The vaccume advance is a power increase device, not a fuel economy improver. When you press hard on the pedal and open the butterfly manifold vaccume drops. The reason you pressed down on the pedal is to obtain more power for increased load, like climbing a grade. You want more Horse power, you do not want the timing to retard and offer less advance and less power. Yes the 79/80 is drivable with vaccume retard, but at times it becomes a real dog and will surge. What you will notice as you take your foot off the gas and increase the manifold vaccume is that the engine will actually run better and deliver more horse power.
With the 2 systems working against each other you end up with little or no advance. The mechanical portion moves the timing about 10 or 12 degrees advanced, but the 79 Dizzy directly connected to manifold vaccume at full throtle will retard the timing by 10 or 12 degrees, the two cancel each other out.
Take a look at a 78 dizzy with points and then look at the 79, you will notice that the vaccume motor is mounted opposite and the travel is reversed.
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