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Mercury outboard won't stay idling

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  • Mercury outboard won't stay idling

    My Mercury 40hp 2 stroke (2001) won't stay idling and I really need an advice as to what could be wrong with it. Let me provide you with all the facts. I have also attached a video of it dying while it idles for about a minute (sometimes shorter).

    - The outboard keeps idling when I push the "choke/enrichment" button before it's about to die by pressing the key in. Yes, it's an electric start. When I press the key in, the engine starts revving higher in rpm for a bit and returns to the normal rpm and then the engine dies.

    - I have cleaned the carburetor (WME 2XK) very thoroughly. I used a carb cleaner, air blow gun and a piece of wire to clean out all the jets and holes. They didn't seem blocked but I made sure anyway. Before I cleaned the carb, I noticed that it was actually very clean without any varnish or gum. I properly set the float per my Mercury Repair Manual. Replaced all the gaskets and the needle.

    - I doubt it's the fuel pump and here is why. The engine keeps running so long as I push the choke button. Also the primer bulb stays firm and the fuel filter stays full of fuel while running the engine. I can see the fuel level in the semi-transparent fuel filter canister. Also, keeping pressing the primer bulb, which is already almost firm does not prevent the engine to die.

    - After starting, when I push the throttle lever up in neutral position, it still dies. So probably no IAC issue?

    - It dies even if I use fresh fuel. Whatever is in my old fuel tank is only about one or two months old.

    - There is fuel in the carb bowl when I unscrew the nut.

    - Possible fuel hose blockage? When I press the primer bulb in the beginning, it fills the fuel filter right up so I don't feel there is any blockage from the fuel tank to fuel filter. After the engine dies, when I unscrew the carb bowl, enough fuel spills out so I don't think the hose from the filter into the carb is blocked either.

    It sure sounds like a classic blocked carb jet issue, but I swear the carb is clean and EVERY hole and jet is unblocked. So at this point, I really have NO idea what's wrong with this thing.

  • #2
    You haven't mentioned if you tried adjusting the idle mixture?

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    • #3
      Yup I tried adjusting the idle mixture screw but it doesn't help. It's currently set at the number of rotation based on the manual. I turned half turn counterclockwise each time and tried until the screw was getting loose and it still died.

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      • #4
        The setting in the manual is an initial setting and the final ideal setting is usually leaner

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        • #5
          I just adjusted the idle mixture at leaner settings clockwise 1/4 turn each until I couldn't start anymore but each time it still died.

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          • #6
            How does she run at wot on the water ?

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            • #7
              It keeps running fine at wot on the water. As soon as I reduce the speed to near idle, it sputters and dies again.

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              • #8
                Need to post results of a compression test.-----Followed by a spark check.------Does spark jump a gap of 3./8" or more, yes or no ?

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                • #9
                  You don't mention that you changed the spark plugs ? Or did you ?

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                  • #10
                    I'm getting a spark tester this afternoon. Will update. As far as the compression, 110 and 115 psi for both cylinders. Spark plugs are about two weeks old.

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                    • #11
                      Here is an update to what I found out today.

                      I get good sparks as they jump the gap of at least 3/8" or more.

                      While idling before it dies, when I pump the throttle lever up and down, I am able to keep the engine going.

                      Also, I have a Mercury kicker that uses the same fuel line connector as my big Mercury. So I swapped the hose and primer bulb and tried. Same result. It dies during idle. The kicker motor never had ANY issues and the bulb and hoses are pretty new. So I believe I eliminated the primer bulb / hose issues too at this point.

                      Still very puzzled about this...

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                      • #12
                        Have you checked the timing at idle? I had an Evinrude outboard that would run fine at WOT but would die at idle and only survived with constant choke bumps and the carbs were spotless. Do you own a timing light?

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                        • #13
                          No I haven't checked timing at all. I don't have a timing light... Your evinrude's issue sounds like mine. Was it bad timing that caused your engine to die at idle?

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                          • #14
                            it was a 1984 evinrude 70hp and the idle timing was supposed to be 4 degrees TDC (Top Dead Center) but somebody had messed with the linkages and it was now -3 degrees TDC, meaning that combustion was starting after TDC. His engine ran great at WOT because the idle timing and WOT timing are 2 different settings. People often mess with the timing linkages because they're trying to adjust the idle rpm. Does your boat have a tachometer? What is engine rpm at idle in neutral? It should be around 750 rpm in gear, so around 900rpm in neutral. I think it would be worth your while to purchase a timing light. Very useful. Have you ever timed an engine? Do you have the service manual?

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                            • #15
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                              Thanks for the idea about the idle timing. I do own a cheap Kedsum tachometer and last time I was idling at around 850-900 rpm. When I adjust the idle speed using the idle speed screw, it still stalls at idle. I don't own a timing light and have never timed an engine before so I'll study this and get a timing light if it's something I can tackle on my own. I guess I understand the concept that it could stall during idle if the combustion timing is off but it still goes on for about a minute before it dies. I'm not a mechanic so excuse my lack of knowledge. Just trying to understand. It's been a long discouraging process.

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