2003 Mercury 25hp long shaft-low rpm at wot

10 FOOT INVADER

Petty Officer 3rd Class
Joined
Apr 29, 2007
Messages
94
Back in 04 we mounted a mercury tracker 25hp 2 stroke to my kids 10 foot invader. At full throttle it will only tack to 4200 and hit 21mph. The 1966 20 hp evinrude would hit 22 mph. The merc is rated to 6k rpm. The prop is the only one you can get and is 13 inch. The tach in the dash is a teleflex signature series. We followed the instructions and i think its correct. Out of the water and no load she will go above the 5k mark. The boat speed has remained constant at 21 mph regardless of weight. Switched from a 6 gallon temp tank to a 12 gallon perm mount tank. The motor was purchased new and has been like this from new. Is this normal for the rpm to be low?
 
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