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  1. #1

    STX-R, 2003 Still surging

    Many of you were kind enough to respond to my issue ten or so days ago. This ski has again landed in my shop, and I am working on it because I want a long time close friend -- the owner -- to join me and my buds on occasional 'ski adventures. The dealer to which he took the boat appears from all indicators utterly incompetent, saying that the middle cylinder lacks compression. When asked how come the boat is able to start easily, idle w/o issue,and hit 70 mph with bad compression, he says that the boat can easily do 70 or two cylinders because it has a "governor on it and would easily do 100 mph with all three cylinders working, were it not for the governor." I, of course, called BS, and the 'ski is back in my shop. My own compression check shows no material difference among the three cylinders. But I must ask, how much should it have -- throttle wide open, all plugs out. I never had much confidence in two stroke compression checks -- except to show difference among the cylinders -- but maybe it's time to change my ways.

    Crank the 'ski up in the water, accelerate normally, it will effortlessly bust up to a speed just short of 70mph. Stay there for 45 seconds, or so, and then begin to surge. Not to buck, but to surge. The surging does get worse to the point that it is just short of a buck. A "buck" to me is as if you were hitting the kill switch intermittently. It is not like that. It's much more gentle, and gets worse. It is most assuredly all three cylinders. It's not just one cylinder cutting out.

    I have been completely through the carbs and found nothing amiss. The fuel supply seems sure, the strainers in the tank have been removed and (on the good advice of this list) the 'ski now has only the replicable in-line fuel filter.

    In my earlier two threads on this issue, folks mentioned a leak down test. I am familiar with a four stroke leakdown test which checks the sealing capacity of the rings and the valves. Sorry to say that I do not know what a "leak down" test on a two stroke is designed to show.

    Are we looking to test the integrity of the crankcase seals, or what? My limited searching skills answer neither the 'why' nor 'how' questions.

    Any help much appreciated. This ski seems so close yet so far away. It will haul booty but only for a while.


  2. #2
    Yes to test integrity of case seal and crank seals, but honestly do not feel that is your issue, im starting to lean towards something electrical. But for shits and gigs have you inspected the rubber exhaust hoses?

  3. #3
    steve45's Avatar
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    I think it's a fuel supply problem. Ride it with the seat off and watch the fuel filter (I assume it has a clear filter). It's normal for the fuel filter to be less than full. Actually, I think I'd try putting in a piece of clear fuel hose and watching that. It will give you a better view. I suspect you might find some bubbles forming from a leak in the system somewhere.

  4. #4
    SplishSplash's Avatar
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    Artie, is this machine stock or are there modification like aftermarket flame arrestor or modified intake manifolds?
    I think you mentioned you tried a run with the fuel cap loose to verify the check valve was OK.
    At this age, the fuel pump diaphragm can fail, but there is no service kit, just a new fuel pump.

    I know this surging is an issue when the stock airbox and carbs are tampered with.
    Have you checked the jet pump bearings. Maybe the are bad bad getting hot after wot for a while and loading the engine.

    Sure there are no exhaust leaks into the hull? Try a run with the seat off

  5. #5

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    I'd try keeping a lower speed....if the ski can continually hold say 55 mph I would then assume it's starving for fuel at a higher rpm.

  6. #6

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    If it's really starting to boggle you, you could fashion a quick temporary inline fuel pump with a switch. ...if it surges and then stops with the pump on then go after the fuel system. ....just my two cents.

  7. #7
    Quote Originally Posted by Jamescmei View Post
    . ....just my two cents.
    Y'all's (pronounced yalz-ez)two cents are precisely what I hope to get, and for which I thank each of you.

    The ski is box stock. Once it begins its surging, it will surge, worse and worse, at any speed; and as the surge increases, the speed, of course, decays. Have not checked the interior of the exhaust hoses, but gonna do so. On the issue of drive section bearings, there is no noise coming from that area as one might expect, but that doesn't take those bearings off the table. Just makes it slightly unlikely that they are my issue.

    I have run the 'ski w/o a seat w/o a change in symptoms. Today it occurred to me to run a long plastic hose from the input to the fuel pump, with the other end dangling down in the fuel tank, below the level of the fuel. The fuel pump pulls a decent vacuum (for a fuel pump) but I have not measured it. Think I will.

    This ski will accelerate like nobody's bidness. Impossible for me to believe that it has a mechanical issue, such as reduced compression.

    Has any of you authored a step-by-step "pressure test" (or was it leak down test?) that is designed to check crankcase seals? I too doubt that as an issue, but I'm running out of other items. In all my two stroke experience I have encountered crank seal issues only on a supremely half-assed Yankee 500 that was (more or less) two Montessa 250 engines cobbled together to make a two stroke, two cylinder dirt bike. It's shortcomings varied with the square of the number of cylinders present, and it had a habit of consuming its own transmission juice. Which, of course, was 90wt.

  8. #8
    Let me out of here......I need to go on Greenhulk!
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    A leak down is on a 4 stroke to check rings and valves. A pressure test checks for a airtight bottom end on a 2 stroke. Bottom end on a 4 stroke is not pressurized during operation like a 2 stroke. I would think a crank seal would present like a vacuum leak.

    To pressure test the bottom you can sandwich inner tube between the exhaust manifold and cylinders and between the intake mani and case. Block off any other holes that could leak and add air to around 7 pounds. I think you have to take the front oil fill plug out

  9. #9
    yup, I have a leak down test device for four strokes. So the correct term for two strokes is "Pressure test" and essentially what you do is to stop off both intake and exhaust, pressurize the crankcase, and hope that it holds 7psi, and if it doesn't crankcase seal(s) are the likely leak. -- got it and thanks.

    Assuming one would add air to a single crankcase at the time, yes?

    Any convenient port on each of the crankcases to add the 7psi?

    Lastly, this procedure looks like quite a bit of work so let me ask this question: Would the surging that I'm experiencing be a likely symptom of leaking crankcase seals? And if yes, does not easy starting and normal idling suggest elimination of the crankcase seals as the culprit? (my asking this question is not intended to be argumentative -- it's designed to avoid hard work for which I've always had a keen dislike)

  10. #10
    Time for me to eat a huge helping of CROW!

    I checked the compression again today on this 'ski and found ~70# on numbers one and three, but number two would barely bump the needle. Either something happened since I checked the compression, or -- much more likely -- I somehow didn't check the middle cylinder. I've been checking the compression on engines since the Eisenhower administration, but nonetheless f'd this one up.

    My bud is on his way back to the Kawasaki shop. He too has some crow to eat. Perhaps lots of it.

    Much thanks to those who pitched in, and apologies to those who squandered some brain power trying to help me out.

    How the heck zero compression on one cylinder could cause the surging is beyond me, but I'm ready to learn a lesson. Another lesson, actually.

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