Mechanical help needed - poor starting and running

[casper]

Hi , never tried any of these forums but here we go.
I am a Kiwi living/working in China.
i bought a Chinese Zongshen bike 80cc, 4 stroke , it's about a year old.
When it is cold it starts no problem but after about 30 sec it starts to lose power more and more and then dies , after this it does start but not easily.
It may do this two or three times , then it will run but with very reduced power until it runs for 15 or twenty minutes the power will slowly increase , so now at the lower speeds it is ok but at the higher end it still has little power.The carburetta and petrol tankhave been striped and cleaned.I live in a small city and believe me not one of the many "repair shops" has any idea what they are doing , please help,