912 UL Low Fuel Pressure and Power Loss

Thank you for all your suggestions. 

The mechanical fuel pump was replaced, therefore I don't think that the problem lies there. 

@Rotax Wizard: Does the routing of the carburetor vent lines affect the fuel pressure? I thought it only affects the mixture ratio

@Sean Griffin: That indead sounds very similar to my problem. However, when I do nothing, the pressure stays low and after around 10 seconds, the engine starts sputtering.
I have very large cooling openings in the cowling, I don't think that it gets to hot in there...however, I have not meassured it.

I am thinking about reducin the return orifice to #30...