Re: WOT screw?
I have never worked on that engine but that does look like the limit screw for the timing BUT you don't want to just turn your timing higher, if you change it there you change ti everywhere and it is tied to the throttle so you could really open a can of worms here.
Search "link and sync" and "Joe Reeves high speed timing", it is a way to set your high speed timing with the engine idleing and works great and the link and sync ensures that the throttle and timing are set correctly, if they are off even alittle the boat will run really bad.
Having said that timing doean't change so if the engine worked last year I really doubt timing is your issue unless you have changed it or messed with the linkage, once it is set it never changes, get all the free stuff first, confirm spark to all cylinders, do a compression test and confirm you have gas to all cylinders.
Normally when an engine suddenly starts running bad it is a carb problem or compression but rarely does timing change by itself.
