[Roadsters] Vacuum advance?

[Graeme]

As the name implies, it is a mechanism which will introduce a measure of
ignition advance when the engine generates high vacuum.
It is PORTED vacuum i.e. only available when the throttle butterfly has
opened past a certain point.
It only has any effect in part throttle applications, which is usually
CRUISE.
It is primarily an ECONOMY/EMISSIONS feature.
Under part throttle the intake manifold generates high vacuum and
compression pressures are low.
With this lower pressure the mixture does not burn as rapidly.
To obtain maximum efficiency under such conditions, and economy, the spark
event should be advanced more than is possible by the centrifigal system. 
That extra is provided by vacuum advance system.
Jump on the loud peddle and there is no vacuum in the system, [everything
goes to atmospheric pressure instantly] to act on the vacuum motor to alter
the timing, so it relaxes and you fall back on the centrifugal system only.

If you have got a "hole" at off idle, maybe there is a lack of suitable
accelerator pump function, or a jetting problem?


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