79' Johnson 25 h.p. overheated now won't start

[kcasper0315]

I replaced a broken prop shaft. When i took it out to test it started just fine. It went in to gear and propelled the boat fine. Slowly eased it into high RPM and after 1-2 minutes it overheated and cut out. Not sure i got the water pump line re-connected to the engine. Had to tow it in because starter rope was very hard to pull and i couldn't get it to fire. Took it apart and made sure water pump line was connected. It didn't seize because i could turn the flywheel by hand. Read a post saying to take spark plugs out and see if it will turn over, it did. Put in new plugs but still having the same problem. Turns over fine with spark plugs out, but with them in it won't. Super hard to pull starter cord. Could it be a clogged exhaust? It just seems like the outgoing pressure has no place to vent. NEED HELP.

[boats.net]

Remove the lower unit and see if can pull rope, if engine turns over good with lower unit off then have problem in lower unit, check lower unit for water in oil.