1983/88 Mercury 115 Technical Questions

Good Day, my brother (mainly) and I have restored a 1966 Boat Locker "Malibu" The boat originally had a 1965 Mercury 1000 6 Cyl. Unfortunately we could not save the powerhead.

We opted to get an 80s model 6cyl with PTT, and the newer ignition system, in order to make it a bit more practical to use. After a year we managed to build a inline 6 with the 88 lower unit, PTT and gearbox, and an 83 Powerhead.
I have experience with OMC andthe Japanese brands, but 0 on Mercury and even less on inline 6s

here is a pic,with 66 decals on, contemplating if we must do the 66 engine coversand chrome beadings.
Attachment 427809Attachment 427810Attachment 427811
issue 1:

Cooling system
Engine running water is coming out of pee-hole AND a tube on the water jacket between the spark plugs

put a hose on it routed to the engine tray to prevent water from being sprayed over plugs
, what needs to happen here, and why is there an outlet on the water jacket?
Attachment 427812

Issue 2:

Motor starts, idles, a bit rough, and runs great for a few minutes, then it goes into a limp.

Action taken
Swopped out coil packs
New Sparkplugs
Sprayed a whole can of Quicksilver Power Tune to dicarbon
Cleaned Carbs
any ideas will be appreciated

here are some pics of the maiden voyage, after a 4year restoration project
Attachment 427813Attachment 427814Attachment 427815

Don’t have the old books handy, but first guess is water pressure gauge tap.

Very nice boat and background pixs..The back plate water is for PSI hookup. Them newer Merc got Pee water from top and bleed off steam. The running issue could be a hole in the fuel pump rubber or a Stator getting ready to go bad. Have you checked for good spark? Also pull the coils off and pull the rubber boot off and check for cracked carbon.... cracks weaken the field.

Water peeing on the spark plugs won't help the way it runs. Block that fitting and see if it helps.