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.

 

[Project_Mayhem]

Did you rebuild or simply spray out the carb?

 

[Carl Wonderful]

I think I will go back and give the carb a complete cleaning. I assumed because he used ethanol free and because it was only a couple years old, the carb might not have been the issue.. but Its definitely a suspect I see now.

As to is it revs before it stalls.... I don't think it did that. It was a very inconsistent idle though... and like I mentioned at only like 5% throttle it was going wild! We were thinking it was more of an issue with the throttle cable/linkage somewhere. Maybe its all of the above, lol.

I did adjust the 2 nut/screw adjustments where the throttle cable goes forward (with an adjustment) then turns around 180 to another nut/screw adjustment which did give is a little more play. We could pull the throttle maybe 10-15% but that isnt saying much. Still very messed up. We basically just trying to get the throttle cable to not pull so much so fast., to give it some slack I guess you could say. Could this be the problem? I think there's another area of the throttle cable, little harder to get to I think, that also has an adjustment for the throttle cable? Any thoughts?

:O

 

[Davidasailor26]

If you could try it someplace under a real load that might give a better idea. If it’s revving because it’s too lean a load will probably keep it down. If it keeps revving that high under load then I’d suspect something with the throttle linkage. Like Project_Mahem said I’d try to disconnect the linkage and adjust the throttle directly on the carb to troubleshoot too.

 

[MaryEllaRose]

A vacuum leak will also cause erratic/high idle. Check the connection between the carb and the manifold and the head and the manifold. Loose bolts? Shrunken gaskets?

Tom G

 

[Project_Mayhem]

Get a carb kit instead of just cleaning. Follow the instructions in the service manual. Many carbs have two circuits -- one for idle and one for every speed above that. This is why you sometimes see two rich/lean screws