Gas these days has a low boiling point and evaporates much quicker than it used to.

• After turning off the engine look down the carburetor and see if gas is dribbling from the venturi. If so, then the check ball & weight might be leaking. Test it by holding down the check ball and pump the gas. You should feel pressure and little to no gas will get by.
• The venturi gasket might not be sitting flat, or needs to be trimmed.
• One or more small passage on the venturi might be plugged. Spray carburetor cleaner through each passage to make sure they go through.
• Check any vent line and gas cap going to the fuel tank. If plugged a vacuum will be created that sucks the gas out.
• Make sure the vent on the top of the float bowl is not blocked.
• Pull the carburetor off, sit it on paper, fill the bowl with fluid. Look for leaks out the bottom overnight.
• May be percolating. Engine gets hot, turned off, the fuel boils and evaporates out. Try other brands of gas. Look for ways to cool the gas. Rerouting gas lines, etc.
• Needle may not be seating. If new wipe the viton tip with mineral spirits.
• Test the fuel pump for the correct pressure. Around 4.5 lbs would be good, but refer to your motors manual.
• Installing an electric fuel pump with a regulator would be the last thing to try.