Offshoreonly.com - Problem with fuel system

I have a interesting problem. My engine won't start. Gives the symtoms of a clogged fuel filter. I change the fuel filter and the boat starts right up. This happens after about evey two weeks. The problem is that today I dicected the fuel filter that I pulled out and found it to be clean as a wistle. I thought it was the fuel filter but obviously not. What else counld it be. Maybe a check valve coming out of the fuel tank? Its a carbed engine with mechanical. I'm thinking that when I change the filter it is really depressuring the line and releasing the check valvue? Only happens after the boat sits for a few days between runs. Never dies while under opperation. What do you think???