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  1. #1

    Uneven Compression after rebuild and carb diaphragm metal ring rusted, SLX 780

    I bought a project 96 slx 780. I rebuilt the top end .5 over and got it ready to break in, the engine idles fine but bogs off idle and wont much over 4500rpm. No red light on MFD. After the rebuild, compression cold was 145-150-150, after the engine warmed up, compression was 120-145-145. And then it would return once cooled off again. What is causing that?

    It has a new fuel pump, petcock, battery. I bypassed the fuel/water seperator too. Can I do that? When cleaned out the carbs, i noticed the 3 metal rings attached to the back of each rubber diaphragm had rusted through in spots. Are those rings essential to running properly? Possilby causing my poor running issues.


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    It sounds to me like the engine isn't getting as fast of a RPM hot VS cold when cranking. Not sure why one is dropping so much psi though.

    Did you properly gap your piston rings?

    You can bypass the water seperator, but why? It'll catch any water in the fuel system, and contains a reusable fuel filter.

    If you have ruse inside your carbs, you will DEFINATELY want a water seperator installed. That means water was already inside the fuel system.

    If you rebuilt the top end and haven't rebuilt the carbs (especially with rusted parts), you're asking for trouble.

    Yes, rusted diaphragms can give you problems. More than likely they are damaged and causing you issues, not to mention potential for rust to clog a jet or get inside the cyl and scratch your cyl walls,

    Do them before you do anything else.

    Also take a fuel pressure reading and see what you get at idle and WOT.

  3. #3
    No i didnt gap the piston rings. was i supposed to? I had the jugs bore to the specs that came with the piston and i just threw everything together. Both rings for each piston were the same, I installed letters facing up and turned the openings to the little pin that is in the ring groove. Then popped the jugs on.

    The water seperator is gummed up and corroding, thats why i bypassed it. The ski was sitting for years and im assuming thats why diaphragm rings were all rusted.

    Can the engine run with warm readings at 120-145-145. Its sucks because Im in the processing of trying to break the engine in and I dont want to do any damage while dealing with this other stuff.

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    Quote Originally Posted by Beachbum3478 View Post
    No i didnt gap the piston rings. was i supposed to? I had the jugs bore to the specs that came with the piston and i just threw everything together. Both rings for each piston were the same, I installed letters facing up and turned the openings to the little pin that is in the ring groove. Then popped the jugs on.

    The water seperator is gummed up and corroding, thats why i bypassed it. The ski was sitting for years and im assuming thats why diaphragm rings were all rusted.

    Can the engine run with warm readings at 120-145-145. Its sucks because Im in the processing of trying to break the engine in and I dont want to do any damage while dealing with this other stuff.
    you actually have to same problem as i do, I am also getting help from these guys, make sure you gap your rings, and change diaphragms, else you might ended up with $500+ parts replace like i do.....

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    If you don't properly gap your rings, you will either A, do cylinder damage if the gap is too tight, or B, have low compression if the gap is too big.

    If the water seperataor was gummed up, chances are the fuel filters inside the carbs are too. Definately go through them.

    You're taking a big chance by leaving the rings unmeasured, but asking for trouble if you don't address the fuel system. Prob most likely cause of you needing to rebuild the top end in the first place.

    Pistons don't fail unless something CAUSES them to fail. So just replacing them without fixing the cause will only lead to another failure.

  6. #6
    I bought the ski as a project needing a top end because the fuel system failed. The only thing bad in the entire fuel system is the diaphragms. I went through the carbs completely.

    So I guess the gap is too big the cylinder with lower compression. Is it too late to do anything about it, or can I take the jug off and set the rings to the correct gap?

  7. #7

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    Quote Originally Posted by Beachbum3478 View Post
    I bought the ski as a project needing a top end because the fuel system failed. The only thing bad in the entire fuel system is the diaphragms. I went through the carbs completely.

    So I guess the gap is too big the cylinder with lower compression. Is it too late to do anything about it, or can I take the jug off and set the rings to the correct gap?
    Well from what I have read so far, you can only increase the size of the gap by filing thus cannot reduce the gap.

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by birdgod View Post
    Well from what I have read so far, you can only increase the size of the gap by filing thus cannot reduce the gap.
    Correct. But you can buy another set of rings to close the gap.

  9. #9
    So did i do damage if I have low compression? I would assume the rings didnt come with too big of a gap from the manufacture. So maybe the gap was too tight at first. What kind of damage can I do if I run it with 120-145-145 readings?

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    cylinder damage if the rings broke off. need to change the cylinders, piston, rings. and hopfully the broken parts didn't get into the crankcase.....

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