Be VERY careful of fire when this happens.  The very same thing happened to me with an old vintage 3hp Johnson.  Fuel sloshed out on the cowl during this episode and I thought it was wiped clean.  Just enough went down in the cracks to ignite when I started the motor.  It flashed up and I actually unscrewed the motor and dunked it in the water to stop the flames.  This was on a ply skiff and I had no fire extinguisher aboard.  The motor got some "char" marks but otherwise nothing was hurt.  This makes a good case for external tanks.

Bill