stroke twin issue

Discussion in '2 smokers' started by kellymac530, Oct 10, 2012.

  1. baloneyskin daddy

    baloneyskin daddy bikaholic

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    Are you absolutely sure the pistons are in correctly, not backwards, and are the correct ones. It sounds like port timing is way off. Its also possible the crank is indexed wrong and the timing is off. Is it a new or rebuilt crank. 2 Strokes are so simple it almost has to be something stupid.
    #21
  2. kellymac530

    kellymac530 motorcycle addict

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    Yes I am sure that the pistons are the rite ones, for a piston port, and I matched them up next to the OE pistons to make sure they are the same port styles. I have pulled the head and barrels to double check that they are oriented correctly.
    Arrows point towards exhaust, and looking thru the intake and exhaust manifolds the ports line up propperly.

    As far as timing, a light is really hard on a jetski because the magnetto is inside a sealed case on the front of the motor, I can see it now since I have been running it with the cover off, but I do not see any timing marks off hand, I will look better. It is a magnetto and not a distributor so it only fires the plugs while the motor is turning quickly enough to generate spark.

    The spark trigger plate only has a few degrees of adjustment, maybe 15* total. I have tried it at every increment possible, all very similar results.

    As far as coils, I have swapped out the ignition from another known running ski and no change. I have swapped out all of the ignition components to no avail. Put the old ones on the other ski and it is fine. I am losing it. Never been stumped like this.

    I really appreciate the help though and I do go and verify all replys just to make sure...
    Sure feels like it is something simple or dumb.
    #22
  3. baloneyskin daddy

    baloneyskin daddy bikaholic

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    I know its a rebuilt crank but it might be a bad crank seal, the one between the 2 cylinders. you'll have to pressurize one cyl and see if air leaks into the other.
    #23
  4. kellymac530

    kellymac530 motorcycle addict

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    I think that is my next test then.
    Thanks a ton Baloney and everyone else.

    Any more help or ideas are still very welcome.
    #24
  5. baloneyskin daddy

    baloneyskin daddy bikaholic

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    Any news?
    #25
  6. kellymac530

    kellymac530 motorcycle addict

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    It held air in all test...pissed me off so I stripped it down, easy on a Jet Ski motor, no trans, and it looks like the bore may have been too tight. They are Hyperutectic pistons and had a coating on intake side and exhaust side that was probably a few thousands thick. I think they bored it to the piston top measurement and did not account for the coating thickness.

    This has now worn off almost totally and scored the cylinder walls badly and the pistons are all scored like a dry seizure looks when you run a 2T without oil. It dropped to about 90 psi compression before I stripped it so I am guessing that may be adding to it.

    I am going to try honing it and see if I can get out the scratches and then gently sand the scratches out of the pistons and check clearances, if decent I will reassemble and see if that runs. If that helps I guess I can bore it over and get new slugs and try again.

    I do have one question, the new pistons are the same height, shape, skirts everything, but there are 2 small holes on the exhaust side of the pistons just under the rings that go thru to the inside of the piston. The old pistons do not have these. I am curious if that is just a modification from the high performance piston manufacturer or if that could be part of the problem here bleeding off too much bottom end pressure??????

    Maybe the pistons are for a different year...they are deffinately labeled for this year motor, and not for the reed valve model 550.

    One more stupid mystery...I am going to call the manufacturer and ask....been really busy with a kitchen remodel for the little woman...that will buy me some garage credit....lol
    #26
  7. baloneyskin daddy

    baloneyskin daddy bikaholic

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    I can't imagine holes below the rings at the exhaust port affecting anything, probably allow a little extra oil at a notorious hotspot. Rings with insufficient end gap will scar cylinder walls also.
    #27
  8. kellymac530

    kellymac530 motorcycle addict

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    Thanks Baloney,
    I will check that before reassembly.
    I do not think tight rings would eat the hyperutectic patches on the pistons, but I guess if they scored the cylinders then the scoring could grind up the patches.

    I thought that the holes might just be a lube vent for better performance/cooling/modification.
    #28
  9. ragtoplvr

    ragtoplvr Long timer

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    Is there a time when the holes are open to the exhaust port. If so then this is an issue.
    #29
  10. kellymac530

    kellymac530 motorcycle addict

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    Yes they are open to the exhaust port. They are immediately under the ring. They are about 1/4" or less in diameter and are on the exhaust side, at least according to the arrow on top of the pistons which is supposed to point the direction of flow....at least it has on every other ski I have built...That would suck if that was the entire answer after all of this time.
    #30
  11. baloneyskin daddy

    baloneyskin daddy bikaholic

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    A1/4 inch hole! hell I thought you were talkin about a pin hole fer chris sake. Yea that'll bleed off alot of air -fuel mixture thats supposed to come up thru the transfer ports to the top of the pistons. I don't think I've seen this before but then I don't mess with jet skis .I've seen these holes but lower on the skirt on the intake side on many pistons though.
    #31