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  1. #1

    Burnt Piston 95 SL750 Need Help

    I just removed the cover on the MAG cylinder and it has a hole on the top of the piston (see Pic). I think it was related to the gas. I was wondering if there is a service person in the Dallas/Fort Worth Area that could help me out. I just started working on it and maybe it would be better to have someone that knows more than I do help me out. Thanks
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  2. #2
    I am also open to any other info on fixing the burnt piston

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk jason.

    Many, many many, threads are on here about the dreaded holed piston.

    Try a search for hole in piston.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome to the Hulk!

    Click here for Polaris service manuals, and other info.

    The classic cause of a holed piston is lean burn - not enough fuel flowing through the carburetor and into the cylinder at high throttle openings. The piston top then overheats and melts.

    The usual causes are;
    Clogged or gummed up carburetor(s)
    Clogged fuel filters
    Old, single outlet original fuel pump, weakened over time
    Old fuel lines, air leaks into fuel lines
    Air leaks in fuel selector valve
    Missing fuel flow restrictor in return fuel line
    (This restrictor is either a small brass fitting located inside the return fuel hose near the carbs, or inside the last carburetor flow exit nipple. Sometimes when the fuel hoses is replaced, the restrictor is discarded with the old hose. Without the restrictor, fuel pressure at the carbs will be too low.)

    mrjetskey may be able to do the repair work, if he is close enough to you.

    Many owners have been able to repair this type of damage themselves. If you are willing to learn and do the work, you will find support and advice on this forum.

    Have you checked compression in the other cylinders? Before the piston failed, was anything else giving trouble with the machine?

  5. #5
    Thanks for the help. I did check the other pistons and they read ok, 135 psi. Would I need to replace all three or just do one?

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by jasonhughes67 View Post
    ...I did check the other pistons and they read ...135 psi.
    Would I need to replace all three or just do one?
    You can probably get away with just replacing the one piston that failed. Probably should have that cylinder checked for damage, and it may need honing or boring oversize.

    BUT, the big question is how much of the melted aluminum got down into the crank case, and whether any of it has contaminated the steel roller bearings down there.

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    You can do just the one cylinder.

    BUT, you need to fix the problem that caused the hole in the first place.

    Otherwise, you'll be right back all over again.

  8. #8
    I have recently put a three port fuel pump on it. I think I burned the hole in when it had the one port on it. I am just trying to find out how much it will cost to replace the cylinder and make sure the crank case is clean of any metal. i have already replaced the fuel lines and cleaned out the carb. The carb looked really go, it was recently rebuilt.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    The triple outlet fuel pump is a good thing. Good chance it had something to do with it. It was known to hole the MAG piston.

    One option is to buy a used set of a piston and cylinder from someone, usually around $100

    If the cyl is in GC, you may be able to just hone it and replace that piston. Hone $15, piston kit $75, gaskets $80 = $170

    If the cyl is damaged. Bore/hone $70, piston kit $75, gaskets $80 = $225

    Rebuild entire top end. bore/hone 3 cyls $180, piston kit with gaskets included $280 = $460

    I'm willing to bet your crank will be OK. Most of the aluminum prob went out the exhaust opening. Many have installed another piston and had no issues after fixing the problem.

    Fuel line replacement is another good thing. Did you re-install the fuel restrictor inside the return hose?

    Also look into cleaning/inspecting the petcock valve. The seal may leak air into the system causing problems.

    The carb or carbs were recently rebuilt? Sounds to me like someone may have had previous issues, then decided to rebuild the carb/s to try and fix it.

    Do a couple tests before buying parts first.

    Crank index test. Possibly the crank is out of phase and causing timing issues on that cylinder.

    Next try a leak down test. That will tell if air is leaking into the engine and causing problems.

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by xlint89 View Post
    ...Crank index test. Possibly the crank is out of phase and causing timing issues on that cylinder...
    Checking the crank index is a good thing to do, but the MAG cylinder alone cannot get out of index relative to the flywheel (which controls the ignition timing).

    jasonhughes67 => Is there any history on this engine regarding previous repairs before you got it?

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