This may be nitpicking here, and I certainly hope it is not taken as such,
but the recent chatter about where the advance or retard port is on SU's
and Strombergs could be little misleading. The port is NEITHER advance nor
retard. It is simply a port that is "timed" by throttle postilion relative
to this port. It is active either at idle then inactive, inactive at idle
then active off idle, or entirely active depending upon where this port is
in relation to the opening characteristics of the throttle plate. Whether
it is advance or retard is ENTIRELY up to how the vacuum diaphragm is
configured on the distributor. The former discussions may be a little
misleading with regard to timing advance or retard to a novice, or someone
that doesn't fully understand the operating characteristics of a typical
internal combustion engine that we are all familiar with here - One way to
tell what you have is to note which way the throttle plate rotates. Then
note where the ports are and you can have more than one. For example, if
the throttle plate rotates clockwise, the port is just on the air filter
side of the throttle plate and the throttle plate, when rotated, exposes
the port to the manifold side then you have a port that is inactive at
idle, but becomes active just off idle and thereon. If however this same
port were in the same position, but exposed to the manifold side (I.E.the
port is open to the manifold regardless of where the throttle plate is)
then the port would be active at all engine speeds. Now, if the port is on
the opposite side of this example, exposed to the manifold at idle, but
when the throttle is rotated it is now exposed to the air cleaner side you
now have a port that is active only at idle or low throttle opening
position and not active thereafter. Typically this is used for a vacuum
retard, but as I mentioned the timing advance or retard is entirely up to
the way the diaphragm is fitted to the distributor - The easiest way to
tell what port you have is simply run the engine and either using a vacuum
gauge or your finger note the reading or feel of the end of the tube under
test. If there is full vacuum at idle and suddenly drops off just as the
engine speed slowly increases then it is a timed "on idle" port. If there
is no reading or feel at idle but then suddenly jumps to full vacuum just
as the engine speed is slowly increased then it is a timed "off idle" port.
If it is active at all times, well then it isn't timed at all -
Barry Schwartz (San Diego) bschwart@pacbell.net
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