On Tue, 20 Dec 1994, Randy Wilson wrote:
> There was one variation on the theme that a few used. I call it the Ford
> system, as the first time I ran into it was while sorting out a modified
> 351C. It is manifold vacuum, but the passage into the manifold is actually
> *just* upstream of the butterfly. When the butterfly is closed, the passage
> is covered; no vacuum. But as soon as the throttle is opened, the port is
> uncovered, and acts just like a manifold tap. This is what Greg has on
> his shiney new XJ12-HE.
And that's the system used on many other pre-smog control LBCs as well.
Actually there is always _some_ vacuum at the port since the butterflies
are never completely closed, but the vacuum advance diaphragm on the
distributor can't operate until the vacuum is high enough to overcome the
diaphragm spring. I've occasionally wondered (when I had nothing else to
do) how much difference it would make to tap the vacuum off the manifold
instead of off the carb, but I've never been inclined to experiment.
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Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily transportation)
fold@mail.bcpl.lib.md.us
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