After 50 or so flights I noticed engine running lean and fuel dripping from the tank. As it turned out I had a crack in the fuel tank around the fuel stopper hole. The problem was when the tank is under pressure (it’s a lot as I’m running a OMI Cline carb mod) and expaning it caused the plastic fuel stopper cap to press against the top engine mount bolt. In turn the fuel stopper hole is levered and all the engine mount vibrations where going into the fuel stopper hole. Obviously the tank eventually gave out and broke. You can see from the picture of the tank below which I epoxyed where the crack developed.
A simple solution to the problem is to swap out the offending engine mount bolt with a 4mm rounded head bolt. The fuel stopper cap then easily clears the bolt when the tank is pressurised.