Richard,
Are you sure it's a vacuum advance fitting on "top" of the intake manifold and
not part of the smog equipment.
Vacuum advance is generally taken where the quickest reaction to throttle
movement is.
WFO Herb
Richard Shipman wrote:
> Can anyone explain to me why my '74 Midget with SU's has a simple vacuum
> advance fitting right on top of the intake manifold and yet so much care and
> attention is being taken to locate the vacuum pick-up at a specific point on
> a sidedraft weber??
>
> What am I missing??
>
> Thanks in advance
>
> Richard
> '74 Midget
> '68 MGBGT
> '58 Morris Minor
>
> At 09:54 PM 8/3/99 -0700, you wrote:
> >Bryan:
> >
> >Just went out and took a good look at exactly where I drilled the hole. It
> >took me two tries to get it right. The spot is at 11:00 on the front
> >throat, and goes through the bore only a few thousanths in front of the
> >butterfly. Too far away from the butterfly and there won't be enough vacuum
> >to opperate the advance. The choke (enrichment hole) is a few thousanths
> >after the butterfly and would be subject to vacuum at all times. If I
> >understand vacuum advance correctly, it's purpose is to improve fuel economy
> >under light load only, like at highway cruising speeds. At Idle, an engine
> >has to have minimal advance to prevent backfiring and hard starting.
> >Centrifugal advance corrects the timing necessary as engine RPM increases,
> >and Vacuum advance allows the timing to advance or retard based on engine
> >load (how am I doing so far?) On a race engine, you would be under load
> >most of the time, so vacuum advance wouldn't be necessary (hence Cooper
> >Distributors with no vacuum advance provision). Manifold vacuum will not
> >work correctly because manifold vacuum is too high at idle and under
> >acelleration, resulting in timing that would be too far advanced in both
> >those situations.
> >
> >That is how I understand it, anyway. My fuel economy improved about 4 mpg
> >highway when I hooked up the vacuum advance.
> >
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