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On 23 Mar 2003 at 13:49, Mark Seaton wrote: > I have just had a good look at the carbs in my collection again. I have a > few odd carbs here that came with very tight venturis (21.5 instaed of 24) > which I guess are not Type3 carbs at all - though they are called 32 > PDSIT-2s. One of these is what I'm using at the moment as my left carb. The small venturis were used up thru 64. > One of the early (type3) sets I have had the correct venturis so I swapped > them over. This one had a drilling straight through the carb wall from the > take off tube, which mated up with a hole in the venturi- straight through > to the air flow. It ALSO has a passage way leading down through the carb > wall to small hole in the throttle plate. The throttle disk partially covers > this hole when fully closed, with part of it above the disk. As the throttle > opens the disk moves above the hole (creating a strong vacuum) I don't know > the function of the upper hole though, but I guess it will diminish the > vacuum signal. The venturi port sounds like what I was hoping for. I don't understand the lower port, but that's not too surprising since I'm sure there's LOTs of stuff about carbs that I don't understand. I just remembered that I have a Solex book here, so perhaps I should just look in there, too. It sounds like they made some extra provisions to modify the vac signal that they got from the venturi. Perhaps this gives some extra advance while cruising. I find the whole concept of "mixing" vacuum signals to get a composite vacuum output puzzling, however. > Well when I took my NOS other carb and swapped the venturis, I saw that > there was no hole in the wall to mate with the hole in the venturi. So I > drilled it! Thinking that there was no vacuum take off! This sounds like it is exactly what I was hoping to see in the late carbs! ;-) > In the later carbs, there is no hole in the venturi, just the passage down > to a similarly located hole in the throttle plate. The throttle disk has a > cut out that keeps the hole fully open above the disk at throttle closed, > and gradually transfers it to the vacuum side of the disk as it opens. > I'm a bit confused now as I have quite a few carbs in pieces and there seem > to be quite a few differences. If someone else could confirm what I'm > thinking here I would feel much better! > The rectangular hole I was talking about is I think actually for the piston > in the choke mechy. > > I guess I should block the hole I drilled in the body as I'm sure I can't be > getting the right vacuum signal with both holes- or try and find a good pair > of late carbs. I wonder if you could just swap that venturi with the one from the right carb. > Sorry if I've caused unnecessary confusion- I guess I should really check my > facts before opening my mouth. No problem, I think you've helped us get a really good start on understanding all of this. I just wish I had looked at this when I had Bob's carb here. ;-( BTW, I just looked at the microfilm to see how many different dual carbs VW supplied over the years. The total is 52, including lefts and rights, 6V and 12V, OE and repair carbs. This doesn't count any of the single carbs. -- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------------------------- List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org