Vespa T5 172, why isn't it fast?

Hi all, I'm looking for a bit if help to try and find out why my 172 worb 5 kitted T5 isn't as fast as a standard T5! My set up is a malossi 172 worb 5 kit with the matching head a 30mm dellorto carb cosa 2 sports clutch 21 tooth and a PM tunning exhaust and it just doesn't seem to fly as I feel it should I just manage to keep up with a standard T5. 

Carb set up is main jet - 122, have try'd jets up to a 128 but was running rich,is there something that I'm needing to check with the engine?

Can someone advise what would be a good carb set up on the worb5 172 kit with 30mm carb and PM pipe!