Thank you BLUE and all that helped in good will. After installing a newe spark plug and charging the battery, more like in an act of desperation, the engine started at once. I should have done this from the beginning, but I was afraid the charger was unfit to charge a scooter battery. Now is over.
Thank you for all your suggestions. I only have to tune up the carburator because the aceleration goes up and down, and you have to be like a musician to get the right tone.
happy week end on the road !!!
Aquaviva