87 Mercury 20xd not fueling properly

[zackduncan40]

i have a 1987 2 stroke Mercury 20xd, It starts up and runs fine but it's like when you get going it is like vacuum sucking the whole gas tank in on its self and it will die if you don't keep pumping the primer bulb as you are going, but as long as you keep pumping the bulb it runs fine. Do I just need a new primer bulb or am I having a problem somewhere else? If so how can I test to see where my problem is? Thanks

 

[211libwtfo]

Do you open the vent screw on the tank.

 

[zackduncan40]

My tank does not have a vent screw and this is a new problem, have used the same stuff for over a year and it has just now started this