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40 HP mercury outboard RPM too high

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sparkie78

New member
I have a 1995 40 HP 2 stroke 4 cylinder mercury outboard. In all the years I've owned it, WOT tops out at 5200 RPM. The last time out I glanced at the gauge and it read 6500 RPM! The motor sounded and was running good (or well?). Still though, I backed off the throttle some cause I knew this wasn't right. Any ideas on what would cause such a jump in RPM? I believe the specs call for between 5000 and 5500 RPM max. I've done no recent work on the engine. I did fill up the gas tank, maybe I put in premium by mistake (LOL). Seriously though, I would worry about the engine flying apart at 1000 RPM over spec. I appreciate any help on this.
 
If you trust your tach, then you are "now" running "not enough prop" (need more pitch).

Did you change anything "on the boat" - remove some gear, clean the hull, one less battery etc?

Less weight on board would effect the "efficiency" of the motor as would less "hull drag".
 
As a matter of fact, I was alone, whereas I nearly always have a 200 lb fishing partner along. I'm surprised it would make that much of a difference. So if I fished alone a lot, I should change my prop to stay within the RPM spec range? Thanks, and I plan to check it with another tach ASAP.
 
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