Hi, Dave.
My '69 spit (2xSU-HS2) had a similar arrangement. The way it works is
that at idle, the vacuum port is _above_ the throttle blade, so it
sees no man.vac. Idle spark is retarded; well, it's not advanced.
Upon throttle opening, the vacuum is applied, spark is advanced into
operating range. My experience with engine behaviour as a result of
all these changes is verrry old, but ISTR that the retarded idle spark
reduces heat losses and HC production at idle, while the advance
off-idle gets you into the proper spark range for power (such as it
is) generation.
Timing should be 2 deg ATDC at idle (850 RPM). Off-idle, this is
advanced to about 10 BTDC, depending on exactly how "off-idle" you
are.
Hope this helps.
Donald.
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