cam timing or ignition timing or things like that.
 
Still plugging away but now it's the little things....as in getting the linkage to work right. The carbs came with a hand made interlink which actually worked fairly well but carb balancing was fiddly to do. I went to Pierce Manifolds and got the standard DCOE interlink hardware. When the two pieces arrived I installed them and reinstalled the carbs on the manifold (I bet I have installed and removed the carbs at least 10 times so far). Found out the arm with the ball connector was not in the proper location for my linkage, too far down and to the back, so I modified the piece with a welded on arm in the proper location. Hooked everything up and found some resistance when using the gas pedal which turned out to be the fitting on the end of the cable getting caught on one of the mounting bolts for the cable mounting plate on the cam cover. Bent the plate and solved that. Fired up the car and couldn't get it to idle down....turns out the interlink was hitting the manifold. Off came the carbs and I filed down the interlink and the intake manifold flange to solve that problem. Reinstalled the carbs and fired up the motor. After warming up the motor crept to about 1200rpm and I could not get it to idle any lower even though the idle speed screw wasn't even touching the linkage but clearly the throttle plates were closed. My guess is the handmade plastic carb mount gaskets are leaking air so I ordered new carb mounting gaskets.

All this sounds like a major pain in the ass, especially when you remember the car was running wonderful with the single carb, but I'm used to this with all the other carb setups I have played with on Fiats so it's more a challenge of problem solving. I'm sure I'll have this sorted by the time it's too cold to drive a heaterless car.
 
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Hi Dallarax19,

If you had to start again, anything you would do differently about the carbs installation?

What is the model of the carbs. I'm running 40 DCOE 18 while also rebuilding a pair of 40 DCOE 2.

I noticed your fuel pump is in a high location. That's where I put mine when running DCNF's. But after 4 pumps in less than 2 years, I finally decided to give a consideration to those people saying the fuel pump should be mounted low so it wouldn't had to "pull" the fuel all the time.

Any reason for the special nuts you used on the carb to manifold studs? More easy to remove?

I like your solid dog bone engine mount.

I Wonder if it is better to feed the carbs as you did (separately) or like myself (from one carb to the other, pic on page 1). I started collecting AN fitting but choosed to invest my time (and money) in making the car run first. Regular hoses will do for now.

I also noticed the bumps were cut on your shocks apron. Are you using Carrera camber plates like me (can also be seen on page 1) or something custom made?

No progress on my side. I wanted to do a video of the carbs running and discovered my fuel pump wasn't working anymore. I think I probably pulled to much on some wires at the ignition switch while working under the dash to move again one of the master to change for new hoses. I didn't like the new Tygone ones I fitted, too much prone to kink and leak. So I'm one step backward, having to find what is the problem: ignition switch, wiring of the fuel pump itself.
 
Hey Daniel

1) i would do 45s
2) DCOE 40 151
3)fuel pump is low but regulator is high
4) nuts came with grommet refurb kit
5)Maximum Motorsports camber plates, i wouldnt use them again
 
Thanks for all that great info. What publication did you get that DCOE circuit explanation from?
I have not gotten there yet but what is your fresh air supply for the carb intakes, do you have an opening in the trunk lid? I was thinking of some kind of vent just above the aircleaners but offset so any rain would not fall onto the aircleaners. I don't plan on driving in the rain but there will be days when it's parked outside.
 
First drive. Took the car out for a short four mile drive. First impressions are the car is rather "energetic". This is typical of all my dual carb conversions on Fiats, the throttle response becomes instant and makes single carbs and FI feel numb by comparison. The throttle is actually a bit hair trigger and I need to work on the linkage ratios to slow it down a bit, fine for the race track but a bit touchy for the street. The sound of course is adrenalin inducing, again a typical symptom of dual Webers. Redline seems to come quicker than I'm used to but I'm going to try fatter jetting as I think it's running a bit too lean for full throttle, this is all in my head. Idle sit's at around 1,000rpm full warmed but with the throttle stop screw all the way off so I still have an air leak, I guess. I installed new carb to manifold gaskets but I have no clue about the manifold to engine gasket so I'll replace those next.

This whole car is a fun project. I drove a friend's stock FI X a week ago and it felt really numb compared to mine. Stripping the car, adding headers and dual carbs will put a smile on your face even if the practicality aspect goes down.

I have not driven my X in a few weeks and I'm reminded of how driver inputs are much sharper on an X than on a 124, they are really different generations in design....now if the X shifted like a spider ...........
 
Air to the carbs... I started thinking about that also and decided to wait until the car will be running.
Option 1 (easiest) I already removed the rain tray of the engine lid and won't put back the access plate between the engine and the trunk. So there will be some air coming in.
Option 2: Building an air box over the air horns and having ducts coming from my Abarth Snorkel.
Option 3: Raising the front of the trunk lid to have it sitting like 1/2 or 1 inch higher to let air coming in. But there may be too much air entering. So I would add two 3 inches circular holes behind the licence plate and put spacer on the plate so it will stay 1/2 away from the wall.
Option 3b: Raising the rear of the trunk lid to have it sitting like 1/2 or 1 inch higher to let air coming in.
Option 4: grill/scoop on the trunk lid.

Each one had advantages and drawbacks, but this is fun tuning, euh, I mean fine tuning.
 
I actually thought about raising the front lip of the rear trunk....wonder if anyone has done that.
Since my car get parked outside I removed my raintray delete engine cover and installed a stock engine cover but removed the rear rubber seal and cut out the two end closure pieces to let the cover vent engine heat a bit without letting in a lot of rain water.
 
FWIW I had built ducting work from the side vents, built a fiberglass airbox and closed off the trunk with fiberglass and observed no improvement. I ditched it all and run UNI filters with a very thin stainless heat shield between the carbs and header and removed the rain tray from the engine cover. Academically you could probably make some rational about cold air ducting but I was never able to see an improvement on the dyno nor any "seat of the pants" improvement. As it is I never have had any hot idle issues or carb problems from heat soak as well.
 
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