I have a 1984 Mercury 9.8 outboard and it runs fine on idle but when you go to full throttle it goes for a couple of minutes then starts to lose power dramatically until in konks out. It will restart but then won't idle. Any suggestions would be appreciated. Also is there any place you know of to get the service manual online for free

Look for some paper or other foreign object in the fuel tank.

Evinrude 50 HP Bogs at WOT - MarineEngine

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Aug 6, 2002 - 17 posts - ‎3 authors
83 50 HP Evinrude - sat out in weather for 4 yrs. Been cleaning ... At WOT, she bogsdown. Up to. ... Yes, this boat/motor is new to me. Actually ... Mercury 50 1987 bogs down when full @ full throttle
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Is this motor used in salt water?
That warning buzzer on your motor is really only tied to the temperature. In order to triage sensor or overheat condition, I'd recommend an infrared thermometer to get a reading on the engine as the buzzer is sounding. There is a product that is like a glue stick which can be pressed against the engine which melts at a very specific temperature... not sure if it's around anymore as the remote thermometers are pretty inexpensive now. Is there good water flow out of the ports below the cowling on the back?

check when its on full throttle that the carb butterfly is at 90deg and not over as this will reduce throttle but max advance on ignition !

Have your spark plugs changed AND carbs cleaned and adjusted.
Usually that can also be caused by a seized advance system ( plate under the flywheel, commanded to turn with the throttle linkage... )

Check valve lash

Remove Carbs And Clean Then Test Again

Dont have the manual in front of me...recollect that there's two types of warnings. The pulsating warning is oil (starts with 30 minutes of oil left) or the fuel filter (or sperator, something like that) may have water in it. Running outboard with cover off, is a little black box that would have a red light lit if that's the case. If oil full & no red light, may be oil injection system...would also check to make sure lines aren't pinched, etc. Mine has been specific to oil the two times its gone off.

The full non-pulsating is engine overheat. Not sure beyond that.

Not much, but hopefully gets you going.

Sound like the fuel pump is not keeping up with the engine's demand. Check it out. Good luck. Hope this helps.

I'd be checking fuel flow. Usually if you have to choke or squeeze a primer bulb to keep it going, the motor isn't getting enough fuel. Check all fuel lines and connections are air tight, then if problem persists, investigate fuel pump.

A good carburettor cleanout and new carb overhaul kit wont hurt either.