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  1. #1
    Why not? EthanYQX's Avatar
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    Question for the gearheads here

    My neighbour's snowmobile, an 800cc two-stroke with a carb, recently had a new head and forged pistons installed, and the cylinders ported. This thing had ridiculously high compression, to the point that it was virtually impossible to pull over.

    Well after the break-in period, it stopped dead on a lake. A piece was gone out of the piston, out of the edge so it was no longer round. I'm thinking it was filled with the wrong fuel, i.e too low of an octane, and the thing detonated. What do you guys think?
    "It is not the critic who counts."

  2. #2
    Why not? EthanYQX's Avatar
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    Should probably mention he's pretty hard on it, usually wide open or stopped.
    "It is not the critic who counts."

  3. #3
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    My gut says that the carb was never rejetted and as a result the motor was running lean on the main. Chances are good that the metal missing off the piston is in the area of the exhaust port.

  4. #4
    Why not? EthanYQX's Avatar
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    Quote Originally Posted by Stacey View Post
    My gut says that the carb was never rejetted and as a result the motor was running lean on the main. Chances are good that the metal missing off the piston is in the area of the exhaust port.
    Yep. Exhaust side. I would think if the carb was not rejetted, it would have detonated sooner, especially with the ridiculous-high compression, no? Correct me if I'm wrong. Is it possible they just got the wrong octane gas somewhere?
    "It is not the critic who counts."

  5. #5
    The Site Administrator: Currently at home recovering from a couple of strokes,please contact my assistnt admins for forum issues Tom Stormcrowe's Avatar
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    Does it run premix or have an oil injector system?

    If premix, another guess was not enough oil in the fuel mix and if injected, the oil injector may have fouled or partially plugged. Too lean an air fuel mix will do that AND not enough oil/fuel mix will also do that w/ a 2 cycle.
    on light duty due to illness; please contact my assistants for forum issues. They are Siu Blue Wind, or CbadRider or the other 3 star folk. I am currently at home recovering from a couple of strokes. I am making good progress, happily.


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  6. #6
    don't misunderestimate me BoSoxYacht's Avatar
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    Quote Originally Posted by CrankshaftYQX View Post
    Yep. Exhaust side. I would think if the carb was not rejetted, it would have detonated sooner, especially with the ridiculous-high compression, no? Correct me if I'm wrong. Is it possible they just got the wrong octane gas somewhere?
    I think you could be correct. Without knowing the octane of the fuel in question, and the compression ratio, it's hard to say for sure.

  7. #7
    Why not? EthanYQX's Avatar
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    It should run premium. 91 octane at least. That's all I know.

    Tom, oil injected.

    What's odd, and what I just found out, is that he ran for three hours on the same tank before it exploded. Clog in a carb jet?
    "It is not the critic who counts."

  8. #8
    The Site Administrator: Currently at home recovering from a couple of strokes,please contact my assistnt admins for forum issues Tom Stormcrowe's Avatar
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    Very, very possible!

    Quote Originally Posted by CrankshaftYQX View Post
    It should run premium. 91 octane at least. That's all I know.

    Tom, oil injected.

    What's odd, and what I just found out, is that he ran for three hours on the same tank before it exploded. Clog in a carb jet?
    on light duty due to illness; please contact my assistants for forum issues. They are Siu Blue Wind, or CbadRider or the other 3 star folk. I am currently at home recovering from a couple of strokes. I am making good progress, happily.


    . “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche

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  9. #9
    Why not? EthanYQX's Avatar
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    From what I understand, a high compression engine has very little tolerance for running lean.

    Also, it wouldn't run on one cylinder. Shouldn't they compress seperately?
    "It is not the critic who counts."

  10. #10
    don't misunderestimate me BoSoxYacht's Avatar
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    Quote Originally Posted by CrankshaftYQX View Post
    From what I understand, a high compression engine has very little tolerance for running lean.
    correct

    Quote Originally Posted by CrankshaftYQX View Post
    Also, it wouldn't run on one cylinder. Shouldn't they compress seperately?
    .
    I have zero experience with snowmobiles, but the outboards I've worked on will not run on 1 cylinder.

  11. #11
    ♋ ☮♂ ☭ ☯ -=(8)=-'s Avatar
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    The compression can make a ring hang up on a port , too...
    Is the ring chunked ? The edge ripped off the top of the port ?

    Is it an ATK motor
    -ADVOCACY-☜ Radical VC = Car people on bikes. Just say "NO"

  12. #12
    You Know!? For Kids! jsharr's Avatar
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    I say the snow genies cast a spell on it.
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    Quote Originally Posted by colorider View Post
    Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.

  13. #13
    Why not? EthanYQX's Avatar
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    Quote Originally Posted by -=Łem in Pa=- View Post
    The compression can make a ring hang up on a port , too...
    Is the ring chunked ? The edge ripped off the top of the port ?

    Is it an ATK motor
    Nah, just the piston screwed and the cylinder wall scored, presumably thanks to debris.
    "It is not the critic who counts."

  14. #14
    Why not? EthanYQX's Avatar
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    Quote Originally Posted by BoSoxYacht View Post
    I have zero experience with snowmobiles, but the outboards I've worked on will not run on 1 cylinder.
    Why is that? Do the cylinders have to compress together to run, or are they totally isolated?
    "It is not the critic who counts."

  15. #15
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    Probably because the flywheel isn't heavy enough to support a 360 degree ignition cycle.

  16. #16
    Senior Member no motor?'s Avatar
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    Quote Originally Posted by CrankshaftYQX View Post
    Why is that? Do the cylinders have to compress together to run, or are they totally isolated?
    They're connected, and the higher compression makes it hard to run on just one cylinder. As would the other changes they probably made to increase the performance.

  17. #17
    Why not? EthanYQX's Avatar
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    Quote Originally Posted by Stacey View Post
    Probably because the flywheel isn't heavy enough to support a 360 degree ignition cycle.
    No flywheel, Stace. Snowmobiles don't have transmissions, just clutches and sprockets.
    "It is not the critic who counts."

  18. #18
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    Trust me... They do have a flywheel. Usually it's disguised as part of the (magneto) ignition system.

    They have transmissions too. CVT's, more commonly referred to as torque converters.

  19. #19
    Why not? EthanYQX's Avatar
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    Where would I find the flywheel?

    And yeah, I guess the torque converter is a transmission...
    "It is not the critic who counts."

  20. #20
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    The end of the crankshaft. As either part of that torque converter or as I mentioned the magneto/alternator.

    What kind of sled... specifically?

  21. #21
    Why not? EthanYQX's Avatar
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    2006/7 Ski Doo MX-Z X 800, on the Rev chassis. Has a higher-compression head and some clutch work.

    Drive looks like this: Motor drives one clutch, that drives the belt, which makes a second clutch move, which drives the jackshaft (runs sideways across the engine, has the brake on it) to a sprocket, bathed in oil in the chaincase, which spins the chain, driving the driven sprocket, which drives the track. As near as I can tell, that's it. That's what I can see in my 600, and his looks pretty much the same.
    Last edited by EthanYQX; 01-13-08 at 01:28 PM.
    "It is not the critic who counts."

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