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Discussion Starter #1
Firstly, the issue. The car starts and idles. That's not the problem. The issue arises when the gas is pressed. From about 2000-3000 rpm the car sounds like its being choked, and performs similarly. The problem arose after I had attempted to adjust the idle mixture. Of course after the problem started I adjusted the mixture back to where it was.

About the carbs/timing setup. I have a custom distributor running Pertronix Igniter III with mechanical advance. The carbs are early 40 DCOEs equipped with brass floats and no air bleed screws. They are jetted as follows:
Main jets - 115
Emulsion tubes - F16
Air corrector jets - 150
Idle jets - 50 F9
Accelerator pump jet - 35

I've tried and failed for two days now to solve the problem. Here's a list of what I have done.
-Removed the Air Filters
-Cleaned the spark plugs
-Cleaned all the jets, Idle, Accel, Main
-Checked fuel pressure(3.5 PSI to the first carb, 3.1 to the second)
-Checked action of the Accel pump
-Cleared throats of all four cylinders
-Double checked initial timing
-Cleaned Spark plugs a second time
-Double checked the synchronization and balance of the carbs(~7 Kg/h on the synchrometer)
-Checked the internal carb fuel filters
-Changed the oil (it needed it, I honestly don't think this had anything to do with it for obvious reasons)

None of this has resulted in a successful solution to the problem.

My next thought is a vacuum leak of some sort in the system. How would I go about testing/fixing this?
Is there something blatantly obvious I'm missing?

I'm starting to feel nostalgic for my 32/36 DGEV
 

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Opel Rallier since 1977
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  • What did the plugs look like when you had them out? Carboned up? Did they smell of fuel? Did they all look the same?
  • Are the idle mixture screws adjusted all the same number of turns out from the fully closed positions? Are they around 1/2 to 1-1/2 turns out from the stops? (Since you have the older carbs.)
  • How warmed up is the engine when you try to drive it?
  • A check when not running: If you pull the plugs off of the top of the progression holes (which are the brass plugs on top of each barrel just in front of the mounting flange) and look down through the 2-3 tiny holes, and move the throttle blades ever so slightly off of the stop, do you see the edge of the throttle blade move past the tiny hole closest to the engine, and then the next one as the throttle is opened more and then the 3rd one? (This is easiest to see if you have the carbs off, but if you shine a light down the carb throat from the inlet, this will help.) This is a check to see if the basic idle setting is right. all 3 holes should be blocked by the edge of the throttle blade when the throttle is closed to the idle position.
  • What model number is stamped on each carb?
  • What chokes (venturis) are in the throats? These are the aluminum 'rings' inside the throats, and have a small number cast on the bottom edge; they will be something like 32 or 34 or 36....
  • Does the 'choking' occur regardless of how slowly or rapidly you press the throttle?
  • Have you adjusted the float rise and drop per the manual?
 

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Discussion Starter #3
  • What did the plugs look like when you had them out? Carboned up? Did they smell of fuel? Did they all look the same?
  • Are the idle mixture screws adjusted all the same number of turns out from the fully closed positions? Are they around 1/2 to 1-1/2 turns out from the stops? (Since you have the older carbs.)
  • How warmed up is the engine when you try to drive it?
  • A check when not running: If you pull the plugs off of the top of the progression holes (which are the brass plugs on top of each barrel just in front of the mounting flange) and look down through the 2-3 tiny holes, and move the throttle blades ever so slightly off of the stop, do you see the edge of the throttle blade move past the tiny hole closest to the engine, and then the next one as the throttle is opened more and then the 3rd one? (This is easiest to see if you have the carbs off, but if you shine a light down the carb throat from the inlet, this will help.) This is a check to see if the basic idle setting is right. all 3 holes should be blocked by the edge of the throttle blade when the throttle is closed to the idle position.
  • What model number is stamped on each carb?
  • What chokes (venturis) are in the throats? These are the aluminum 'rings' inside the throats, and have a small number cast on the bottom edge; they will be something like 32 or 34 or 36....
  • Does the 'choking' occur regardless of how slowly or rapidly you press the throttle?
  • Have you adjusted the float rise and drop per the manual?
I'll try to answer your questions in order as best I can

