Weber DFTH 30/34 rebuild question

[Sparepart]

I am servicing a DFTH 30/34, it was well used and then been sitting around for a long time so its got a bit of corrosion on top of tar on the surfaces. All is going well so far, I have obtained a service kit and am helped by the exploded view that is attached below. I am puzzled by something. I see the automatic choke and how the bi-metal coil will unwind pressure on the choke butterfly etc this is item 4 in the diagram. I also see a part, item 14, which contains a diaphragm that is not in the service kit. Item 14 appears to be intimately involved with the choke, mounted at right angles to the bi-metal coil axis. I have not dissasembled this yet. Does anyone know what this part does? On the parts list is is "Kit-Choke Control" part number 6187682. Does it operate the choke at sometime after the engine is already hot. ?

 

[Foz]

I found this page in the Weber manual which is generic for their carbs.  I had to read it several times before it made any sense to me, but I think it answers your question.

Have a read and see what you think.

Steve

 

Carb.pdf

[Sparepart]

It does indeed explain, thank you very much for sharing this. In short when the engine is cold starting it prevents too rich a mixture (i.e. flooding) if the throttle is used to take revs above the idle rate. (I think). If the diaphragm fails then I assume there would be a tendency to get a rich mixture when driving with a cold engine, until the bi-metal coil expands enough to release the choke fully. So I notice (now) that some of the service kits that come from Germany do have this diaphragm, just not the one I bought, ho hum. .... just looked at the mechanism and I see it's the opposite of what I said above. When there is good vacuum below the throttle butterfly (idling) the diaphragm is compressing a spring that wants to release the choke. So if the diaphragm fails, the spring will constantly push the shaft that tries to release the choke (against the bi-metal coil that is hold in the choke on). This means that when the engine is cold it will tend to cut out if you try to rev it due to insufficiently rich mixture. There are so many parts to this carb, it looks like it was designed by the same committee that thought up a camel.

Edited December 9, 2021 by Sparepart
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