1989 Mercury 90hp Low RPMs

enggass

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Sep 14, 2010
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So I just got a new/used Whaler Montauk boat w/ Merc 90hp with 17Pitch prop. RPMs top out at 4000 and top speed around 25MPH. Thoughts? New fuel lines/filter. Should be around 5200 RPMs and 40mph.
 

ondarvr

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Apr 6, 2005
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11,527
Lots of things to check.

First make sure the boat isn't full of waterlogged foam, it's a common issue on these boats.

​Check compression, spark, clean the carbs, check filters, etc. once you know the motor is running correctly then you can proceed with the correct plan.
 

Silvertip

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Sep 22, 2003
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Very often people feel the engine is running fine when in fact one cylinder may not be firing or it has a mechanical issue an is not making power. Always start with the cheep/free checks first. Compression checks require a compression gauge but you can rent or borrow them at many auto stores. Checking plugs requires a plug wrench which you should have anyway and they cost less than lots of other stuff.
 

jimmbo

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May 24, 2004
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The 90 merc has a WOT range of 4500 - 5000 rpm, above 5000 the power curve drops off very quickly. Propping for about 4700 -4800 will give the highest speed, as that is about where that engine makes it peak power. 4300 is about where my 115 ran when it lost spark to one cylinder
 

ondarvr

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Apr 6, 2005
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On my chart it shows the RPM range is 5,000 to 5,500 RPMs
 

enggass

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Sep 14, 2010
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UPDATE: Broken plastic connector to one of the Carbs so only 2 of 3 cylinders were firing. Thus causing low RPMs and terrible speed production.
 

tommarvin

Ensign
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Nov 22, 2015
Messages
999
YES, something easy,
Yes new USCG A-1 fuel lines from tank to motor add a fuel water separator, new primer ball, new on motor fuel filter,
Clean fuel tank, pump out old gas, add stabil marine 360 every time you add gas, and more for long term storage.
And a whole lot more,of things to do.,
I used to crab with two guys in a 17 foot Montauk great boat, I will never forget it.Be safe out there.
 
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