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Thread: Piston question

  1. #1

    Piston question

    I have 2 identical skis. 94 750SL's One has a broken case, piston, and rod. I looked down the spark plug hole of the MAG on the running ski, and saw a perfect circle on top of the piston right under the spark plug. I figured it was starting the "hole in piston" condition I've read about.

    SO I checked to see which of the 2 pistons was best (from the busted ski) and thought I would swap one out. I noticed they both had the identical circle on them as well. Ran my finger nail across the the top of the piston and can NOT feel anything. I've read on here about some pistons having a number on the top of them. I wanted to check and make sure the piston is not supposed to have this circle before I start swapping parts. If these circles shouldn't be there, looks like my projects just got bigger.

    Included some pics. What do you think?
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  2. #2
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Those circles you see are casting marks on the pistons. They are normal. If those are oem pistons, they wont have #'s. They are standard size. If they were over sized, you would see .25, .50, .75, 1.0. Those mark the oversize in mm.

  3. #3
    Well that answers that. Seems like that front piston on the running ski is good afterall. Thanks

  4. #4
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Here is a standard oem 780 piston. It has the same circle in the middle.
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  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Correct, those are casting marks. So, maybe your "suspected" piston is good afterall?

    There are numbers on the tops indicating piston size. 78A in Stars pic means it's for a 780. Yours should have a 75A for a 750. Any other numbers stamped does mean oversized .25mm = 10 thous. .50mm = 20 thous. .75mm = 30 thous. etc....

  6. #6
    Moderator beerdart's Avatar
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    Looking at the pics it looks a little lean. You should turn the high screws out 1/4 and recheck the wash after a good ride.

  7. #7
    Took off the carbs and they are needing a rebuild for sure. The diaphragms are shot. Someone has been in here before cause I see stripped screw heads. I have to take out those two screws to get to the bowl right? Looks like I may have to drill that one out. Wish me luck!

    Do I really need to remove those and check out the bowl? I'm thinking I should make sure there isn't any sand or junk in there while I have it torn down this far. Any thoughts?

    Where can I buy replacment screws for the inside of the carb? The seats and needle's look fine, so I guess I'll just order the diaphragms. Anyone else with carb rebuild experience should feel free to chime in.

    Thanks again fellas
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  8. #8
    Moderator beerdart's Avatar
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    Yes you need to remove them that is where the jets are. You need to remove the needle seat and inspect the O-Ring.. I have some carb parts here. When you rent the alignment tool I can send some carb stuff if you let me know what you need and I have it..

  9. #9
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    You can still possibly remove those screws w/ one of these... http://www.sears.com/shc/s/p_10153_12605_00947641000P

    Looks like water has been sitting in the right carb,and with that, I'd recommend you replace the float valves and springs. How did the strainers look on the inlet side of the carbs?

  10. #10
    FLjoyrider's Avatar
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    those little impact drivers work like a charm! If the screws are already stripped you can take a flat punch and smash the little edges back into place to get some extra grab with the impact.

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