Vespa T5 Malossi 172 kit carb set up

I have a Malossi 172 kit the exhaust is an SIP performance nt pipe Dellorto phbh 28mm carb and standard clutch. When i get to 55mph it keeps holding back. The needle is a x2. Idle jet 52. Atomiser AV264. Main jet is now a 138. Have tried all jets from 122 onwards. Any help please.