fuel problem

[lathehand]

I'm helping a friend with his 1986 90hp merc inline 6 .It starves out of fuel every so often.When it dose that the primer ball colapses and stalls out .At first I thought it was a clogged vent We've put it on a seperate gas tank and no change. Then we changed fuel lines and primer ball and redone the carbs.Still ng.Haven't touched the fule pump except to check the straner it was clean.I'm thinking if the diaphrams were bad it probably wouldn't run as good as it dose .sometimes it runs for a hour and it's fine sometimes it acts up after only a few minuets