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Thread: Budger's SLT780

  1. #1
    johnsonmtz's Avatar
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    Budger's SLT780

    So Cyndi has some decent carnage on the MAG piston. I knew from the piston wash (looking through plug hole) this cylinder was a little lean and had opened up the high needle to compensate. During the Midwest gathering weekend it must have gotten even more lean and finally toasted the piston. The crank and rod looks perfect so it seems this is a top end job only. Here are pictures of the carnage.

    MAG Piston
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    MAG head
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    Cylinders still on:
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    Piston Wash on CEN and PTO are perfect:
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    KJ


  2. #2
    johnsonmtz's Avatar
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    Cylinder

    I'm pretty certain this cylinder is done. It was an SBT motor that was already punched out to 1.00mm oversize. I could chance getting a 1.5mm oversize piston and bore this out but there are sizable nicks on the port chamfers and I'm not certain an extra 0.5mm will clean it up sufficiently.

    I know I have to get a new head.

    On option is to get a used cylinder and piston to match. I already have a line on a cylinder and head combo, I'm just waiting to hear if it's standard or oversized because I also have an offer on a piston.

    The other option is to resleeve this cylinder. SBT has sleeves for $80 and I'm sure I could get a local shop to press out the old one and press in the new one.

    I hope to figure out a solution tomorrow so I can get things in the shipping channels to arrive early next week and start putting the ski back together.

    Cylinder bore pics:













  3. #3
    johnsonmtz's Avatar
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    Once I get my parts figured out the next step will be to find the cause. I should have done a leakdown test before removing the cylinder. However, I wanted to evaluate the damage and make sure the bottom end was okay before I went any further. From the piston wash it's obvious the CEN and PTO are in great shape. While I'm waiting for parts I'm going to pull the carbs and check the MAG carb to see if there is a restriction or other explanation there.

    If the carb is okay I'll do a leak down test later and see if I can find the cause. If the problem is not the intake gaskets it's either going to be the sealant on the case or the front main seal. If the problem is there I'll obviously have to pull the motor to fix that problem.

    I'm hoping the problem is only in the carb, but time will only tell.

    KJ

  4. #4
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    Id be willing to bet its a crank seal if its not in the carbs.

  5. #5
    johnsonmtz's Avatar
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    Quote Originally Posted by BryanP View Post
    Id be willing to bet its a crank seal if its not in the carbs.
    Yeah, that's kind of what I"m thinking. I can't remember though, do the cases have to be split to install a front crank seal or can it be installed with them together?

    KJ

  6. #6
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    I hope those port edges were damaged by the bad piston and didnt look that way from SBT. Those dont look too good at all.

    I'm pretty sure you have to split the cases. Might not be a bad idea anyways so you can make sure everything is absolutely clean in there.

  7. #7
    Cases need to be split. And chances are there is metal and what not in the case. clean and replace seals and gaskets

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