New dr75 kit on Vespa 50n

I have installed a dr75 kit on my Vespa 50 smallframe and I have a feeling it is lacking some power. I also read a post from someone with a similar problem on their pk50. My set up is slightly different. 

Dr75 cc kit, 16/16 polini carb and reed manifold, have upgeared the main gearing, new polini banana exhaust. , 74 main jet.

After all this I am only getting 40kph which seems rather slow. Does anyone have and ideas what could be restricting it and what speeds I should be getting. 

Thanks