Carburettor Pierburg 175CDUS, engine B230A

[LaRy]

I have a problem that my Volvo 740 from 1985 won't accelerate as it should when reaching over 4000 rpm, it hesitates.
It has to do with the carburettor, a Pierburg 175CDUS. The float chamber is normally ventilated through the air cleaner when the motor is running. When ignition is off, the float chamber has atmospheric ventilation. This is automatically selected by an elctric valve, and that valve is working. If I loosen the rubber hose from the air cleaner to this valve, the engine is working perfectly. So if the carburettor has atmospheric ventilation also connected to the float chamber, when running, the engine works perfectly. PS, the air filter is brand new.