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On 28 Apr 2004 at 8:09, Mark @ Mo-Sys wrote: > I would like to understand more what is going on with the vacuum signal. The > port in these throttle bodies is just a small hole in the wall just below > the throttle butterfly in its idle position. As the throttle is opened > slightly the tip of the butterfly passes in close proximity over the hole > and then the hole is above the disc so I'm guessing that at that point there > should be low vacuum in the port. But at idle I would have thought there > would be quite some vacuum there? Are you sure this is how they are? I'd expect the port drilling to come in just ABOVE the butterfly. Do you see a large vacuum at idle? Your description actually sounds like the way the port would be drilled for the vacuum retard found on distributors with vacuum advance AND retard. Forgive the stupid question, but is there any chance that the throttle bodies have gotten misassembled and are not upside down? That would put the port on the wrong side. Is there any chance that there is another port up in the carb, in the venturi? Or there could be a second port somewhere on the throttle body. > If my vacuum signal is too strong all the way through the rpm curve it > suggests to me that there must be some kind of restriction above the carbs > that is creating an unnaturaly high depression within the carbs, or that the > vac cans are too soft. I will check but I don't see how the air cleaner could > get that dirty by its design to cause a restriction? and the air passages are > so large to the air cleaner that it would have had to suck up something huge > to block them! So I'm perplexed! Actually thinking about it I could easily > check it with the aircleaner off to see if the problem persists. Yes, check it with the air cleaner off, but I agree that it's very unlikely that there's a blockage this significant in there. Another test that I'd suggest would be to put a Tee in the vacuum line and then run an extra line up to a gauge next to you while driving. This way you could note how much vacuum you actually get in various driving situations. You should be able to make a chart of vac vs. vac. advance and then tell us how much vac advance you're actually getting under various situations. You should also be able to take your Bosch distributor part number to a Bosch dealer somewhere and get them to check to see if the 507 vac can is the right one for it. My Bosch parts catalog just lists the US parts, so the 507 doesn't appear in it (but the US FI 73s got a 505, so we're close enough that this seems likely to be correct.) > Before I re-fitted the original carbs I had some other older carbs on there > and those did an even stranger thing with the vacuum in that the advance > would rise rapidly on acceleration but then would take a long time to get > back down again like there was a vacuum lock somewhere. It's quite possible to get dirt in the drilled port which would make it slow to come to equllibrium. I suppose it's possible that some loose dirt in there could make a sort of check valve. -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org