  • The plugs were carboned up. I didn't think to smell them. All 4 looked identical
  • I have the idle mixture screws turned 1.25 turns out from the fully seated stops. One of the issues I've had tuning these specific carbs is turning the mixture all the way down doesn't see to produce any change in engine rpm. When I was adjusting the before the issue arose, I would blip the power a little bit to see if the idle would adjust, but this too resulted in no change
  • the engine has varied from from cold to normal operating temp throughout the time the issue has been present, no change
  • I can check this tomorrow in the daylight
  • the model stamped on each carb 40 DCOE 2, 9 C
  • I'm having trouble finding the venturi size on the carbs. I'll try again tomorrow in the daylight
  • the choking is occurring no matter the rate throttle is applied
  • the last I adjusted the floats was when I rebuilt the carbs about a year and a half ago. I used the paper included in the Redline rebuild kit
 

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Opel Rallier since 1977
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1,710 Posts
  • The plugs were carboned up. I didn't think to smell them. All 4 looked identical
  • I have the idle mixture screws turned 1.25 turns out from the fully seated stops. One of the issues I've had tuning these specific carbs is turning the mixture all the way down doesn't see to produce any change in engine rpm. When I was adjusting the before the issue arose, I would blip the power a little bit to see if the idle would adjust, but this too resulted in no change
  • the engine has varied from from cold to normal operating temp throughout the time the issue has been present, no change
  • I can check this tomorrow in the daylight
  • the model stamped on each carb 40 DCOE 2, 9 C
  • I'm having trouble finding the venturi size on the carbs. I'll try again tomorrow in the daylight
  • the choking is occurring no matter the rate throttle is applied
  • the last I adjusted the floats was when I rebuilt the carbs about a year and a half ago. I used the paper included in the Redline rebuild kit
OK thanks very much. Is the 40 DOCE 2,9C on each carb or is one a '2' and the other a '9'? (And I am not expert at all the different model numbers!)

Your reply to the 2nd bullet is the most important above IMHO: The fact that the mixture screws are losing 'command' of the idle mixture. This almost certainly means that when you check the progression holes (by viewing via the plugs above them), you are going to find at least one of the holes 'behind' the closed throttle plate; i.e., the edge upper throttle plate when closed will be forward (towards the inlet) of more of the progression holes than it should be.
When this happens, then you are idling mainly off of the progression holes, not mainly off of the idle mixture hole behind the closed throttle plates, and the mixture screws cannot vary the mixture properly...they lose 'command' of the idle mixture. (Note that the same fuel-air mixture is fed to the progression holes as to the idle mixture screw.)
This could be due to the throttle plates being set too far open, or you may have vacuum leaks that are forcing you to open it too far to just get the car to run. What manifolds do you have? Is there a vacuum connection to the brake booster? If so, if you can set up a tee between the check valve and the brake booster, then you can see the peak value of the vacuum level at least on that one carb throat. (You need a very small air-bleed hole on that tee line (maybe pin-hole sized or a bit larger) to bleed off excess vacuum above the idle vacuum level that occurs when you open and then close the throttle.)
And when you look at the throttle plate edge through the progression holes, make sure that all 4 plates move past the same progression hole in each throat at exactly the same throttle angle. This is a good way to know that they are truly sync'd.

This is all step 1.

Step 0 is to go back and adjust the floats; that is fundamental to every phase of carb operation. Here is what looks to be a pretty correct video in how to do this.
When the closed position float setting is done, it has to be where the the needle is just barely closed as noted in this video. And you measure to the brass float main body, not to the seam.
 

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Opel Rallier since 1977
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OBTW, you should not adjust the idle mixture until the engine is fully warmed up.... if you adjust it when the engine is cold/cool, then it will be much too rich when warmed up. Keeping the engine running while warming up without the cold start enrichment on (the traditional 'choke' in US parlance) will require you constantly pump the throttle lightly to make the aceelerator pump shoot batches of fuel to engine to enrich the mixture for cold start. This takes a bit practice. So if the engine starts and runs well cold with no pumping or cold start enrichment, then you know your idle mixture is way too rich.
 

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Just Some Dude in Jersey
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A DCOE 9 sounds like an older Weber. I have one. On the bright side, they're easier to tune or, rather, there's less fine tunability. The latest version, I think it has 3 numbers, can adapt to various conditions better, but requires a finer amount of tuning for perfection.
 

