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  1. #1
    bwhitsonjr's Avatar
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    Piston Wash? Burn through?

    I've seen how to determine if the cylinder is lean or too rich. However I can't seem to determine which mine is doing. I look in the #2 cylinder or the MAG cylinder and it looks like a small ring the size of a dime not as deep as a dime directly in the center. This isn't present in the middle but it is on the front cylinder as well. I don't know what this is. Burn through? I'm wanting to take the top of the cylinder heads off and take a picture for everyone to see and help me out. I'm at the point where I've removed the carb and it's being rebuilt. I've removed the intake and inspected the reeds and they are flawless. The compression on all 3 cylinders was 110 psi on each after 2 tests. While I'm at this stage do I need to undergo a top end rebuild or put the intake back on, followed by the carb and then I'll be replacing all fuel lines, oil lines, and select valve. Then inspect impeller and drive shaft. If all goes well I'll try it in the water granted it starts and runs better. Please let me know you thoughts! Thanks!


  2. #2
    john zigler's Avatar
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    sounds like you have detonation, probably from a lean condition. What ski are you working on?

    post up some pics.

  3. #3
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    The mark in the center of the piston could be from the original casting. Its best to pull the heads and have a look. Post pictures and there are plenty of people who can tell you what you are looking at.

  4. #4
    bwhitsonjr's Avatar
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    Hey John this is Bryan W with the carbs that I just mailed to you. Your going to be rebuilding them first of next week after Easter. 95 Polaris SLT 750. Bryan last you told me to see how the ski works before I get into any major engine work. I did want to do that but figured I would have to follow with a new gasket set. As you can see I got John Z rebuilding my carbs and when I get them back I'm going to install, replace all oil lines, and fuel. As well as fuel pump, fuel select valve. Afterwards I'm going to see what this thing does. I was really just curious if at the stage I'm at if I found a problem with the way the top of the pistons look should I undergo engine work and then reassemble everything and by that time I would've gone through everything. Not sure if I should undertake that at this early of a stage. But if it's not that much trouble I can take the heads off post a pic and see what you guys think. Your the expert here not me! =) Let me know.

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Is this a blue Fuji engine ir red domestic engine?

    Do you trust the compression gauge you have to be accurate? If not, can you retest compression with a better gauge?

    Remember with compression testing;
    Battery MUST be healthy and strong, otherwise readings will be low with a weak battery.

    Also, all spark plugs should be removed and the throttle held wide open while cranking.

  6. #6
    john zigler's Avatar
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    Quote Originally Posted by K447 View Post
    Is this a blue Fuji engine ir red domestic engine?

    Do you trust the compression gauge you have to be accurate? If not, can you retest compression with a better gauge?

    Remember with compression testing;
    Battery MUST be healthy and strong, otherwise readings will be low with a weak battery.

    Also, all spark plugs should be removed and the throttle held wide open while cranking.
    Actually, you should leave all plugs in, when testing compresion. motor will spin faster.

    Hello Bryan, I did not know this was you. Can you see an "indent" in your piston tops? or it may be just a casting mark, like Bryan P said.

    A pic will tell us. I have new head gaskets in stock, if you want to pull your heads.

  7. #7
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by john zigler View Post
    Actually, you should leave all plugs in, when testing compresion. motor will spin faster.
    I brought this up before as per Group K, but was questioned because when Harry wrote it he was working on a twin. He does not specify that a triple is different. I would like to have a definitive answer to this.

  8. #8
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    I might be doing a test on a new motor today. I will try it both ways and see if it makes a difference. I have done it both ways in the past and never really noticed a difference in readings.

  9. #9
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by BryanP View Post
    I have done it both ways in the past and never really noticed a difference in readings.
    Same here....

  10. #10
    Tony's Avatar
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    Quote Originally Posted by CrazyA View Post
    Quote Originally Posted by john zigler View Post
    Actually, you should leave all plugs in, when testing compresion. motor will spin faster.
    I brought this up before as per Group K, but was questioned because when Harry wrote it he was working on a twin. He does not specify that a triple is different. I would like to have a definitive answer to this.
    The key is to perform the test with the same variables on all cylinders. If you test one cylinder with plugs in/out, test all cylinders with plugs in/out.

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