My 502 does not like to start after it runs for a while. When I first take the boat out it will run fine with no issues, But as soon as we stop and sit at the beach for a while and try to restart the boat it will run for a 30 seconds then die. I have dumped everything known to man in my gas tank thinking it was a fuel issue, changed seperators, I'm stumped here. Any ideas???

I think there were vapor lock issues on merc efi that did not run a mechanical pump. The symtoms sound very similar. I think merc sold a kit to add the mechanical pump back in.

Both my 502's did this also this summer. Vapor lock. I'm going to running my return lines back to the tank instead of to the separators. Many claim this to be the easiest best fix. Do a search here.
A buddy installed the merc check valves. His still vapor locks

i kinda thought that was the problem, when I take my separator off I can hear a release in pressure but it still takes a little while before it will start to run right. I am hoping this is the issue but I think my ecm is going bad also because the motor shuts off on its own when running at high rpms for any long period of time. It starts right up after it dies then but I am starting to not like this 502

Have you looked at the ICM inside the distributor cap? When mine failed it seemed every symptom was fuel related...not so much.

It finally died on plane one day and was hard to restart. About another .5 hours run time and it wouldn't start at all. Don't get mad at the 502, it's a great motor. Good luck.

Well my dumba$$ had the same issue, Put a pressure gauge on it, got 30psi running, about 6 low. OK....start searching for new pumps and in the mean time I pulled the old pump out of the VST and looked it over thoroughly. Come to find out I was missing the O-ring that seals the pump to the brass adapter that the fuel line hooks to. Basically, if the enginer was cold it'd start and run albeit kinda weak, but would vaporlock almost instantly and that's because the fuel rail had no way to hold any static pressure.

THis probably isn't your case, but I wanted to write down for future searchers.

Thanks for the reply, I have not had time to get out and check yet because either weather or work keeps me away from the boat this week. UHHGG....