Bob is 100% correct in that the vacuum advance mechanism hose should
only be connected to the carb mounted port below the throttle plate. At
idle, small throttle openings, and down hill off-throttle positions the
manifold vacuum below the throttle plate is at it's highest, and the
engine is under practically no load. The tube is isolated from the
outside atmosphere. It can take a lot (20 degrees crank angle) more
advance with no harm, and increase in fuel economy.
Most non throttle position vacuum ports are located on the manifold and
always read manifold vacuum. There may be carbs with two small vacuum
port tubes, I am just not aware of them.
Never let "never" be said, however, as some remote carb builder, for
reasons I don't know, might have put one above and one below the
throttle plate(s) . Use the one just below the throttle plate. Closed
throttle, high vacuum, high advance. Staged reduction in vacuum to
equal atmospheric as throttle opens. This happens relatively quickly.
Put plug on tube not used.
Steve
Bob Palmer wrote:
>Ron,
>
>This is a nice article, but I disagree with their recommendation to connect
>the vacuum advance to the "tuned port", which is located above the
>carburetor butterflies. If connected here, the vacuum at idle and low
>throttle is zero and increases progressively as the throttle opens. This is
>how vacuum advances were hooked up years ago. What we need now on our motors
>is more advance at idle, and less advance as we apply throttle. This is
>especially true for high duration cams. To get this functionality, you
>should hook the vacuum line to the carburetor below the butterflies, or
>directly to the intake manifold. For an article that discusses this, follow
>this link.
>
>http://www.centuryperformance.com/timing.asp
>
>Bob
>
>-
>
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Steve Laifman
Editor
http://www.TigersUnited.com
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