Seeing the condition of spark plug, I can say that the mixture is rich, and that is also a thing that push me to idea of air leak, because i need to run enough rich that engine will not stop when warm...the sensor plate is not deflecting so much...the gap that is created when is sucked down is some about 1-1,5 mm maximum.Fuel strainer is ok.Fuel tank was empty and cleaned, fuel in the tank is new, and when I was checking the fuel flow to fuel distributor I have seen that fuel is perfect clean, and fuel flow is exactly as specified in WIS, about 1 litre in 40 seconds.