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  1. #1

    STX-R, '04, hits 69 mph, soon starts to surge, gets worse.

    Working on a good bud's '04 STX-R that had been stored inside and dry for a good while, at least four years.

    I have made progress: it will haul booty, fine as you please up to a respectable 69mph (indicated) but after a little while -- say 30 seconds -- begins to surge. The surging is at first barely discernible, but within a few moments, gets worse and worse until it should be called bucking -- almost. If one were to graph the surging/bucking it would be a sine wave of increasing amplitude but somewhat steady frequency. Never a square wave.

    The surging is somewhat delayed if one starts, applies the throttle more and more until mid range -- say 40mph -- is reached, but the surging begins and continues much as above.

    If I slow to idle, then take off again, either to full or mid, the surging begins almost immediately.

    If I let the boat sit for two or so minutes, engine off, then the first scenarios recur: great running, followed by the surge scenario.

    No warning light, no indication on the dash that anything is wrong. I don't think it's going into any limp mode, cuz is it not true that the 'warning' light always accompanies the limp mode. (?)

    Here's what I've done so far:

    1) ensured that the fuel supply and return hoses are unrestricted to the fuel tank, including the path through the res/on/off valve -- in fact, the fuel will siphon out of the supply hose into a container on my shop floor very freely, and does so in both the 'on' and 'res' setting;

    2) Ensured that the tank vent is not a factor by taking the cap off, running the 'ski with no change in symptoms;

    3) removed the carburetors (they come off as an assembly) inspected, removed the R& R'd each jet and each needle seat and ensured that the screen attached to the needle seat was clear, and each was;

    4) Inspected the carburetor diaphragms (both top and bottom) inspected the check valves in each carb, as well as the float arm etc. and nothing was gummed or visibly out of spec;

    5) Removed the float arms (which aren't really float arms) from each carburetor, and removed the fuel pump from the carburetor assy, and ensured that the supply and return lines that are integral with the carb assembly were not clogged;

    6) drained all stale gasoline and replaced with 93 octane.

    QUESTIONS:

    Is there any way to test the fuel pump? it's $175 and reputedly very low failure rate.

    What am I missing?

    Note, I addressed a different problem with this ski here http://www.greenhulk.net/forums/showthread.php?t=236411 but since I screwed up the model, began this thread.


  2. #2
    When was the fuel filter last replaced?

  3. #3
    Probably never, but it flows fuel very freely, and when pressurized, it doesn't leak. In the siphon exercise I mentioned above, the flow was through the filter.

    Keep 'em coming, I was hoping you'd lend your expertise to my issue.

    BTW, this is a gorgous 36 hr ski

  4. #4
    steve45's Avatar
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    If it had stale gas in it, I would look at it very closely, and backflush it if you see anything in it. I would also pull the fuel pickup from the tank and check condition of the screens. In fact, I'd take them off and put pieces of rubber fuel hose on in their place. No reason to have two filters in the fuel system, especially when the ones in the tank are so hard to get to.

    I would also ride it with the seat off. You might have a small exhaust leak that's putting fumes in the hull, or you might have a water leak that's spraying water into the flame arrestor. The driveshaft coupling will spray water everywhere.

  5. #5
    I'm real sure that the gas tank screens are not an issue, given the free flowing fuel siphoning into an Early Times bottle on the floor through the tank filter and the in line, tho I do like your suggestion re just one filter -- for the future.

    But that exhaust leak issue is real fascinating. I have heard of that on skis.

    If I ride it w/o a seat how do I avoid an interface between the drive shaft and my family, uh, . . . well, what would I sit on?

    Could I accomplish the same thing by propping the front bonnet open w/o the big ole bucket n place?

    I'm also thinking of taking the glove box out, running a hose to the input side of the fuel pump, with the other end in my Early Times bottle full of gas such to eliminate all of the tank and associated crapola. Comment? Do you wanna watch?

  6. #6
    These ski do pull alot of fuel, even the free flow test that is not nearly enough to prove anything. I have had issues with my ultra fuel filter was clogged and it would do exactly how you described. Stumble bog ex.

    When steve said ride with thr seat off that means you only, he wants you to pay attention to what the engine is actually doing, if there is an exhaust leak you will see it and the ski will run great with the seat off. Its a diagnostic.


