Yep...
There's ported vacuum "before" (the butterfly) and there's ported vacuum
"after" the butterfly (intake manifold).
You see the former on pre-emission
Spridgets and the latter on the later emissions engines, usually.
And of course, they work differently,
the latter being more of a vacuum
retard rather than advance I think but,
I'm not positive on this 'cause I always
got rid of the emissions stuff and used
a mechanical dizzy (#40819 - Cooper S
mechanical (only) curve) in place of
whatever was in there originally.
And of course, I know virtually nothing
about 1500's anyway, really.
As I understand it, the "retard" system
(ported after the butterfly) starts fully
advanced and retards (to the mechanical
curve only, at some point) depending
upon "load". The more load you have, the
less vacuum advance you get. IOW, it's
really a vacuum "retard" unit.
In the pre- (or "early") emissions engines,
the vacuum works in accordance with
what other listers have already stated
within previous posts....vacuum advance
being available under partial throttle or
light load for greater fuel economy
(fire can be lit earlier in the cycle)
under partial cylinder filling circumstances
(lower effective dynamic compression ratio...or something like that!:):)
Cap'n. Bob
'60 :{)
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