mixture setting
 

The CO specs for my 83 model is 0.5 to 1.5% CO. When I measured it the reading was 4.5% CO. I have tried to set it leaner according to the Porsche workshop manual (without the oil temperature meter) but cannot change it much. Setting it richer increases the CO % to 6%, setting it leaner brings it back to the lowest of 4.5% CO. This is all at 850 rpm, at idle. The CO reading goes down to 2% at a constant 2500 rpm.
It has new injectors, fuel pump, filters, coil, plugs, rotor, dist cap, runs well. New vacumm hoses, no vacuum leaks, new inlet manifold gaskets, new intake boot.

What could be the cause?

Would the aux air regulator under the manifold influence the mixture setting? The valve was not working properly - I opened it and found a broken bakelite plate inside which I attempted to repair. It stays open when cold and closes after 2 - 3 minutes when the engine is running. The repair was not that good as some air still escapes past the valve because my "repaired plate" does does not seal the opening properly.

Could it be the the MAF ?

Would a too rich mixure indicate a too high fuel pressure, would the fuel regulator be too weak?

Measuring the fuel pressure is the amount of cc fuel delivered in a certain time - am I correct?