Mercury Very high revving at Very low throttle? (small outboard)

[Carl Wonderful]

My buddy has a 2023ish mercury 4 stroke. I got it running for him (sat for a year and a half). We only tried it in neutral. Let it run for a good 10- 15 minutes in water bucket, just trying to keep it running to see if it would get better but did not. But at say 1/10 of the throttle,( just above starting area) it is going nuts like really high rpms. Its like we have the whole travel of the throttle, compressed down to a 1/8" turn...instead of the full ( oh I dont know, 2 -3").... It's almost impossible to just get it settled down to stay running cause there's no "room" on throttle. A hair of a turn revs it like wild and a hair of a turn back it'll almost want to stall.

I'm having a really hard time finding info on this.

I''m guessing its a idle adjustment or a throttle linkage adjustment or something but cant figure anything out.

There is a little brass (flat brass, no screwdriver option) on the top/side of the carb that has a green line in marker on it. Then just to the left of that is another brass screw that can be adjusted.

Just lost here. Any info and or diagrams to help me understand whats going on and what does what would be great!!

 

[east coaster]

Had the same issue with my tohatsu 6 hp on my dinghy. Turned out to be the idle jets were gunked up (technical term) with ethanol residue. Used carb cleaner to remove the material and now it runs fine. When I finish with the outboard for the day I turn off the fuel valve and let the engine idle to empty the carb bowl to prevent gunking up the idle jets. Works for me and carb cleaner is now onboard the dinghy for when old age catches up to me and I forget to turn the fuel off.

 

[Carl Wonderful]

Had the same issue with my tohatsu 6 hp on my dinghy. Turned out to be the idle jets were gunked up (technical term) with ethanol residue. Used carb cleaner to remove the material and now it runs fine. When I finish with the outboard for the day I turn off the fuel valve and let the engine idle to empty the carb bowl to prevent gunking up the idle jets. Works for me and carb cleaner is now onboard the dinghy for when old age catches up to me and I forget to turn the fuel off.

He has only used ethanol free gas in it he assured me.

I could be wrong here.... but wouldn't gunked up jets usually slow the idle or not even start... instead of speed it up? If i am wrong please help me understand.

 

[Project_Mayhem]

He has only used ethanol free gas in it he assured me.

I could be wrong here.... but wouldn't gunked up jets usually slow the idle or not even start... instead of speed it up? If i am wrong please help me understand.

Ethonal free gas can still clog a carb. That's what happened to my two stroke Evindrude 140 (yes, very large engine for a two stroke).

Once upon a time a small airport by me had an open house. They had a handful of four seater prop planes giving short rides. Every time they shut off the engine the RPMs increased before stopping. I asked about it and it turns out that when they cut the fuel, the air/fuel ratio went lean. I believe this could be a contributing factor for you. I would also disconnect the throttle linkage at the carb to verify that no other adjustments are off. I've seen a few engines where the throttle and linkage were connected by a spring. In that case, if the butterfly valve was gunked up, the engine wouldn't respond well to input from the user

 

[rgranger]

A high engine rev. is a sign of running lean (oxygen rich) so "gunk" in the fuel system is a likely suspect.

Let me ask a question. When it stalls, does the engine rev up a little bit more just before it quits? If so, that is a tell-tale indication of a fuel starved system.