Starting Problem

I have a Yamaha T50D Outboard Motor.
If the boat is stored on the rack for a day or two the motor starts only after a few starting attempts. If the boat is stored in the rack for more days the motor will not start. When I then unscrew the fuel filter cup and some of the fuel spilled out and then screw the filter cup on, the motor starts right away.
After that the motor starts on each first try.
Why is the fuel prevented from reaching the spark plugs after the motor ihas been stored for some time ?

[boats.net]

Try pumping bulb till firm and then squeeze hard. Tilt motor all the way down and start without giving it any throttle, start at idle speed.