Doug---I think the only purpose of the TVS is to provide vacuum retard
to the distributor timing. If the switch was bi-passed and the vacuum
line connected straight to the distributor, there would be no
"automatic" way for the timing to advance at idle speeds, when
necessary. We know for instance that retarded timing can cause an engine
to overheat during prolonged periods of idling in a hot environment.
When the coolant exceeds a given temperature, this switch interrupts the
vacuum retard, and the timing then advances.
I can't say why there is an extra port on some of these, nor why Triumph
didn't use this system at all on their last models.
Yes, the timing retard operates when the throttle is closed, and vacuum
at the throttle plate is at the highest. (When the thottle is opened,
vacuum at this port is reduced, and the timing restricton goes away). It
is said this had to do with reducing the NOx pollutant.
Dick
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