Todd Mullins wrote:
>
> Robert Allen writes:
>
> > Vacuum advance increases fuel economy and provides for smoother running
> > when cruising. It does nothing (for or against) performance. When using
> > Vacuum Advance, it must be connected to *ported vacuum* which is a port
> > just aft of the throttle plates on the carb -- not manifold vaccum as it
> > used by the power brakes. You want vaccum advance to go away as soon as
> > the throttle position increases.
>
> For reference, the box-stock Lucas dizzy on my box-stock '74 uses
> manifold vacuum, taken from one end of the balance pipe. It's
> definitely closer to the block than to the carb butterfly.
By golly, you're sure right. MGs use manifold vacuum for their
distributor and the carbs are CD -- constant depression (or continuosly
drips) so there will always be vacuum on each side of the throttle
plate. Curious. That would imply that vacuum advance contributes to
total advance even under periods of moederate acceleration. From a
traditonal carb perspective, that is pretty strange.
D.D.B. said he has a downdraft Weber which is a traditional venutri carb
where ported vacuum might be available. Mechanical advance is set
(easily with a Mallory) to maximize performance and, then, vacuum
advance, while cruising at a steady part throttle, would increase timing
under light load conditions which, as in my original post, is supposed
to provide increased economy and smoother engine operation. Transition
to full throttle would negate vaccuum and drop total advance to the
mechanical advance setting -- thus preventing engine knock.
Now I'm wondering how much vaccuum is present in the log manifold under
acceleration. Maybe I'll go lift the hood of the Triumph with the two
Strombergs and see if I can figure it out.
Also, there is the perverse "vacuum retard" on emission MGs (that I have
zero experience with) so I'm curious where that vacuum source is. I
understood that the "retard" feature was supposed to be active only at
or near idle so I wonder where the vacuum source sees idle. Or I could
be wrong again.
Bob Allen, Kansas City, '69CGT w/DCOES, '75TR6 w/Strombergs
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