Fuel tank lid O Ring worn, fuel tank lid spring weak, crack in tank, crack in internal fuel tank pick up stem, pin hole in fuel line from tank to motor.

Take your tank out. Empty it completely of fuel. Seal the outlet nipple with some fuel line. Submerge the tank in a sink of water and blow into the inlet line. If you got bubbles anywhere, there is your problem. If not, flush the tank out with a bit of fuel and refit it, then change all your fuel hoses with some new stuff.

Also check the fuel inlet banjo on the motor (HSN assembly) is nipped up tight. These seal on alloy washers not rubber ones, so this assembly can vibrate loose on ocassions.