Weber 28/36 DDE Carburettor (Part 2) - Carburettor Set Up

The correct carburettor set up is important on any car but particularly so on a bvh DS where there are two idling speeds and so much of the hydraulic wizardry is dependent on a hap[py carb.

Most settings should be done when the engine is hot – so running. At this stage of my rebuild that isn’t possible for my car, so I just needed to do an initial set up before bolting the carb to the car. I turned my attention to the primary butterfly throttle stop screw: the screw that was originally factory set by Weber.

Citroen Operation DX.142-3 of manual 518 (applicable to my DDE carburettor) offers no set-up instructions. It tells the reader not to tinker with the primary butterfly stop screw:

"IMPORTANT NOTE: Under no circumstances must anything be done to the stop screw for the first (small) butterfly, individual adjustment of this screw having been carried out by Messrs. WEBER on each carburettor"

Indeed, on my 28/ 36 DDE carb, it is a screw with an additional lock nut – whereas the screw for the secondary butterfly is a screw and spring.

The primary stop screw is on the right - with the lock nut

I have no more than a basic understanding of how I think the carb is working. Here we go:

The reason that this setting of the primary butterfly is critical is that it determines the degree and source of the fuel/ air mix when at idle, and ensures the optimum and smoothest acceleration as the throttle is opened up - which is achieved through a series of ‘progression holes’ in the side of the primary throat/ shaft: the successive exposure of the progression holes allows the initial and gradual introduction of additional fuel into the low/ mid revs driving range.

The cluster of three offset progression holes can be seen

With the engine running, and throttle closed, a vacuum is created below the butterfly. The principle is that, in it's 'stop' (idling) position, the edge of the butterfly should just block the 'first progression' port so that, in idling mode, fuel flow and mix is achieved only via the vacuums' draw of fuel through the idle jet.

As the throttle is opened, the primary butterfly flap moves such that one, then more, “progression holes” are gradually exposed and are also subject to the vacuum: additional fuel is pulled into the manifold as more air is introduced - thus the appropriate fuel/ air mix is maintained. If the primary butterfly is not set correctly then either the car will have ‘lumpy’ and/ or delayed acceleration (dead spots), or will not idle correctly because the idling mix is already partly via the progression holes.

I THINK that's the idea......

It is generally accepted is that it is very likely that the butterfly settings will have been tampered with in the past and so there needs to an understanding of how to reset these. Other than use of a CO2 meter, I’m aware of three approaches:

Operation D.142-00 in the later manual 814 offers general advice on adjustment of later variants of the Weber carburettor:

"With the butterfly closed, the edge of the (throttle stop) screw in contact, turn the screw 1/3 of a turn. At this moment, a feeler gauge of 5/100 (0.05mm) must pass between the butterfly edge and the bore"

This is only applicable to Weber 28/36 carbs where the stop screw doesn't have a lock nut - and so not my 'DDE' variant. I didn’t like the sound of this one anyway: my feeler gauges are flat. How could I possibly get an accurate and reliable reading pressing it against the curved bore of a carb throat? The fingers of different gauges might be wider and so sit in the curve in a different way?

Rotate the primary butterfly stop until it hardly touches the lever. After that rotate the screw by a further 1 1/4 turns inward. 

This sounds a bit arbitrary and assumes the stop lug hasn't been bent. It assumes standardisation across all carbs. If this was the case, then why would each carb have had a unique factory-determined setting in the first place? I've also seen similar instruction - but which instruct you to turn the screw in 1 1/2 turns......

3.   Adjust the locking screw for the primary butterfly so that, with the throttle in the idle position, the first progression hole is ‘just covered’ with the throttle in the idle position.

I liked this. Since the setting was all about fuel mix and acceleration, this was the approach seemed the most logical one and would be geared to the individual carb. But how to judge ‘just covered’? The advice I have read was to use a magnifying glass….

I came up with a different approach….. Likening the flow of fuel/ air to access for light (yes, I know how wrong that is!!!) I did the following: I removed the brass plug in the side of the carb that covers the progression holes. This tucked around the back of the carburettor.

The brass screw plug covers the progression hole passage

Removing the plug revealed the progression holes.

Progression holes revealed.......

Shining a torch in the bottom of the primary barrel, I looked through the small hole where the brass screw had been and adjusted the stop screw so that it ‘just’ cut out the light.

Adjusting the primary butterfly to cover the progression holes using a torch

I found the optimum point, beyond which any further minor tightening of the adjuster screw immediately began to allow light to show. I tightened the lock nut and blipped the throttle a couple of time to check that light was still cut out and that any miniscule movement of the throttle began to expose light. That was it. I'll have to see if that works......

Choke and Secondary Butterfly
Pulling on the choke lever turns the spindle of the choke flap/ strangler - but then further also starts to pull on a linkage mechanism that opens the primary butterfly. With the choke fully engaged, the primary butterfly is meant to open by no more than 0.9mm

I found that, if the choke lever/ was fully engaged, after the choke flap had closed........

Choke flap fully closed.

......the linkage still had scope to open the butterfly by a good couple of millimetres.It’s meant to be just 0.9mm.

Any further pulling will operate the linking mechanism

I reasoned that, when the engine was dropped in the car, by adjusting how I trapped the choke cable outer in the clasp on the carb, I could limit the length of pull on the choke cable so that it fully closed the choke flap – but only opened the secondary butterfly by just under 1mm. 

I aim to adjust the choke cable fitting to limit the  travel of the linkage

I just hope I’ll remember this when the time comes!

Refitting the Accelerated Idling Device
Following a re-zinc, the long studs were refitted to the carburettor body.

The spring-loaded valve was refitted......

.....and secured in place with its brass collar.

Plunger and collar refitted

I had cleaned and repainted the main body several years ago. I coated the slide valve inside with LHM and made sure it could move freely before bolting the body to the carburettor. 

There is no gasket between the accelerated idling device and the carb, so I'm not sure whether I should expect any LHM to leak past the slide valve?

UPDATE: I noticed a groove in the body of the idling device and a similar recess in the carb. Since there is no seal on this thing, I have a theory that any fluid that gets past the slide valve goes down a drain in the carb into the inlet manifold, and then drains away through the long hose on the bottom - just as excess fuel does. Just a theory....