Weber DCOE issues

[MostlyOpels]

Are there any twin sidedraft gurus here? I'm having some issues with my 40 mm twin Weber DCOE's, on a stock Opel 2.0 CIH engine. I can get it to run OK but couple of things bother me.

First, the mixture screws have no effect whatsoever. Doesn't matter if I close them all the way, there's no effect on how the engine runs.

Second, there's a stumble when accelerating carefully, from a steady cruising speed on light throttle. The engine just falls flat on it's face, before it gathers some revs after witch it accelerates just fine, all the way to the top end. To put it another way, I guess this issue manifests itself when transfering from the idle circuit to the main circuit. I'm not an expert on tuning carbs though, so I'm not sure.

The car starts fine with a few pumps of the gas pedal when cold, and stays idling without the need to keep it running. Works great when accelerating hard, from stand still to redline. One other thing is, that I can't get the engine to idle slow enough. The engine is idling at around 1200 rpm, when the idle speed screw is not even touching the linkage lever.

I've disassembled the carbs, cleaned them, and changed all the consumable parts before putting them on the engine. Here's the specs on the Webers.

Idle jets: 50 (Have tried F4 50, which caused the engine to "sneeze" through the carbs. Too lean?)
Main jets: 130 (Have tried 140, 145, and 125 but none of those fully cured the stumble. I THINK 130 and 125 performed the best out of the four. Putting on 120's made it considerably worse.)
Air jets: 180
Emulsion tubes: F16
Chokes: 34
Cold start mechanism is eliminated.

[retroray]

Have you used an air flow meter to balance both carbs?

Is the linkage between the two carbs in good condition aswell?

[scimjim]

If it’s idling at 1200 rpm, you’re on the main circuit, so the idle mixture screws won’t do anything. Have you checked and balanced the progression holes?

[metalshapes]

First of all, 34mm venturies on a stock 2 litre is way too big.

For a street car, I like to err on the small side.

That gives a relative small HP loss at the top for a big drivabillity gain everywhere else.

My guess for that engine would be somewhere in the 30mm range.

That will make a big difference of how the carbs read the signal...( how it responds to tuning )



When I put twin 40's on my completely stock 1600 Kent, experts told me " it wont work without lots of porting and a big cam" and " you need at least 34mm venturies", sometimes in the same sentence.

I used 27mm, on a bone stock engine.

It gained a lot of torque, drivabillity, and even much better mileage over the stock progressive twin choke downdraft carb.
( I had one set of jets on emulsion tubes for mileage/ daily driving, and a second set on emulsion tubes, ready to go when I wanted it to go faster )

[metalshapes]

The way I see it, you have 2 things going on with the venturi size.

I think at 34mm they are to big for a stock tune 2 litre Opel, and also they don't choke down enough from 40mm for a strong signal ( Bernoulli effect? )
They only ramp in 3mm along the inside of the carbs.

A stock Alfa Romeo 2 litre ( back when they came with Webers ) used 32mm if I remember correctly.

And that is about as big as I would go in a 40mm Weber, for a street car.



34's would be fine in 45DCOE's on a bigger engine, and in 40's in a racecar that spends most of its life at Wide Open Throttle.

[MostlyOpels]

Oct 3, 2020 20:56:56 GMT retroray said:

Have you used an air flow meter to balance both carbs?

Is the linkage between the two carbs in good condition aswell?

I have used a flow meter to balance them, and the carbs are in sync. The linkage is in good condition too. Thanks!

[MostlyOpels]

Oct 3, 2020 21:07:37 GMT scimjim said:

If it’s idling at 1200 rpm, you’re on the main circuit, so the idle mixture screws won’t do anything. Have you checked and balanced the progression holes?

That was my suspicion too.. I think I checked the progression holes for blockages, when I cleaned the carbs but I don't know how to affect their balance. Other than filling them up, and drilling new ones. That's something I wouldn't want to do. I guess the position of the throttle blades on their axles has to do with the balance also. Do you mean, I should check that the blades sweep past the holes evenly?

[MostlyOpels]

Oct 3, 2020 23:01:56 GMT metalshapes said:

The way I see it, you have 2 things going on with the venturi size.

I think at 34mm they are to big for a stock tune 2 litre Opel, and also they don't choke down enough from 40mm for a strong signal ( Bernoulli effect? )
They only ramp in 3mm along the inside of the carbs.

A stock Alfa Romeo 2 litre ( back when they came with Webers ) used 32mm if I remember correctly.

And that is about as big as I would go in a 40mm Weber, for a street car.



34's would be fine in 45DCOE's on a bigger engine, and in 40's in a racecar that spends most of its life at Wide Open Throttle.

Yes, I've been thinkin about this too. I've kind of known, that my venturis are a bit too large but have not paid much attention to it, since the car has performed pretty well, apart from the issues I mentioned. I'm not sure if it has anything to do with my current issues but I agree with you, I should step down on the venturi size anyway.

[scimjim]

Oct 4, 2020 5:55:52 GMT MostlyOpels said:

Oct 3, 2020 21:07:37 GMT scimjim said:

If it’s idling at 1200 rpm, you’re on the main circuit, so the idle mixture screws won’t do anything. Have you checked and balanced the progression holes?

That was my suspicion too.. I think I checked the progression holes for blockages, when I cleaned the carbs but I don't know how to affect their balance. Other than filling them up, and drilling new ones. That's something I wouldn't want to do. I guess the position of the throttle blades on their axles has to do with the balance also. Do you mean, I should check that the blades sweep past the holes evenly?

