Hello Bob and others:
Vacuum advance on the MGA comes off the rear carb.
David F. Darby
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On Tue, 08 Apr 1997 10:05:36 +0000 Robert Allen <boballen@sky.net>
writes:
>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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