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Can Opeler
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The DCOE 9s also have an issue where the fuel enrichment (cold start choke) circuit leaks constantly. If that is leaking you’d be running dog rich and the idle mixture would do nothing. Sounds like you are lean to me but doesn’t hurt to check.
It’s a well known issue. Just search fuel enrichment circuit leak Weber DCOE and you’ll probably find some info about it.
 

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Opel Rallier since 1977
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BTW OP, what fuel pump do you have and do you have a pressure regulator between the pump and carb's? If so, is the pressure set to 3-1/2 lbs or less?
 

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Just Some Dude in Jersey
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Hey man, any chance that you could make a video of your engine running and post it on YouTube for us to watch?

Does your engine do that misbehavior in the driveway when you rev it and stuff or does it only do it when it's under load while driving?
 

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Can Opeler
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He posted to facebook. It’s a pretty catastrophic lean condition the instant he gives it any throttle. I’ve had this happen before. Removing and remounting the carb fixed it whatever the issue was.

When the carb is off I always check everything so there’s no telling what I changed to fix it.

9 times out of ten my DCOE carb issues are blamed on a clogged idle jet fuel hole if lean or air hole If rich at cruise. 1/10 times it’s mounting gasket failing, a bolt too loose, or the cover over the progression holes leaking.
 
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Discussion Starter #11
Sorry it's been awhile since I updated this post. I took both carbs off the car and readjusted the floats. They were both about 1-2mm too low. While I was at it, I used some copper gasket spray on the header gasket to ensure it would seal. With the carbs were off, I discovered the 3/4 of the machine screws for the inspection plates on the back (engine side) of the carbs were missing. I don't know if this was the main cause of the problem I was having but it probably contributed. I have everything back on correctly torqued and ready to go, just waiting for some gasket maker to dry. Thanks all for your help
 

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Just Some Dude in Jersey
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You're right! My diagrams call it the inspection cover. Doesn't seem like it's a big deal, there's a big hole in it on the 3 carbs I have.
 

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Can Opeler
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It’s just the cover for the internal throttle springs and such. It would have had no effect on your issue.
 

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Just Some Dude in Jersey
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It's a really big deal WHERE you bend the float. You would think that just bending the tab that closes the fuel bowl valve would git the job done. Or bending each individual float arm. Apparently not. I'm pretty sure that you should only bend where the arms come together. It's not just the height of the highest part of the float, the float has to be LEVEL with the lid, not at an angle to it.

I could be wrong about the particulars above. As I said previously, I found the float adjustment maddening, especially since I was bending it in the wrong location. My tuner guy gave me a tongue lashing about it.
 

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Discussion Starter #19
So after all that work, the situation hasn't changed, which leads me to a couple theories. I'll elaborate on each as best I can.

1. There is still something wrong with the carbs. This one is low on the probability scale as I just spent a weekend checking and adjusting everything on the carbs

2. Something wacky is going on with the fuel pressure. When I checked the fuel pressure, it was with the engine off with my electric pump on. The pressure was 3.2-3.5 psi. My fuel system is currently using both the factory engine driven pump and an electric pump mounted back under the left rear wheel well. Since I know the output of the electric pump is good, could a failure of the factory pump result in enough of a fluctuation to starve my carbs?

3. Timing chain/head gasket some other catastrophic issue. In theory, an engine could idle if the timing chain had slipped a tooth, or if the head gasket had a hole in the side, right? I had a stick S10 that skipped a tooth on the chain once and it too had a similar loss in acceleration/power. Also probably unlikely because I would have seen other indicators of such failures by now as well(water in the oil, horrific metal on metal sounds, etc)

I'll try to get a video of what's going on posted in here somewhere as well.
 

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Opeler
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Where do you have the fuel pressure gauge located?? I’d put it right before the carbs if it isn’t there already.
I had issues with DCOEs and it was several things: multiple small vacuum leaks, a bad pump jet (took the carbs off and pulled the throttle levers and one shot much shorter and less fuel than another), double check your timing curve (are you using stock distributor?), and ultimately after verifying everything under the sun I did more compression checks and found my compression to be off between cylinders. Turned out to be improperly installed & failed valve springs.
I hate you’re chasing issues down, I hope you can find the culprits, because it may be more than 1. I’d also call Pegasus Racing, they have a dedicated tech line and are very helpful. Pegasus Auto Racing Supplies

Good luck!

Eric
 
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