I have a late 90's 25hp Mercury Outboard, it is on a 15' wood rowboat, weight approx. 350lbs. Run with two adults (not large) 2 children. Problem I'm experiencing is it does not give me good top end. Starts right up, idles fine, in neutral I can get top RPMs from motor but under load, on the boat I cannot. Compression is 120 each cylinder, new plugs, etc. checked linkage - ok, Prop is a 13P. Tried adjusting motor angle on back but does not make too much of a difference. Years ago I ran the same boat with an 18hp with no problems, boat got up and went fine. Motor issue?? or one last thing but couldn't imagine it would make that much difference in performance, the motor is a long shaft and really should be a short for the boat. Suggestions?? Thank you.

Too long a shaft will definitely affect performance, may be too much drag to plain boat out.

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could be fuel to air ratio. When i was working on mine i had it set wrong but still sounded ok but it was too rich and i couldn't get on plane till i changed it.