"One-way" fuel line valve..?

Yes, as soon as the engine starts, the idle is fast (circa 1000/1200 rpm - no tachometer). I let it running 20 or 30 seconds, then I give a brief impulsion onto the pedal, the choke disengage and the idle drops down circa to 650 rpm. The engine doesn't die, the idle is smooth and regular.

You start with the gas pedal fully depressed? I can try that