What we need to know is, is the fuel getting out?
To properly test the fuel system and the air system we need high pressure fuel injection gauges. They need to read in excess of 100#. We take a reading of the fuel pressure and the air pressure, each has a schraeder valve to check. Same as a tire valve. this will tell us if the high pressure pump and the aircompressor is delivering the proper amount of fuel. You can get some inexpensive sets at stores like Harbor Freight, not great quality but fine for the occasional use.
However before we get that far lets see if the high pressure pump even turns on.
With the engine cover off locate the VST, the silver colored tank with the fuel pump. Have a helper turn the key to the ON position NOT start, just ON. You should hear the fuel pump run. If you don't hear it then it is probably either bad or gummed up.
Mercury, and the other manufacturers have always had a problem with these pumps when they sit with old fuel in the VST. Mercury has a procedure to try and clean the pump with Quickleen, a fuel cleaner and treatment. Sometimes it doesn't work, particularly if there was any water in the VST.
So lets see if the pump is working and post back your results, we will go from there.
Just a note you can manually drain the VST at the end of the boating season to try and prevent this in the future.
Also get me the serial number when you post back.