The fact that the engine died when at low speed is the key. Did you set the low speed mixture? I had the same problem yesterday, my engine would die when i lifted the throttle, so I did the 'pinch test' while it was on the box and it was rich so I adjusted it so it took ~5 seconds to die after pinching the fuel line. They engine ran great the entire main, never stalled or flamed out once. As far as leakage, IDK, I have two of these engines and I never noticed any leakage, but i never looked because they always ran good.