The gas leak is in your tank seam near the seat due to rust. Trust me I know. Had the same thing happen to me in April. I had to completely restore my tank and rebuild the carbs. The gas seeps into the air box. The engine vent exits into the air box at it´s lowest point. When the engine is running, the positive pressure from the engine blows the gas fumes out of the air intake making the leak smell even worse. If your tank is not too far gone, I suggest you remove the rust from the tank correctly and seal the tank. Do Not use Kreem TM, to seal your tank. It does not work. Use either Red=kote or Craswell 2 part sealer.