1995 Formula with 502 and VST
 

I know the VST has been covered a lot in this group, but I haven’t seen my specific symptoms. Hoping someone here can help me out.

Take the boat, run it around the lake for a while, head to the idle zone to swim/hang out. Could be 10 minutes, could be 4-5 hours.
start the engine, it runs fine for 30ish seconds spits, sputters and will die. Immediately hit the key and it will fire right off, run for 30ish seconds, spit, sputter and die. Will do this 5-6 times. All the while you can hear loud hissing like the engine is trying to compensate and take in more air, as soon as the hissing stops, the engine idles down and will run fine. If I throttle up a little when it sputters, I can sometimes keep it running. But until the hissing stops, it will not stay running on its own.

Any advise?

BTW, I have already rerouted the return line directly back to the tank.

Following

Merc gave up on this system long ago. Best advise here would be to do the same. Search the Merc fixes.

Vapor Lock
Can fix with return to tank instead of return to filter

Quick fix, need a rag, safety glasses, small screw driver
Remove the shrader valve cap off the valve to expose the shrader valve. Put on safety glasses and place rag around shrader valve. Cycle key and hear the pump turn ON and shut OFF. Press in on valve and allow air from fuel rail to escape. Cycle key a couple more times until no air comes out. Replace the cap and fire it up

I already rerouted the return back to the tank. A couple of years ago actually.
If it’s vapor locked, how does it immediately restart when I hit the key right after it dies? Multiple times. I’ve tried cracking open fuel lines to relieve pressure with no luck as well.
Never has been a deal breaker, just annoying more than anything.

Check your fuel pressure
The loud hissing leads me to the IAC is wide open trying to get more fuel. It only happens after its stopped.
Would need to get hands on to see if there is something else, but return to tank most always works