I had this problem on the first ARF I bought which was a H9 Cherokee. The fuel tank was ok but the outside of the stopper was a soft black rubber. It passed the leak test at the sink but would start to leak when you throttled up and the tank became pressurized. It was too soft to tighten up against and get a proper seal. The tank was a non standard shape so hard to replace but I just bought a replacement stopper from another brand and it worked fine.