Well, time for an update. Have not made any progress on solving the inoperative OB circuit. The car is running well, have driven it approximately 30 miles with the new microprocessor controlled vacuum canister (see picture).
There is however one small problem. When power is applied, the new canister goes through its start up routine, which is to advance 7 degrees and then retard 3 degrees to establish its reference location. In my distributor the outer plate rotates 7 degrees but when it reverses to rotate 3 degrees in the other direction, the outer plate and the star wheel appear to be locked together.
So it would appear when I disassembled the distributor earlier in pursuit of freeing up the advance/retard mechanism, I incorrectly reassembled it somehow. I’m pretty sure I installed the little oblong steel key correctly otherwise the star wheel wouldn’t turn with the shaft. I will have to remove the distributor again and inspect. There is plenty of axial play, so that’s not the problem.
Managed to install the distributor one tooth away from its prior location. Unfortunately, one tooth appears to be a big angular difference so the distributor is rotated all the way to the other end of the slot with only a tiny bit of slot left to turn clockwise. But the timing is right on the TDC mark on the fan shroud after the thermo time switch activates and the idle is steady at 750 rpm. Not sure why Porsche couldn’t come up with a design that allowed 10-15 degrees of adjustability in either direction, perhaps I’m still doing something wrong.
That’s it for now, will update when I have the distributor removed and inspected. May have to send it to the vendor who has kindly offered to go through it to ensure all is well.