Firstfives.org

This year I've put the original engine in the car - a 528 evil Zenith carb e12.
And I have a strange cooling issue. When it was hot outside, the engine would warm up, the thermostat would open, temperature would raise a bit more, a bit over the first line above the middle. Just enough to panic me. But then the fan would kick in and the temperature would stay around that line, while caught in city traffic. As long as I drive, the temperature needle stays around the middle.

And then it got colder outside. Instead of heating up less, the temperature now goes up more, mostly right after warming up. Once the fan kicks in the temperature drops, and then things settle to something acceptable.

It is worst when the car is not moving. Saturday I worked in the carbs, I started the car, but did not move it, temperature went up steady close to red. Then the fan clutch engaged and things cooled down.

The fan clutch is the really old type http://bmwfans.info/parts-catalog/E12/E ... n_coupling
I have changed the clutch #6 with a new one, I have added some washers in an attempt to make it earlier, when it couples it cools well.

Could this be a radiator issue? No coolant flowing in front of the fan, causing the clutch to stay too cold?

If your radiator was an issue the engine would remain hot.
Does your radiator warm up gradually due to coolant bypassing the thermostat, or does it warm quickly due to the rush of coolant when the thermostat cracks?

Gradually.
I was thinking that maybe if the rows near the fan clutch would clog up, that might these kinds of issues..?
Clutch seems to work fine as long as it actually receives heat. I might test this with a heat gun now that I think of it..?

Wow, I haven't seen one of them since the one on my old E3. But think about it, there was probably a reason they discontinued it. I didn't have any problems like that because the one on mine had already been bypassed and locked on by the time I got it. Unless you are trying to keep it purely original, I would put a later fan clutch on and a 8 or 9 blade fan.

Well the original clutch was in there from 1976 up to 2012, that would be 36 years, and covering 2 or 3 hundreds of thousands of kilometers. Not exactly what I would call a weak design. I've seen several newer viscous models go wrong - but I see their advantage, they always apply some torque to the fan, pulling air through the radiator.
I'd give it a got with a new radiator if there would be any available locally at a reasonable price.

Can you get your radiator re-cored? It's a disappearing art, most radiators are throw-away now. See a place that does truck radiators, it's less costly and all they save are your side tanks.

Today my fan dug in to my radiator when accelerating to 6k rpm while coupled.
So I think I'll give the upgrade to viscous coupling a try, after I manage to find a new rad.