Help w/ 40DCOEs - Alfa Romeo Bulletin Board & Forums

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Old 11-01-2005, 05:07 PM
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Help w/ 40DCOEs

Hoping for some help. I have been restoring a 74 Spider w/ Weber 40DCOEs and having trouble getting the car to run (idle or otherwise). Anyway, tonight, I found the car was running on the front 2 cylinders only! I assume this is a carb problem as it is simply not running on cylinders 3 & 4 at idle. I found this by pulling off the plug wires. The back carb is getting fuel because there is plenty of fuel in the bowl. Any ideas why the fuel is not getting to the cylinders? Thanks.
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Old 11-02-2005, 11:48 AM
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I am no expert, but I'd check to see if you have the carbs so far out of sync that it has the back carb almost shut down. I have seen this in other carbed Alfas before. When you pull the wires, the engine doesn't seem to be affected any more (in a bad way, that is) than if the wires are connected. Here is a link to a simple method for synchronizing the carbs: Shortcut to: http://www.teglerizer.com/dcoe/tweakingdcoes.htm Another way to do it is to remove all linkage from the carbs, turn out the idle screw till the throttle is closed, then hold the front carb closed and screw the sync screw down till it it just barely seats on the tab for the front carb. That isn't perfect, but it should be good enough to let the motor run.

I am assuming from your post that you have some firing on 3 and 4 above idle, is that true? Also, you mention that you have fuel in the bowl. Did you check to see if it is enough to cause the carb to operate? That article tells you how much you should have in the bowl. If it is low, perhaps the little screen filter is clogged up. Other possibilities might include the choke being stuck or trash in the idle circuit.

Hope my ramblings give you some ideas.

Good luck.
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Old 11-02-2005, 10:53 PM
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you might also want to remove the carbs and check if the throttle plates are properly centered, and if the progression holes for each carb are in the same spot. Throttle plates not being properly centered will ruin the proper airflow at low throttle/idle. Progression holes beig different from carb to carb is a sign that your carbs are not a matched set and that someone patched them togeter at a previous date. They may never run well together. What do you know about the history of the set-up?

Pat Braden's book on weber carbs is a nice thing to own.
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Old 11-03-2005, 01:53 AM
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First thing to check is the mechanical aspects of fuel delivery to the carb and the obvious vacuum issues.

Check the filter, the float and the valve in the front carb - you need to ensure gas is getting into the float chamber.

Check the linkage as described earlier.

The other thing you need to check is that the fat rubber mounting seals properly against head and carb - if it's holed, perished, loose, cracked or badly seated then there wont be enough vacuum to pull fuel through.

After that it's all pressures and vacuums and other esoteric hard to understand stuff. But at least there's no more moving parts ....
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Old 11-03-2005, 05:55 AM
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Help w/ 40DCOEs

Sono,
The Weber conversion was a kit done by a previous owner who was an Alfa mechanic, but he is no longer around and his old shop partner knows nothing about Alfas and doesn't care to! I checked the throttle plates/progression holes and feel confident they are close from carb to carb.

Othercarsaford,
I do have firing above idle on 3 & 4. Last night, I put in new plugs and tried adjusting each idle mixture screw individually just to see what happened and the engine ran much stronger, but the idle mixture screws were 4-5 turns out. That seems like a lot to me? After doing this, I could pull any of the four plug wires and have little effect on the engine at all at idle. I have eliminated the choke as suggested by several posts on this board and the screen filters are clean. I will check everything again per the article you linked.
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Old 11-03-2005, 06:03 AM
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4 to 5 turns on the idle mixture screws is huge. I seem to remember that new carbs may use 1.5 to 2 turns, and my '68 Junior takes 3/4s to 1 turn to be about right. It sounds like you may have a vacuum leak. Here is another link to a carb tuning article that might help...Shortcut to: http://www.geocities.com/motorcity/d.../webrtun1.html

Also, I have assumed that everything else about your motor is working properly. Unfortunately, valve and compression issues greatly affect how well carbs will tune. If you have an older, tired motor, the carbs may be showing you a larger problem with the motor.
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Old 11-03-2005, 07:16 AM
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The base number of mixture screw turns can depend on the model of 40 DCOE. I've read that the later emission version DCOEs, like the 151/152 series, require 3+ turns while in my experience, the older non-emission carbs, like the 40 DCOE 27, 28 & 32 need no more than 1 full turn. Any turns more or less than the 'norm' could indicate incorrect idle jets (assuming that the engine mechanicals are in good order). What model DCOEs do you have?
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Old 11-03-2005, 08:38 AM
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Othercarsaford,
Motor was rebuilt a "few" thousand miles before we purchased the car, after which my wife had an encounter with a tree, hence the restoration project.
Papajam,
the webers are 40 DCOE 77 models.
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Old 11-03-2005, 08:41 AM
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holtmt

Had same problem twice on different cars with head gasket not sealing; check for coolant presurisation & compression.

Good luck

Richard
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Old 11-14-2005, 03:44 PM
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What F number emulsion tubes does your 40DCOE 77 have?
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Old 11-15-2005, 09:44 AM
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There's a very nice little software program I've found helpful when setting up carbs.
You can find a copy of it here: http://greend.com/carb_software.htm
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Old 11-15-2005, 10:51 AM
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Thanks Gary, I can set them up. Just want to play with a different set of emulsion tubes. Want to buy 4-F34
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Old 11-15-2005, 11:09 AM
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Pierce Manifolds has everything Weber if you're in the U.S.
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Old 11-15-2005, 03:38 PM
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Thanks, will try them.
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Old 11-16-2005, 07:40 AM
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Another thing to check is for air leakage between the carbs and the head. There are multiple gasket interfaces - head to intake manifold, manifold to carb mounts, mounts to carb. The carb mounts can also be cracked and leak. If you are introducing air into the system anywhere downstream of the carbs (and it doesn't take much), you'll go nuts trying to get a stable idle. At higher speeds, a small leak is less noticeable.
Good Luck
Wil Painter
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