Ok, so I have now owned my 350 mariner for 2 years... last year was my first full season on the Great Lakes and LOVE the boat. Last season I never got to use my Gen as the ball valve on the through hull that was busted in the OFF position. Bummer!! But.. over the winter I did:

* New spark plugs
* New Impeller
* New Impeller coupling
* Oil Filter (did not change oil as I could not warm up the engine, figured I'd do that this year.)
* New Ball Valve, of course.

This year, fired it up and she purred like a kitten... then after about 20 mins shut down. I ASSUME it's overheating, so go to replace the thermostat. Only to find the thermostat is missing the guts! like the top 1/2 is there, but the spring and everything behind is ... well, not there! I have noticed that my reservoir is over full, could that be indication of a blockage??

So now my questions are:

1) Did the previous owner take the guts out?? Ya know, just to not have to worry about overheating issues... Doesn't SEEM like something anyone would do, BUT... Ya never know.
2) if NO, on #1, where could the parts have gone? I used a flexible claw to feel around the entire top manifold and did not find anything.
3) have removed most all hoses and did not find any signs of the parts.
4) Can I just flush the Gen and see if I have flow with no blockage? where would I hook up?

And lastly...

5) if YES, on #1, I can toss in a new thermostat, flush the system BUT...what is causing it to shut down?

Does anyone have any experience with these bad boys? Would love some guidance, as I'm grasping at straws.