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Hmmm. Good question.
The '85 uses Volvo/Chrysler "Computerized" ignition timed by a Hall switch in the distributor, so it is adjustable by turning the distributor housing. I've owned two cars using that system. It adjusts timing to engine load through a vacuum hose from manifold vacuum monitored by a pressure transducer right on the ignition computer itself, by the washer bottle.
I suppose, setting idle timing, you would pull that hose and plug it toward the manifold side, but I do not recall needing to do that, whether the presence of idle vacuum changed the timing any, nor any manual reference saying to do that.
The Volvo manual (green book TP30432 p.67) explains the timing procedure like this (my words):
1. Set the idle timing to 12 +/- 2 BTDC at 750 rpm idle.
2. Check "centrifugal" advance by running at 2500 rpm with the hose disconnected from distributor (which of course it does not have!)
3. Check "vacuum advance" by running at 1500 rpm and disconnecting hose (this time it says at the computer) to observe the change in timing.
The only reason I ever had to set the ignition timing was after swapping distributors, otherwise it doesn't ordinarily change much over time like points used to. I just warmed up the motor and set it to 12 without touching any vacuum hoses.
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Art Benstein near Baltimore
MapQuest really needs to start their directions on #5. Pretty sure I know how to get out of my neighborhood.
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