PK50 V5N1T

I am struggling to get my engine running properly and am running out of ideas.

I have totally rebuilt the scooter and everything is working fine except I cannot get the engine to idle properly and it is hard to start when cold. The engine seems to have a mind of its own and revs up for about 20-30 seconds without and physical throttle input. Doesn’t matter what I do it continues to do it, new carburetor, new manifold, new seals and gaskets, I have removed and seals the carb and manifold many times but nothing changes. 

It will on occasion behave itself and idles well, go for a run and it goes back to revving up it will even do do it while on a run especially if your are approach a road junction.

the only thing I can think of now is the choke is some how coming into operation but doubtful.

Any body got any ideas or has experienced this and got a cure.

cheers