Then check compression on each cyl
Have you tried playing with the tilt ? Does it make any difference at all ? Will give couple extra knots.
Starting issues seem normal dependant on it's age but again may not be firing on all 6.... I had one a while back (blackmax V6) sounded great, ticked over well but under load (ie on the water) would give similar results as yours. Turned out to be ring failure on 2 cyl (giving 65psi instead of usual 135ish).
That's worst case though.... Sounds prop related if reverse affected too
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