I have to ask this, even if it sounds dumb and I might even know the answer.
But the car is over 40 years old and that kind of engine is not "standard" here in germany. So I better ask even stupid questions.

I get that you say 13.5 average. But is 13.5 more important at certain rpm or wot?
Maybe 13.5 at idle, then it get's leaner/richer and as rpm gets higher it(should) go back to 13.5 and at wot way over it?
so I could set 13.5 at idle or high rpm without secondaries.

The guy with the O2 Sensor is a specialist but we have (in normal cases) strict emission laws so (normally) theres no question at what point it has to have what AFR.
And mostly it is just control and not tuning.

EDIT:
Maybe I have got something wrong, writing a bit complicated (bad english) or just start to think too much.
The help I get from you guys as always very good.