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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