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----- Original Message ----- From: "Jim Adney" <jadney@vwtype3.org> > 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. I did get it wrong- I just had a look at some identical throttle bodies and found that actually the vac takeoff is just below the butterfly valve at idle, though it would be above if the throttle closed completely. It remains below the valve throughout operation, so I guess it should always have vacuum which is what I'm seeing- but why? I guess that the vacuum would be strongest with a venturi effect when the throttle first opens up and the air flow is fastest and close to the hole, and then again when you're over-running and the throttle is closed but the momentum of the car is driving the engine- which would give you an increase in advance when you're cruising at a steady pace. I took some photos: http://www.mo-sys.com/throttlebody/ The one with the drill is open the .6mm stock idle opening and shows the position of the holes BENEATH the butterfly valve. In there is also a picture of the PTFE bushings that all 12 (including apparently NOS) of my carbs throttle bodies have had. You can see the brass ring that keeps it in place and the white PTFE liner (which is less than .5mm thick) Perhaps they were introduced to later cars after US cars had all changed over to FI? Sorry for the confused mind Mark Seaton, '73 Fasty London ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org