Motor wont go past 4100 rpm at WOT

[starkyb]

I have a 1988 75 horse Mariner 3 cylinder. It will push my boat to 34 mph. All of the sudden it wont go past 4100 rpm at WOT. It idles ok and revs up when in idle but just gets a point and wont go any faster. The gas is fresh and the spark plugs are new and I also have a water separator installed. Any thoughts, it seems like something electrical but I don’t know what. PLEASE HELP!!

 

[jimmbo]

So what rpm was it running at before it dropped to 4100?

I don't understand how it could rev up when it is at Idle? That said, revving it in Neutral is a sure fired way to Insuring you may not have an engine to Rev up.

Does the loss of RPM completely Random, or does it happen under a certain Condition, or set of Conditions?
Examples; After a certain Period of Operation, When it has warmed up to operating temp. When engine was run, then shut sown for a while, then when restarted it acts up?

Have you done the Basics? Compression? Spark that can Jump a 7/16" gap" Fuel supply adequate?

 

[starkyb]

Sorry. I meant it will rev up in neutral. At this point it wont open up completely when hot or cold in gear. I did try something when I got home though. I was able to run the motor on the number 2 cylinder and it didn’t run that much different when ran it on all 3. But when I tried to run it on 1 and 3 without 2 it wouldn’t fire at all. That kind of makes me think that 1 and 3 aren’t firing properly. I can resume testing tomorrow,

 

[jimmbo]

It wouldn't be able to get to 4100 if it was only firing one Cylinder, at Higher RPMs. makes me think Fuel Problems(gummed up carbs), and maybe a Spark issue. But that is a just a Guess. Proper Troubleshooting will reveal more

Do a compression test
Gummed up carbs can run run Lean, and Lean can ruin an Engine

 

[Scott Danforth]

The motor would rev to 5500 in neutral running on only 1 cylinder and a dirty carb. Reving a motor in neutral never proves anything. However some times reving in neutral ends up with motor run-away and then the motor grenades

What is the results of the compression test?
Do you have spark on all cylinders?
have you serviced the fuel system

 

[starkyb]

Update... I replaced the switch box and while I was in there I put three new coils on as well. Now the motor starts immediately, even when cold. It idles smoothly and doesn’t die at all. I think the coils were probably fine but I just call it maintenance on a 32 year old motor. I didn’t give it much gas while running in the driveway but what little I did was smooth and no hesitating or stalling at all like it was before the repair. Thanks for the help guys!!