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On 24 Sep 98, at 23:50, Dave Hall wrote: > NOTE: The vacuum unit does not advance the ignition timing. It > retards the ignition 7¡ at idle speed and returns the plate to the > static position after throttle opening". Dave, As I read your post, it sounds like this vacuum unit is the same as the FI ones that I am familiar with. If that is the case, and the quote you gave seems to me to support that claim, then I don't think you have interpreted it correctly. On the usual dual vacuum cannister there are two diaphrams; one advances the timing and the other retards it. The breaker plate connecting rod is spring loaded to return to a somewhat centered postion if there is no vacuum. Each vacuum chamber is connected to a drifferent drilling in the carb or FI intake air distributor (which incorporates the throttle plate.) The retard drilling enters the venturi just downstream of the throttle plate on the side where the throttle plate moves downstream as it opens. Thus it has full idle vacuum on it when the engine is idling, but this goes away abruptly when the throttle is opened slightly. The advance drilling enters the venturi a bit upstream of throttle plate on the side where the throttle plate moves upstream as it opens. The vacuum it sees is proportional to the air velocity past its opening. Thus this vacuum is zero at idle (drilling upstream of the closed throttle plate), maximum at small throttle such as you would use at cruising (drilling just under the lip of the throttle plate and all the air rushing at high velocity through a rather narrow opening), and zero, or nearly zero, again at full throttle when the air velocity is much less due to the wide open throttle. The additional advance is useful at cruising where the mixture is leaner, the flame front advances slower, and the output, and heat load on the engine is less. Plus it gives better gas mileage. Dual vac adv/retard distributors generally need to be timed with the retard line removed so that you can be sure what the actual starting point is. The extra retard at idle is for the sake of emissions at idle and during coast down. I get the impression that the exact amount of retard is not too important, so it is not well controlled. While all the books say that you should remove the vac lines to do the timing, if you try it with the lines both on and off you will see that the way VW does it, there is no difference, so you can leave your ADVANCE lines on. There are other makes which do this differently so this advice is not universal. Jim - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------------------------- Search old messages on the Web! Visit http://www.vwtype3.org/list/