C85 overheating at wot

[mickybrun]

I have 2 1996 C85 and one engine is overheating at WOT. I can run al day at 4500 rpm but when I push it to 5000 in less than a minute it overheats. New impellers and housings, new thermostats. Does this sounds as a poppit valve issue or as a clogged waterjacket issue?? Thanks

[ausnoelm]

Best guess is "clogged water jacket issue"