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Mercury Sport Jet 120XR Poor idle/missing?

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  • Mercury Sport Jet 120XR Poor idle/missing?

    I recently purchased a 1996 Bayliner with a 120XR Sport Jet (s/n 0e189282). I did purchase a manual (Clymer) but it does not seem to relate specifically to mine since some of the wire colors seem odd and according to it, I should have an extra, large red wire coming out of the voltage regulator (which I don't). I also can't find the switch box, I assume it is the mess of wires with bullet connectors instead of an actual box.The engine used to start up and idle wonderfully, now it barely starts and only if I increase the throttle while starting. After increasing to WOT it seems to start running on all fours, but you can still hear it miss every now and then. Back down to idle works some of the time. It used to run 5000 rpm but now only does 4500-4700. (ok, ok, I miss the extra speed!)Anyways, the motor ran fine up until right around the time I replaced the spark plugs (any correlation?) I am using NGK BZ7HS-10 with a 0.040" gap (tried 0.030 & 0.035 as well). Has anyone had a problem with the spark plug wires? Can I replace them with wires I can buy at the local auto parts store? Should I be using RFI suppression plugs?Around the same time I left the blower on one night, at least 24 hours and the battery was dead. I recharged it with my truck and put it back in the boat. Seemed fine, but the voltage seemed to remain around 11.5V, around 12.5V running. If I had a bad cell in the battery would it affect the ignition?The stator is black and only has four wires coming from it, two yellow and two others whose colors I don't recall. I did a resistance measurement and one set was around 0.4 ohms and the other set was 600 ohms. Do these seem right? I do not have a DVM but do have a Fluke with a max reading indicator, but it probably reads RMS.Does anyone have any ideas? I am going on vacation next week and was hoping to take it along but don't want to risk damaging the engine if I can't get this issue resolved. I would shellout the bucks for a stator if I knew it was definitely the problem, but then again I don't think I will be able to get one in before vacation Thanks in advance for any help!Shawn

  • #2
    Re: Mercury Sport Jet 120XR Poor idle/missing?

    I have a 94 Bayliner Reflexx with a Sport Jet 90 and have also had some electrical problems. Just not the same as yours.The Clymer manual has some very detailed diagnostic steps to determine the cause of electric problems.The manual indicates that it should be started with WOT. I have had problems starting the engine and keeping it running. See my post about an electric priming fuel pump. http://www.iboats.com/cgi-bin/ubb/ul...;f=17;t=015611 It seems that the longer it sets between uses the harder it is to start. From what I have read, hard starting is almost normal for this engine.


    • #3
      Re: Mercury Sport Jet 120XR Poor idle/missing?

      I checked again and it is the green/white and white/green wires coming from my black stator that measure 600 ohms. According to the poor manual that I have, this is way low for a black stator and ok for a red one. Can anyone verify this?Thanks,Shawn


      • #4
        Re: Mercury Sport Jet 120XR Poor idle/missing?

        How long has it been sitting?When was last time carbs rebuilt?I know everyone blames stator for all probs on these engines, but mine was the carbs.Everytime my stator went out (twice) it just died and would never restart.Ken


        • #5
          Re: Mercury Sport Jet 120XR Poor idle/missing?

          shawn, compression test first, and then a spark check....the color on your plugs even, or is one washed off clean? the fact that you are getting almost all your speed says you can ignore the stator...water in the fuel? a cylinder getting water in it from a bad gasket?


          • #6
            Re: Mercury Sport Jet 120XR Poor idle/missing?

            The carbs have never been rebuilt, and it sat most of last year (stabilized and fogged). The engine ran like a top when I got it, and for a few weeks or more. The previous owner used it around 50 hours or so and kept it in great shape.I would almost feel bad if it wasn't the stator, I ordered one and am getting it friday. Maybe I can return it unopened if I solve the problem? Is 600 ohms a valid resistance for my black stator? It only has one ignition coil.The top three plugs are/were all tan colored. The bottom one was darker with some slight oil residue. I checked each cylinder with a spark tester at about 7/16" and got good blue spark while cranking.I am leaning toward cleaning up the carbs, though they look awful clean. Maybe I will pull them and clean them up while on vacation.I bought some dry gas and some carb cleaner (both STP) additive but read that their use was not recommended. Can't I use either of them? How do I treat a problem of water in the gas? I tend to think that it is not that since I have used at least one tank of gas and last time I filled it up I put in 93 octane.One last thing, when I first go and start it up, it idles (seemingly well) a few seconds and then stalls and doesn't start up and idle again. Is this a major symptom for something obvious?Thanks,Shawn


            • #7
              Re: Mercury Sport Jet 120XR Poor idle/missing?

              Oh, and another thing, it seems to be a fairly obvious thing when it starts running 'on all fours', kind of like a switch is being thrown on and off.Shawn