Piaggio Zip 70cc Cuts out at certain speed

If I stay at about 60MPH constantly for around 5 minutes my bike cuts out(as if I pulled my key out). My bike starts straight up again and I've heard this is to do with my mixture screw? I can't full rev either or my bike bogs out.

My current setup:
PM tuning airfilter
Polini Sports 70cc Kit
115 jet
Polini kevlar belt
17.5mm Dellorto Carb


Any ideas?
Cheers,
Wunny

 

this could be a silly idea, but maybe its over heating?
and mixture screw messes with your idle

 

I had a similar issue with my similar set up, i turned out that the float needle wasnt opening far enough and allowing the fuel bowl to drain quicker than it could refill because of the angle the carb sat on the manifold. Some boiling water and leave the float to soften, bend the tag down that the needle sits on, quench it in some cold water to harden and presto, never had a problem since...

 

I thought it was over heating as well but then my mate proved me wrong by saying that he had the same problem (then he messed around with his mixture screw and it worked) and there are other zips going faster but still don't cut out.

@Jonnymarrw00t
Could that actually be the problem?

@JerseyJim
Could you explain it step by step if you don't mind?

Also, could getting a 21mm carb fix this problem?
thanks alot guys,
Nathan