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89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

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  • 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

    Hey guys (and gals)...I'm having a bit of a problem getting my JS550 to run properly.

    The ski has been sitting in a garage for a few years. Everything seems to be well taken care of. The head had been milled and compression is right at ~150 in both cylinders. I bought a new battery and a fresh set of plugs, emptied and cleaned the tank and refilled with fresh mix and away I went.

    Problem #1

    I can get the ski to fire up but it's a rough start. It doesn't fire right up like my brothers 650. I have to primer it and lay on the throttle for a few seconds before it will fire up. Once started I have to feather the throttle to keep it running. It will not idle at all. I've tried adjusting the pilot and idle adjuster with no luck.

    Problem #2

    I haven't had the ski under load yet - just hooked up to a hose while trying to make some carb adjustments. After the ski has warmed up - sometimes I punch the throttle to hit a higher RPM and it will 'runaway'. Basically I have to prime it and flood it out for it to stop running. Pulling the plug wires won't stop it.

    I've read around a bit and it looks like crank seals are the most common culprit. However, I pulled the front flywheel cover and everything looks fine. I felt around on the rear seal and it doesn't seem to be bad either. Obviously a leak down test would tell me for sure but I'm not quite sure how to do that.

    Also - I rebuilt the carb and replaced all gaskets on the intake side just in case there was an air leak somewhere. Obviously the ski is running lean but I'm just not sure from what. I'm thinking my high mixture screw is set too lean because it only wants to run away when I hit WOT.

    Thoughts/suggestions?

  • #2
    Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

    Do a search for how to do a leak down test on a 2 stroke. I would suggest re-sealing the complete engine.
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    • #3
      Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

      Welp - performed a leak down test and the bottom would not hold pressure at all. Lots of air rushing out of the front seal. Replaced the seals and performed the test...bottom end now holds pressure.

      Put everything back together and I still have the same thing going on. Hard to start and will not idle. Backfired a few times through the carb.

      It has honestly passed every test with flying colors. Compression - 150 in both cylinders. Cylinder leak - good. Leak down test - good.

      Any ideas where to go from here? CDI? I'm stumped...

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      • #4
        Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

        Still sounds like carb issues. Go back thru the carb
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        We try to answer each question quickly and accurately.
        Please do not use Private Messaging for Tech Support, use the forums.

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        • #5
          Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

          Been there, done that. I rebuilt the carb and nada.

          I'm just befuddled at this point. The hard starting and idle issues still make me think air is getting in from somewhere. Just not sure where?

          Crank seals were replaced and case was resealed. Held pressure fine. I've replaced every gasket on the intake side so the only other place I can think of would be on the exhaust side. Every gasket looked fine, though.

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          • #6
            Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

            Quick question - the stock compression in a js550 is like 112-120 psi (can't remember exactly) and the previous owner had the head milled for higher compression.

            That's the only aftermarket modification done to this ski. I'm showing 150 psi in both cylinders. Would the carb need some adjusting or should it run on stock settings with the high compression?

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            • #7
              Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

              should be ok, but you could try opening them up a little more and let it run richer see how it reacts.
              SBT Tech Support is here to help with your problems.
              We try to answer each question quickly and accurately.
              Please do not use Private Messaging for Tech Support, use the forums.

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              • #8
                Re: 89' JS550 Engine Runaway - Crank Seals? Carb adjustment?

                Just stumbled across something today...a friend of mine is purchasing a pontoon but they were having trouble getting it to start. After a little researching I found an article talking about gas and carburators. Apparently anything with a carb on it - you should not use any kind of fuel with ethanol in the mix? The ethanol will cause the engine to run leaner and mess up your carb?

                I used 93 octane from Exxon mixed 40:1. I really hope using non-oxygenated fuel solves my problems.

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