Losing Fuel

Before trying to start it after it having sat for a few days, pull the air filter off and work the throttle to see if the accelerator pump is working.
If not, the carb is either empty or the pump plunger is dried up.

Without pumping the throttle, crank the motor over for like 10-15 seconds to fill the bowls.
Again check to see if the accelerator pump is working.
If it works right away, then the carb was likely empty.
Most likely from evaporation.

At that point, you should be able to give it a few pumps, and it should start right up.