Mercury 90 4 stroke running rough at half throttle

[Keith8]

I have a 2002 mercury 90 4 stroke carbureted. It runs good at full throttle but if you back off to 3/4 or 1/2 throttle it cuts up bad. Also when I pump the primer bulb it does not get firm & it don't pump fuel through the inline fuel filter. I was thinking maybe it is getting air from the tube inside the tank. It's got me puzzled why it would run good at full throttle though. Any suggestions???