Old Merc 400 won't rev up

[Desert Toad]

I have a 1960 Merc 400 that I believe was bored out and built for racing like they would a mini hydroplane boat. It seems to rev up decently when it's first in the water and the motor is cold. But then I can't seem to get it revved up to really run. I know the prop is big, but once on plane it doesn't rev up much. Carbs are clean, compression is good, plugs are new. Any tips on adjustments or anything else is much appreciated.