    Your just getting to crazy with this, lets go back to the basics. Spark fuel and compression, have you checked the compression? New set of plugs? Have you looked at the exhaust hoses and make sure none have collapsed? Change the fuel filter, disassemble the fuel pump and make sure the diaphragms is not torn.

  7. #7
    Quote Originally Posted by turboman412 View Post
    disassemble the fuel pump and make sure the diaphragms is not torn.
    Several good points in your post, but I was a bit shy to deal with the pump since the service manual cautions against taking it apart. I was worried it might be difficult to get back together w/o special tools or some such, no? If it's a 'straight forward' deal, it's next.

    Cuz from the very beginning I've suspected the pump.

    Would you, as I have, more or less ruled out the ECU, and associated electronics?

    And thanks for your on point and helpful comments. I do appreciate 'em. BTW, I did understand Steve's suggestions too, and take 'em very seriously, and gonna try the ski w/o the seat, but gotta tell you, it's gonna be an exciting ride. This thing hauls booty, until it doesn't.

  8. #8
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    in regards to the fuel pump, yes a low fail rate unless somebody has cracked the case open. There are no repair parts for that pump and it is pricey

    You can measure pump output by putting the return hose in a safe container and run the engine, but this doesn't seem to be the issue.

    I'd be doing a leak down test.

    The ski sat for a few years, likely it's got a seal leak. That's my best swag

    you can easily check for an exhaust leak by having a help ready to plug the exhaust outlet while you are running the engine at 3k rpm and spraying fogging oil..once it's smoking good..plug the exuast. Any leaks are kinda hard to miss. Needless to say this test has to be done outdoors.

    keep in mind the ski is 11 years old..the carb diaphragms may have lost flexibility

    these problems are difficult to find unless you have a way of running the engine under load while you can fiddle with the engine.

    I use this:

    Click image for larger version. 

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    I'd do a pressure test first. You need to get that out of the way before spending time and money on other things.

  9. #9
    steve45's Avatar
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    I doubt that it's an electrical problem. If it runs strong at first, it's probably OK.

    I've never taken that particular fuel pump apart, but I doubt that it requires special tools. They don't sell parts for it.

    As turboman said, that engine will suck more fuel than siphon action will draw. You said it's been sitting for 4 years, the fuel system would be the logical place to look.

    Look at the photos of the fuel gauge sender on page 3 of this thread: http://www.greenhulk.net/forums/showthread.php?t=235770 This is why you need to pull the pickups and look at them.

  10. #10
    Quote Originally Posted by nmpeter View Post
    in regards to the fuel pump, yes a low fail rate unless somebody has cracked the case open. There are no repair parts for that pump and it is pricey

    You can measure pump output by putting the return hose in a safe container and run the engine, but this doesn't seem to be the issue.

    I'd be doing a leak down test.

    The ski sat for a few years, likely it's got a seal leak. That's my best swag

    you can easily check for an exhaust leak by having a help ready to plug the exhaust outlet while you are running the engine at 3k rpm and spraying fogging oil..once it's smoking good..plug the exuast. Any leaks are kinda hard to miss. Needless to say this test has to be done outdoors.

    keep in mind the ski is 11 years old..the carb diaphragms may have lost flexibility

    these problems are difficult to find unless you have a way of running the engine under load while you can fiddle with the engine.

    I use this:

    Click image for larger version. 

Name:	tanked.jpg 
Views:	39 
Size:	61.5 KB 
ID:	366127

    I'd do a pressure test first. You need to get that out of the way before spending time and money on other things.

    Thank you Pete: That is an enviable setup. The worst part about working on watercraft of any kind, especially two strokes, is what works on the hose may not work on the lake. And it's generally a long trip to the lake! (and I wish it weren't so darn far to New Mexico)

    My naiveté suggest that I ask for some clarification:

    You mention "seals" "pressure test" and "Leak down test" Do each of these refer to testing the integrity of the crankcase seals?

    One more question: I'm thinking that there's not much need to snoop around for an exhaust leak if the surging is present when I'm riding w/o the seat, yes . . . ?

    Incidentally, I inadvertently left out one detail in my initial post: When I first put the ski in the water, it ran flawlessly for about 20 or thirty minutes, and was scary fast. The surging developed only after several scoots of about three miles each.
    Last edited by Artie; 07-22-2015 at 05:15 AM.

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