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  1. #1
    skeletor88's Avatar
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    951 misfire after riding for 15 mins

    I have a 2001 XPL that, after years of trouble free operation, has devloped a problem.

    After riding for about 15 mins at near to wide open throttle, I slow down to talk to a fellow rider. Upon attempting to ride again, the engine begins to misfire and sometimes stop. I have found that if I keep the revs to around 4000rpm, I can get back to the launching area. During this time, the ski tends to misfire and sounds like it is detonating.

    The front piston seems to be nipping up and shows signs of this happening. The rear piston looks perfect.

    The engine has had a full rebuild and is run in (2 tanks of fuel at varying revs).

    However, during the run-in stage, I had an electrical fault and turned the tow tap on while I was towed to the beach. Unfortunately, I forgot to return the water flow, so the engine overheated, blowing the head gasket. This was replaced, all bolts retorqued and now this problem has appeared.

    The carbies (stock) were rebuilt as well.

    Does anyone know what the problem could be???


  2. #2
    KrunchovXPL-GTX-RX's Avatar
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    What is your compression right now?

    How do the plugs look in comparison?

    "The front piston seems to be nipping up and shows signs of this happening. The rear piston looks perfect."

    Does this mean that you looked at the front piston and something is wrong with it, or are you referring to the spark plug?

  3. #3
    skeletor88's Avatar
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    The compression is at 110 for the mag cylinder and 112 for the pto. I first removed the raves and noticed some slight scoring on the mag piston. Then I pulled the head and replaced the head gasket. Both pistons looked okay.

    After writing the post yesterday, i connected the hose to the water inlet line and ran the ski for about 20 mins. I noticed a slight amount of exhaust coming into the engine bay, so I have changed the rubber coupler at the stinger.

    The engine ran perfectly, though it appeared to bogg down after idling for a while. I also noticed that the low speed adjusters were out two and a half turns. These have been reset to 1 1/4 turns. I have no idea what the high speeds are set at. Does anyone know what they should be with 1.5mm over pistons and a stock class mpem?

    After running the ski yesterday, I found that the plugs did not appear to have any discolouration on them at all, save for the usual coating of oil. In other words, they were still a shiny silver. What does this mean?
    I will post a photo of them if I can.

    Thanks for your help, by the way.

  4. #4
    Quote Originally Posted by skeletor88 View Post

    After running the ski yesterday, I found that the plugs did not appear to have any discolouration on them at all, save for the usual coating of oil. In other words, they were still a shiny silver. What does this mean?
    I will post a photo of them if I can.
    The plugs will not colour up when you are running on a hose as the motor is not under load at all. Also, the compression seems to be on the low side and should be around 135 or thereabouts, possibly worth checking with a second gauge or a confirmed calibrated one.

  5. #5
    mibunkerking's Avatar
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    Quote Originally Posted by skeletor88 View Post
    The compression is at 110 for the mag cylinder and 112 for the pto. I first removed the raves and noticed some slight scoring on the mag piston. Then I pulled the head and replaced the head gasket. Both pistons looked okay.

    110 is really low for a 951. stock is 150. it could just be your gauge since they are even. are you running a forged pistons? if so, did you warm it up before you compression tested?

    After writing the post yesterday, i connected the hose to the water inlet line and ran the ski for about 20 mins. I noticed a slight amount of exhaust coming into the engine bay, so I have changed the rubber coupler at the stinger.

    that was probably your problem.

    The engine ran perfectly, though it appeared to bogg down after idling for a while. I also noticed that the low speed adjusters were out two and a half turns. These have been reset to 1 1/4 turns. I have no idea what the high speeds are set at.

    it would be a good thing to know where the HS is set to and what jetting you have.

    Does anyone know what they should be with 1.5mm over pistons and a stock class mpem?

    you have to tune it in based on mods and plug chops. normally you will be 0 to 1/2 turn out on a stock boat.

    After running the ski yesterday, I found that the plugs did not appear to have any discolouration on them at all, save for the usual coating of oil. In other words, they were still a shiny silver. What does this mean?
    I will post a photo of them if I can.

    a photo will help alot. you need to run it underload for about 30mins if they are new plugs to get some color on them. then you want a brown color on the porclein. any hint of a metalic silver on the porclein or electrodes means your seizeing.

    Thanks for your help, by the way.
    .

  6. #6
    flyin' the friendly skies airbornexp's Avatar
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    since this engine has back fired have you checked to see if your reeds are still intact or broken?

  7. #7
    Quote Originally Posted by skeletor88 View Post

    After running the ski yesterday, I found that the plugs did not appear to have any discolouration on them at all, save for the usual coating of oil. In other words, they were still a shiny silver. What does this mean?
    Sounds like it's running lean Check the main filter and both carb filters for black junk. A good carb cleaning and rebuild kit might be wise.

    Good Luck

  8. #8
    96XPSS's Avatar
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    Also remove your fuel selector switch and clean it. Mine was completely plugged at the beginning of last summer, and had already been replaced once by the previous owner. (2000 XP)

    Silver is the aluminum melting off the piston and depositing itself on the spark plug.

  9. #9
    KrunchovXPL-GTX-RX's Avatar
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    HS screws are closed with a stock air box.

    .5 to 1 turn with Prok Flame Arrestors.

    If you ran it on the hose for 20 minutes you may want to go ahead and replace the carbon seal on the drive shaft. They are hot enough to raise a blister after just five minutes so I would hate to see what four times that long a run did to it.

    When you read plugs you are not looking at the metal part of the plug (well, you are to some extent, but that is not where the color is going to be). The white insulator around the ground electrode is where you will see the color.

    You only read plugs off of WOT. If it idles at all the results are useless. That is why you must do plug chops in order to read the plugs.

    If you have a stock air box and jetting there is no reason to mess with the HS screws or be chopping plugs. The over-bore will not effect the carb settings like intake and pipe mods will.

  10. #10
    skeletor88's Avatar
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    Thank you all for your replies. Yes the carbon seal is in need of a change anyway. I have replaced the exhaust couplings from front to back (I am running a through hull setup now). The rear crossover coupling was destroyed only leaving a finger width of channel to blow through.

    I am also running a r&d air filter. I noticed today that after a couple of good revs (on the hose), the engine idles at around 3800 -4000rpm for about 5-7 seconds before returning to the correct idle. Is this a concern?

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