RPMs are 6800 at WOT for a 1995 115 Johnson

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brad77

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Apr 15, 2012
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Hi. i'm new to this forum. Seems like there is some great advice and a wealth of knowledge around here.
I recently was given a boat by my uncle. It sat up for about 4 years without being started. I put it in the shop and got it running. The boat is a 1995 Sunbird Neptune 180 with a 1995 115hp Johnson. I ran it the other day and at WOT the tach was reading 6800. I got 39.9 mph on the gps. It has a 3 blade ss prop with a 17" pitch. The RPM range for this motor is 4500 -5500 per the owners manual. What could cause the RPMs to go so high? Is the tach most likely inaccurate? The tach was holding steady and the boat seemed to perform well. Any help is appreciated.
 
BonairII

BonairII

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Jun 7, 2011
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Re: RPMs are 6800 at WOT for a 1995 115 Johnson

I would borrow another tach to confirm the accuracy of your existing tach. Does the motor sound like it's screaming?
 
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Mikeyboy

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Mar 15, 2012
Messages
475
Re: RPMs are 6800 at WOT for a 1995 115 Johnson

Welcome brad. I agree with bonairll that you should double check the accuracy of the tach first. The motor will not last long at those rpm's. Also a gps is not a great way of measuring speed on a boat as it doesn't take current into affect.
 
Part-time

Part-time

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Jul 5, 2011
Messages
536
Re: RPMs are 6800 at WOT for a 1995 115 Johnson

look behind the tach, you will see a setting for different cylinder motors. It's probably on the wrong setting.
Some tachs will have 4,6,8 cyl those are meant for I/O motors, some will be marked in pulses instead of cylinders.
My tack for example has 2,6,12 pulse settings. A 4 stroke engine will only have half as many pulses as a 2 stroke engine so if you have a 4 cyl 2 stroke on a tach that was meant for a 4 stroke, on the 4 cyl setting you would be reading twice the RPM's...
I hope this helps you.
 
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steelespike

Supreme Mariner
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Apr 26, 2002
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19,069
Re: RPMs are 6800 at WOT for a 1995 115 Johnson

As suggested varify your tach. sometimes if you operate the switch on the back of the tach it will clean the contacts.
A gps for speed; I can't think of any better way to measure speed.If you make a 2 way run you'll get an average.
if you drift in the current you'll get an accurate indication of its speed.A regular speedo will read slower down stream and faster up stream.
Current isn't likely to affect basic calculations.Unless your in a raging torrent.
 
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walleyehed

Admiral
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Jun 29, 2003
Messages
6,767
Re: RPMs are 6800 at WOT for a 1995 115 Johnson

....and 6800 is a tad high but it will NOT kill your engine. Yes, the manual says 5500 for a crossflow but 5800 is best, 6k won't hurt a thing. If it's a looper, 6K would be the target with a normal load, but 6200 won't hurt it either..
And yes, the tach is off if you have a known 17P. with a GPS of 39.9.
 
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