speedfight 2 100cc carb problems?

speedfight 100 dellorto phva 17.5 problems - bike starts well but as soon as you accelerate it starves of fuel but is happy to tick over indefinately- carried out usual tests, fuel delivery good cleaned carb jets etc all fine.replaced e choke then replaced again with manual choke makes no difference. funnily enough if you partially remove choke by 1mm the bike then runs much better , but the minute you reseat the choke full the problems starts again. have never come across a problem like this can anyone help as its unridable. many thanks