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How To Tell If My Sled Is Detonating/Pre-Igniting?

This is a discussion on How To Tell If My Sled Is Detonating/Pre-Igniting? within the The Performance Tuning Shop forums, part of the Snowmobiles category; I just rebuilt my sled with 73MM (+1MM/+.040) and I had to replace the head also, The head I purchased used has .2MM (.007) less ...

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Thread: How To Tell If My Sled Is Detonating/Pre-Igniting?

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    I just rebuilt my sled with 73MM (+1MM/+.040) and I had to replace the head also, The head I purchased used has .2MM (.007) less material between the domes and the gasket surface. (Apperas that .007 has been shaves off, the owner of the head insists that its stock but either way).. I am a little woried about something going wrong with this setup. I do not want anything to happen. So what do I need to worry about with this head and what do I need to watch?

    Thanks


    M[color=red]oved from General. Welcome to the forum! [8D] ACM[/color=red]

  2. #2
    SnoWhore
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    Sledneck26's Avatar
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    knock and ping. and your DET sensor. thats how you tell. umm. if it does ping and knock going to a higher octane will prevent it becuase it wont explode under load as easily. when you run your sled you will most of the time distinctivly hear the knocking and pinging. Its when the gas in the cylinder is compressed and ingnites before the spark plug sparks creating 2 expolsions instead of one sometimes. if you go to a higher octane, if you run 92, buy octane booster or buy vp 95 fuel.

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    Octane's Avatar
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    You cant hear a 2-stroke pinging. Its not like if you put junk gas in your car or truck and can actually hear it detonating.
    A lot of times, when a 2-stroke is getting preignition, the only telltale sign will be a blown engine.

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    thats what I was worried about.

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    Chassis durability tester
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    shortstop20's Avatar
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    What kind of sled? Your main tool to judge your air/fuel ratio should be your spark plugs.

    Once you have a sled's jetting setup correctly, you should be able to notice if the jetting was suddenly lean. It will be much more of a surging raise in RPM's, not a smooth increase. In leman's terms the motor will just sound plain sick. The same is true for a sled that is jetted too rich.

    I just went out riding yesterday, sled felt lean right off engagement, I never even checked my plugs to verify that it was lean at that throttle position, I could just tell it was lean. I flipped to 1/2 choke, hit the throttle and it had much smoother acceleration at the same throttle setting. If the sled runs better with the choke on while you're riding it, then the mixture is lean. If it runs worse when you put the choke on then it's rich. This is one of the many tricks you can learn by reading around on the Internet and experimenting while riding. I would suggest you buy Olav Aaen's carb tuning handbook. There is alot of useful information in there. His clutch tuning handbooks is great too. They are $25 each. There are several things that effect the air/fuel ratio, main jet size is only one part of the equation. Pilot jet, needle clip, fuel/air screw settings, etc. are often overlooked but are just as important to have smooth operation from idle to wide open throttle. Aaen's carb tuning book explains all of this.

    Another thing to keep in mind is that Detonation and Pre-ignition are not the same thing. Search for "Detonation and Pre-ignition" on Google to learn the difference between the two.

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    its a 96 polrais indy 500 SKS. I have a very good guide on carb tuning and my carbs have all the correct setting and jets for my altitude and temp. I dont have the snowmobile together yet. I got the pistons and cylinder on then turned it over and I can hear kinda a click at the same place every time, like a ring might be catching a port, so I took it all appart and all the ports are rounded over prety nice. there is no where that I can see it catching, I ran the piston up and down with my hand but did not hear it (didnt think i would) but now its got me a little confused. it does it almost at the bottom of its stroke and it sounds like its coming from pto side (side with the clutch, thats the correct term for that side right?.. and mag is the other side?).. anyway so its coming from the clutch side only. It does not do it at any other point. I am hoping its the clutch but im not sure. I took a video, its good for sound thats about it:

    video is with the heads off, its hard to hear at first (lots of recoil noise, but it gets more hearable towrds the end)
    I took this video before taking it appart. after puting it all back together it sounds the same, I should have thought to twist the crank and see if its my pistons or not but that would have been hard with the conrods still in.

    http://s16.photobucket.com/albums/b38/subdude17/?action=view&current=S3010063.flv

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    I tried to delete the last post so I could make a thread about my new problem , but I couldnt

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