Yes, is there not a cover you remove and check the butterflies are equal and just covering the first hole, and a balance screw to adjust them?

[MostlyOpels]

Oct 4, 2020 6:35:22 GMT scimjim said:

Oct 4, 2020 5:55:52 GMT MostlyOpels said:

That was my suspicion too.. I think I checked the progression holes for blockages, when I cleaned the carbs but I don't know how to affect their balance. Other than filling them up, and drilling new ones. That's something I wouldn't want to do. I guess the position of the throttle blades on their axles has to do with the balance also. Do you mean, I should check that the blades sweep past the holes evenly?

Yes, is there not a cover you remove and check the burrerflies are equal and just covering the first hole, and a balance secret to adjust them?

Yeah, there are screw in plugs over the holes. I can set the butterflies with the idle lever adjustment screw but I don't think I really checked the coverage, when I rebuilt the carbs. I just made sure that the butterflies moved freely, and looked to sit evenly in the ports when closed.

[melle]

I don't have any hands-on experience with side draft Webers, but I can highly recommend John Passini's Weber tuning book(s). If you PM me your email address I can send you a PDF of the factory tuning manual if that helps.

[jonsey]

I’d make sure the pump jets were working too. You should see a nice stream of fuel down each choke when you press the throttle.

I used 36 chokes in my 40’s on my Vauxhall Red top but it was revving to 8000rpm most of the race. But it was still pretty tractable when driving normally. It had been set up on a rolling road though not just a set of carbs put on off another engine.

[MostlyOpels]

Oct 5, 2020 17:00:15 GMT jonsey said:

I’d make sure the pump jets were working too. You should see a nice stream of fuel down each choke when you press the throttle.

I used 36 chokes in my 40’s on my Vauxhall Red top but it was revving to 8000rpm most of the race. But it was still pretty tractable when driving normally. It had been set up on a rolling road though not just a set of carbs put on off another engine.

I have checked the accelerator pumps, for a steady stream of fuel. I can actually get around the stumble, if I pump the pedal a couple of times when accelerating from for example an intersection. So I think the pumps are working but it's during a smooth acceleration, when the engine stumbles.

Anyway, the driving season is coming to an end, and I think it's obvious that I need to take off the carbs and start checking them over for a few basic things. I guess I shouldn't mess with anything else, before I can get the idle circuit and mixture screws working properly.

[stevo67]

If engine is idling at 1200rpm and the throttle stop screw is not touching lever I would be checking for an air leak between carbs and manifold or from brake servo if used. Failing that remove carbs and check for bent throttle shafts.

[stevo67]

Also make sure ignition timing is set to factory settings as a starting point.

[MostlyOpels]

Oct 5, 2020 22:18:11 GMT stevo67 said:

If engine is idling at 1200rpm and the throttle stop screw is not touching lever I would be checking for an air leak between carbs and manifold or from brake servo if used. Failing that remove carbs and check for bent throttle shafts.

The brake cleaner test was the first thing I did but that didn't reveal any vacuum leaks, the throttle shafts are in good condition also.

[MostlyOpels]

Oct 5, 2020 22:32:39 GMT stevo67 said:

Also make sure ignition timing is set to factory settings as a starting point.

Yep, the ignition is set. I don't know about factory specs but I got about 10 degrees at idle, and about 30 maximum advance.

[stevo67]

Only other thing that I can think of is that the throttle butterflies may not be centered correctly. Only way to check will be to remove carbs to check, once removed look on the engine facing side of butterfly there is usually a small, short horizonal grove which should line up parallel to throttle shaft. If butterfly is off centre this could be enough to allow enough air in to give you high idle even with idle speed screw turned all the way out.

[MostlyOpels]

Oct 7, 2020 21:49:07 GMT stevo67 said:

Only other thing that I can think of is that the throttle butterflies may not be centered correctly. Only way to check will be to remove carbs to check, once removed look on the engine facing side of butterfly there is usually a small, short horizonal grove which should line up parallel to throttle shaft. If butterfly is off centre this could be enough to allow enough air in to give you high idle even with idle speed screw turned all the way out.

Ok, thanks. I'll check that, once I get the carbs off.

[MostlyOpels]

Ok, so this is a bit embarassing..

I was getting desperate, trying to figure out why the mixture screws weren't working. To my defence, the professional tuner I took the car to a while ago, didn't notice anything either. Other than the fact that the mixture screws don't do anything.

I challenge anyone who knows Webers to look at this picture, and spot my mistake. The layout of the screws was the same on all of the barrels.



Anyway, after I realised what was wrong, I started to tune the carbs again. Only this time, with the idle circuit finally in effect, I can't get them synced at idle anymore. The mixture screws work, but I can't get the engine to run smoothly by turning the linkage balance screw. I guess the photos below, could show the reason why. Like stevo67 suggested, there is something wrong with the way the butterflies sit. The rear carb isn't that bad, but the front carb has both of it's butterflies binding in the barrels.

Clynders 1 & 2



Cylinders 3 & 4



I filed down the backsides of the screws, holding the butterflies in place. I was able to get one screw out, but the other seven were impossible to remove with a screwdriver. I doesn't help, that someone has attempted to do the same before, and stripped the screw heads in the process.

Any tips, or should I just try and drill them out at this point? I've tapped them with a hammer, and a screwdriver before attempting to turn them, but to no avail. The ones that still look good in the photos, are not so good anymore.