Ok here is the checklist:
1. Make sure there are no leaks in your fuel tank. Take out the tank and watch for leaks or bubbles.
2. Make sure your clunk is not touching the backwall of the fuel tank. Clunk should always be 3-4mm short of the backwall of the fuel tank.
3. Replace the fuel lines with new lines. Bubbles means that your lines are leaking.
4. Make sure your fuel tank in not vibrating. Start your plane with the wing off and see how the fuel tank is behaving at different throttle settings. I had the same problem as yours. Turned out that the fuel tank was vibrating like hell causing uneven delivery of fuel.
5. Open the needle valve to a point where the engine is running very rich and then start adjusting from there. When you think you have set the needle valve properly raise your plane and if the engine speeds up you are fine if not you are lean. Richen up till you hear the engine pick up when you point the nose of the plane towards the sky.
. If after doing all this it still quits then you should switch to better Abc or Abn engines.