[TR] Vacuum advance/retard

[Randall]

> The ultimate power limit is still 
> the same, VA or no.

Right.  VA just gives better fuel mileage at cruise, and possibly somewhat
improved part-throttle response.  It does nothing at WOT (which is why most
race cars don't bother).

> At the other end, if the port is on the manifold or the base of the 
> carb its vacuum will go down progressively as the throttle is opened, 

Most if not all VA ports are 'timed', meaning they exhibit little or no
vacuum at idle (so no advance at idle).  This is done by putting the port
right next to the butterfly, so with the butterfly fully closed, the port is
on the upstream side.  When the butterfly opens, it moves past the port,
exposing the port to manifold vacuum.

A VR port is also right at the butterfly, but on the opposite side so that
it is exposed to manifold vacuum only when the butterfly is fully closed.
When the butterfly opens, it moves past the port, blocking off the manifold
vacuum from the port.

So for ZS carbs (at least as used on Triumphs), the VR port (if present) is
on the bottom and the VA port (if present) is on the top.  But if the
throttle plate (butterfly) turns the other way (like on an SU H6), the ports
have to be swapped as well.

Randall