Opel Rallier since 1977
The numbers for ignition advance on all other engines form this era are similar. So there is not any real 'out of my depth' on this matter. And yes, the '75 US FI Opels only had a vacuum retard. But, IIRC without looking it up, it is only at idle.low RPM, so really is a form of ported vacuum advance.It sounds like you’re right on it.
I drove around with the vacuum advance to manifold for all the positive reasons that you read about but the engine seemed too have much advancement on higher rpm deceleration (throttle plates closed)with no backing off from the advancement, I just didn’t like that so I moved it back to the carburetor.
I’ve since tuned out the off idle stumbles, I found that getting the mixture screw to 1.5 turns out from bottom enriching my idle AFR to 12.5 or close took care of the little lean spot off idle. Back when I had trouble with that was another reason for trying the manifold vacuum advance.
All this discussion about vacuum advance, and I’m out of my depth here on the 2.4, I thought Opel did away with the vacuum advance in 75’ ? Or was that just on the FI 1.9’s?