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2000 Mercury 90HP outboard bogging at full throttle.

G

Gutsu

New member
Just bought my first boat and owner said it sat for 6 years. its a 2000 mercury 90hp engine outboard. Before selling he took it to a mechanic who cleaned out the carbs and pistons. The mechanic thought he cleaned out the gas tank to empty but accidentally left 10 gallons in there, I filled up the rest so 20 gallons were in the gas tank. there was still 10 gallons with 6 year old gas. I ran the boat for 1 day on the water using my big motor for about 30 minutes before switching to my kicker. I then flushed out the gas and cleaned out the tank and put new gas...

With the OLD GAS at full throttle my RPM's were at 3,200 @ 25 MPH. it would run for about a solid 30 seconds then start dropping to 3,000 and back up to 3,200 RPMs. This would repeat forever untill I drop RPMS a bit to under 3,000 RPMs and the issue stopped.

With the NEW GAS it runs at 4,000 RPMS @ 30 MPH and after about a solid 30 seconds starts doing the same exact issue. Drops to 3,800 or so and back up. It will do this until i drop the speed.

I cannot attach the video but I do not think this is a issue with Old gas causing the CARB to be clogged because Right when i got on the water it started doing it. and if the person cleaned out the carbs or other stuff it wouldnt be consistant. What else can i check without spending hundreds of dollars on a mechanic!

PLEASE HELP!!!
 
On a Lund 2000 17 foot, adventure fisherman. It should still be going that much RPMs?

Does it matter on anything else or should it be running at 5000 regardless
 
Step #1 is to post actual results of a compression test.----Outboards like that should be propped to run at 5500 RPM.----Size of the boat does not matter.
 
Ok! I will definitely do that compression test! Is there a video of anywhere I can see how it’s done? I can’t find any old videos of a similar version of my boat! Also is it very complicated that I couldn’t do or is it simple? I am mechanically inclined so I presume I’ll be able to do it!
 
Interesting. Will check that out, is there a recommended size I should use for fuel lines or are they all the same universal?
 
Checked the gas filter, everything’s good, checked gas lines, they look perfect, Took off the spark plugs. They are all good, did a compression test, all cylinders firing at 90.

When I start the motor and give it “throttle” to raise RPMS by picking up the metal tab on the engine by the carburetor (not through the throttle handle) it randomly will misfire through the filter, not sure if it’s doing this because boat has to resistance since it is not in the water but my father who isn’t a mechanic in boats but cars says you can hear the cylinders misfiring. I could not hear it as I don’t know what to listen to but he says he can hear it misfiring. I’m debating whether I should change the spark plugs and ignition coil, would that help?
 
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