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  1. #1

    STX900 revs dropping

    Hi I have a stx900 (year 2000, I think) at my holiday home. I have been having issues for a number of months now. Full revs is around 8000, top speed shows as 54mph. It will work fine for a variable amount of time sometimes 15 min sometimes 2 min then the speed will drop to 34mph and 5400rpm. It will sometimes come back to life for a while on its own usually I have to get it out of the sea and back on the trailer. When back on the driveway it will usually be perfect and back to full rpm. On one occasion I continued to run it at the slower speed for a while, about 20min, then after a change of rider it would only do 6mph 3000rpm.

    The power does seem to drop in 1/3 increments.

    I don't have the time or expertise to do much work on it myself and am relying on a local motorbike garage that don't speak any English. They do regularly carry out Jet ski work.

    The engine was rebuilt and re bored after a piston failure in 2015 it worked fine for a while after that. I was advised that the failed front piston may not have been getting any oil so as a precaution add some 2 stroke to the fuel tank in addition to the on board oil tank which I do. 1/2 to 2/3 of a litre when refilling the tank. Any opinions on the validity of this advice also appreciated.

    Earlier in the year they gave it a full service including new parts in the Carb (I don't know what parts due to the language barrier with the garage)
    It had new plugs then too.
    I replaced the HT leads with new on our last visit.
    It is mainly new fuel this season 98 octane.
    The garage then cleaned the carbs again and fitted a (aftermarket) repair kit to the fuel pump. Again it worked for 5 minutes or so and then dropped to 5400rppm.
    I have left it to cool with the seat off for 30 min as I suspected an overheating issue but that made no difference it still re started at max 3500rpm as a max.

    Any advice on what to try next would be greatly appreciated before I go back in October.

    I am reluctant to purchase a new fuel pump as the repair kit made no difference and I wouldn't expect a failing pump to produce those symptoms?

    I have read in the FAQ's about the fuel tank vent which I will check when I get back but again if it was restricted I wouldn't be able to run at full speed for 15 min and the cool down time I gave it once should have let enough air back in to restart at full rpm.

    Thanks for any help.
    Mark


  2. #2
    steve45's Avatar
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    Sounds to me like one of the carburetor slides isn't opening correctly. You might remove the flame arrestor and try riding it briefly with the seat off and see if they move, or see if there is anything else going wrong.

  3. #3
    Rodsatheart's Avatar
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    Did anyone address the oil starvation issue ? Has anyone pulled the fuel supply assembly and checked the pick up screens.

  4. #4
    Hi oil starvation was just his best guess at what caused it. He was a marine engine specialist not the guy I am dealing with now. Can you test or see if each is getting oil?

    I had to get back to land on 2 cylinders so it was wrecked by the time I made it back.

    I checked the fuel filter and that was very clean suspect he changed it or cleaned it at the service. I am not sure what you mean by pulling the fuel supply or checking the pickup screens and I wouldn't be able to tell you if they had done it if I understood. If you can give me more info on those I can check myself in October.

  5. #5
    Thanks I might have a go at that although it sounds a bit risky. Why do you say briefly what are the risks?

  6. #6
    steve45's Avatar
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    Quote Originally Posted by mstow View Post
    Thanks I might have a go at that although it sounds a bit risky. Why do you say briefly what are the risks?
    It's not risky, just don't do it unless the water is smooth and be careful of water spray during turns. The flame arrestor is required for proper fuel metering on CV carbureted engine, such as yours. Prolonged running without the flame arrestor will cause it to run excessively lean and could cause major damage.

  7. #7

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    Check for an exhaust leak into the hull.

    Ride it with the seat off. If the problem goes away, it's an exhaust leak.

  8. #8
    Rodsatheart's Avatar
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    If the injection lines are fairly new, they should be either clear poly line or a colored Tygon line, either way you should be able to see they are full of oil and no air bubbles present.
    The pick up is part of the assemble on the tank that was the selector cable running to it, and fuel line coming out of it. Has anyone done a compression check yet to rule that out ?
    If your adding 3/4 of a liter to 10 gallons of fuel, your mixing the tank at 50:1 which is putting your overall mix at 25:1 or richer.
    You should only be adding 1-1.5 oz's of oil per gallon to the tank. That will put the overall mix at 32:1 which is what I use for new engines. Anymore than that is excessive, to much can cause pooling on the pistons. Try pulling the plug caps off, trimming about 1/2 inch off the leads, and reinstall the plug caps. It is also possible that the plug leads are failing and need to be replaced. Has anyone check to make sure that each cyl. is getting consistent spark ? This can be checked by using a timing light. Just attach the positive and negative clips to the battery, and the big black clamp looking clip to each plug wire (wire goes in the U shaped section) while it is running, pull the trigger, and it should flash. If it doesn't, there's no power going through that lead.

  9. #9
    Hi All thanks for the replies. Carb was corded and seals damaged. My Marine engineer replaced the whole carb with a s/h one from southern Cyprus, it was better I got about 10 min at full speed then revs dropped again. New parts fitted in fuel pump. Leads were replaced with new last year, new plugs several times. It was usable just not right. Then the engine blew up again in May, crank shaft was damaged this time. Engine and crank rebuilt by a different Marine engineer. Oil pump now disconnected mixing oil with fuel as before (thanks for the Ratio info Rodsatheart) . New engine is OK, compression tested but it didn't run well. 2/3 speed at best. Middle plug was oiled up other 2 OK swapped plugs around and middle one still very black. So I think we are back to Carb issue. 2nd engineer has just had the Carb to bits and has requested the parts shown. My UK supplier is saying Kawasaki never made them and the CV carb is shit anyway unreliable and difficult to set up. He has recommended replacing the CV carb with a keihin cdk11 from an earlier model. I am struggling to find one on ebay. Any comments or further advice appreciated before I throw more money at this Ski. I also could do with knowing if there is a way to permanently clear the oil warning light (other than filling the oil tank with water.) as that is going to mask any other warnings.

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  10. #10
    steve45's Avatar
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    I've never really had much trouble with the CV carbs. They need to be clean, and a lot of people apparently don't do a good job of cleaning them. The fuel rails need to be cleaned, not just the carbs themselves. Looks like your diaphragm cover has seen better days, along with the O-ring. Don't forget to clean the internal strainers, too.

    What part was your supplier saying was not available from Kawasaki?

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