 Weber carburetor
Please start by watching the first video below. No, not quite, unless you're an expert on Weber carburetors for the Porsche 911. But then you don't really need to read this text here. However,
if that isn't the case, you might find it a bit difficult to follow the individual steps shown in the video.
Nevertheless, we’d like to spark your interest in a carburetor that looks as good as new, achieved through special machining and/or an (expensive) bath, with a quick look at the video. We'll
take it as it is now, and first we'll have to shut off certain lines again. They often run as bores in integrally cast material on the outside of the housing. However, since you can't drill around a corner,
you have to position two holes so that they are open to each other but sealed off from the outside with plugs. They have to be taken out for cleaning and put back in afterward.
But what kind of carburetor are we dealing with here? Well, it's a downdraft carburetor with three air ducts, which means one carburetor is just enough for one side of the six-cylinder boxer
engine. Although the does not specify the exact distance between the air ducts due to its cylinder spacing, it does provide an approximate measurement. To save space, two float chambers are arranged
between
three air ducts, as clearly shown in the image above by the connections and the vents leading upward.
So imagine the line coming from the fuel pump on the left to fill the left float chamber, and a short piece of hose, which can likely only be installed by removing and reinstalling parts, the
connection to the right float chamber. The floats themselves are therefore unusually narrow and also dented due to the device for pre-foaming in the float chamber.
A common air filter is usually placed over the top closure of the air ducts. Porsche sought out this funnel-shaped design very early on because it allows more air to enter the combustion
chamber when the flow does not break away from the wall. Each air duct has a throttle valve at its lower edge, operated by a common shaft located in the center and to the right, and connected to the gas
pedal.
The left throttle valve, on the other hand, can be adjusted using clamping pieces so that all three valves close to the same extent when not actuated.
| You can see this very clearly in the video below. |
Further down toward the throttle valves, there are air funnels. In these, the diameter decreases somewhat more rapidly from top to bottom and then slowly increases again. In the middle, there
is another
elongated tube, supported on the left and right against the edge. Because of the support to the float chamber, the fuel is pre-foamed, meaning it has already been mixed with air. It is sucked
out by
the negative pressure at the air funnel.
For the production of pre-foamed fuel, the wall facing the float chamber is shaped to accommodate a vertical bore. This leads downward into the fuel and upward into the air space above the
float level. Air is sucked in from above through a nozzle, which, depending on its speed, causes the air to dive more or less forcefully into a small, very irregularly perforated tube ('mixing
tubes').
The whole point is not only to combine air and fuel into an initially very rich mixture, thereby ensuring better overall mixing, but also to prevent this mixture from becoming too rich as air velocity
or load increases. That is why the nozzle at the top is also called the 'air correction nozzle'. So if the vacuum system requires more fuel, his nozzle directs a sort of 'braking air' that penetrates
deeper into the surface of the fuel and pushes it back slightly.
The float level is controlled by needle valves, as usual. They can be removed from the outside and extend into the float chamber from above; in the top part of the image, they are only visible
at the screw connection above the inlet. There is a gauge for the float level. The float's mounting bracket should not be bent without precise adjustment. The main nozzles can be accessed
after loosening the screws 1 (see image below). The air correction nozzles and the mixing tubes below them can be accessed after removing the upper funnels.
In the picture, the screws for the fuel mixture adjustment are marked with 2 and those for the air correction with 3. Particularly noticeable in the top image is the accelerator pump, which injects a corresponding
amount of fuel when the accelerator pedal is pressed. Since this carburetor does not have a choke, you will need to pump the accelerator pedal for a cold start, depending on the temperature. The carburetor
was operated at partial load with a relatively lean mixture, but therefore required an additional connection to the float chamber so that it could achieve its maximum power at full load using a richer mixture.
Synchronization remains a fundamental problem. It should always be taken into account, even if it’s just a matter of changing the air-fuel ratio at idle. However, it’s hard to fully agree with this if the
synchronization only takes place when the system is idle. The challenge is to strike a balance between idling and at least partial load.
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