98 force Mercury 90 hp outboard reduces rpm at high-speed

dakotaba5

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Mar 6, 2017
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I have a 1998 force mercury 90 hp motor That reduces rpm to 3000 at high-speed, wide-open throttle. Sometimes when it's running at high-speed it will bog down and reduce it to about 3000. This of course lowered the speed of the boat . What could cause the engine to lower it RPM and not run properly? By the way, it runs fine at low speed's and Idles fine. If I disengage it from the propeller to increase rpm, it will hold steady with the throttle wide open
 

roscoe

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Oct 30, 2002
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Welcome Dakota.


A few questions.
Do you know the maximum rap it will reach?
How long will it run at these rpm before it drops down?

Will it run at say, 2500 or 3000 rpm for an extended period of time without issue.

Does problem seem to occur after engine is run a while and is all warmed up, or
will it happen with the engine fairly cold?

Problem could be several things.

You may have a wire which is shorting out as the throttle is increased and the timing plate advances.
So look for worn or frayed wires up under or around the flywheel.


I can't remember if that motor has separate high and low speed coils in the stator. If so,
the high speed portion may be failing.

Or, you may have a coil that is failing.
Or a loose sparkplug wire, or ground wire on the ignition board.

Best thing to do is a spark test on all cylinders when the issue arises.
Shut it down and test for a good strong spark asap.



"If I disengage it from the propeller to increase rpm, it will hold steady with the throttle wide open"

Best not to rev the engine above 2500 rpm when there is no load on the engine. They can and do self destruct.
 

jerryjerry05

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May 7, 2008
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Welcome.
Roscoe said a LOT of good things to check.

​Also check the tank vent, it can get clogged (wasp)and not allow air to pass.
The inline fuel connectors have rubber in them.
They can go bad and suck air.
The inline filter can be clogging.
​The fuel pump failing and cross feeding and sucking air
​The squeezie has a valve inside.
​It can go bad and stop or restrict the flow.
​Ethanol can melt the inside of the fuel lines and cause them to collapse.
Check for water in the fuel.

​You make any changes since the last time it ran good.
 

pnwboat

Rear Admiral
Joined
Oct 8, 2007
Messages
4,251
Try squeezing the primer bulb when it's acting up. If that temporarily fixes the problem, then you might have a fuel delivery problem. Bad fuel pump, restricted/bad fuel line/fuel line connector, gas tank vent line plugged etc.
 

dakotaba5

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Mar 6, 2017
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I don't know what rap is. Max RPM is around 5500. It will run for extended time no problem at low speed and rpm. Sometimes I can get it to go back to full speed by closing throttle then quickly reopening it. It runs for about 30 minutes to an hour before its dropped down. At least that's what I recall. It's never done it cold. Usually after ran for a while. I haven't made any changes besides replacing the starter a few months back that went bad because it wasn't fully engaging.
 

roscoe

Supreme Mariner
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Oct 30, 2002
Messages
21,657
"rap" is what this site auto corrects the spelling to, for rpm. Sorry, I didn't correct it before posting.

You gave us some good info.
Since it doesn't do it cold, only after running for a while (30 minutes), I am going to stick with an electrical issue, until you can test for spark.
Dropping from 5500 to 3000 rpm would equate to dropping one cylinder.
Possibly from a failing cdm coil, which is failing when it gets heated up.

Also, don't forget to look for loose or worn or cracked wire insulation that may be grounding out somewhere.

Keep us posted.
 
Last edited:

jerryjerry05

Supreme Mariner
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May 7, 2008
Messages
17,923
That also sounds like it might be getting hot??
Test the overheat buzzer.
 
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