Anti siphon/ pickup replacement

Happy Memorial Day ! I have an 82 nautique that starves for fuel after an hour or so of running fine. Took the anti siphon valve out and cleaned with carb cleaner and air and the boat seemed to run better but I'm still having the same issues. I have about 20 hours on a new fuel pump that I purchased 8 months ago. It is definatly fuel related because when it stalls I pull the fuel line from carb and turn the motor over and no fuel or very little fuel is pumping. Let the boat sit for a few minutes and then fuel is delivered properly .. I also notice that when I shut the motor off I   hear the ball or something in the anti siphon valve rattle around with a vibration noise for a minute, like there is some type of vacuum on the tank . The valve is threaded into a 90 degree fitting that is attached to the tank. Is there a pickup screen attached to the 90 degree fitting inside the tank that could be clogged ? It appears this 90 dg fitting is welded to the tank or my eyes are just going bad. Is the 90 dg fitting designed to be taken out of the tank? Thanks and happy boating this Memorial weekend.

Thanks for the reply. Ill see if I can bust the 90 dg fitting loose and dig some more for the clog and yes the vent tube flows air freely.