New to all of this...1st post . picked up a 15' sears supposedly a 1974 with a 1965 33 Ski-twin. Not at all like my 40' sea ray LOL
Ran it in barrel when looking at boat, idled nice...
Cleaned up and got ready to run new plugs fresh gas, changed lower unit fluid tiny bit of water -(more later) got to lake wont run. no fuel, replaced fuel pump and take off from marina, runs nice make a few passes and go back for kids. ran all day fine.
Take a buddy out late, after about 10 min she start loosing power and back firing violently.
Buddy who is small engine mechanic says order the lower unit seal kit you need any ways, get a tune up kit and impeller and bring to the shop these are simple to work on.
we do that, in the process he finds no gasket between carb and engine (replaced), looked clean anyways, replaced tune up kit impeller and seals.
He calls me and says come get it all went well. I get there and he is pulling his hair out, the engine ran fine for about 8 min in the barrel pumping water nicely. All of a sudden a bang, then a hiss, barrel full of milky oil and water and engine hot.
Pulled it back apart entire lower unit full of milky residue, impeller fine!
maybe a coil for what seems to be a heat related ignition issue ?? they "look" newer but who knows.
I keep hearing that the 33 is a good old motor but how much do I invest.
Just a wild guess here but I would think your friend did not line up the copper water tubes properly during assembly and water was not getting to the motor or maybe some junk from the old impeller plugged up the tube hence good flow at first and then the overheating. Impeller would be fine but engine would overheat. Pull the water jacket and check for obstructions.