Phoenix Models P51 Mustang 55cc - Egine failure on Re-Maiden

This engine has been in my bigger planes for 10 years. Never ever had any problems. It has run about 10 liters of petrol in 9 years. But this evening it started to fail and after a roll it went down to just a quarter of throttle and up to half. After a bad attempt to land i went for another round but had almost no throttle. I will try to see if there is something loose inside the tank or if the membrane in the carb is broken. It idles on low throttle but dies out when i give more. But only when the engine is warm. On start up it runs clean and with full effect.
If you have an idea of what it could be please share.