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  1. #1

    SL 750 Possible Bad Piston...Need help...

    Hi guys. Well first of all my neighbor is selling me his 95 SL 750 for 250 bucks....that is the good news. Bad news is it probably has a bad piston. But i'm not sure (he certainly has no idea). Where do you all suggest I begin trouble shooting. I was planning to run a compression test on the cylinders. I understand they are suppose to be pretty close in compression to each other. What is the recommended range? 120 to 150? Let's say they all register in the safe range where do i go next? Carbs? Supposedly he was running real hard one day all was good. Went out the next day and ran a while and it slowly died and had to be towed. I guess he asked a mechanic friend and he said it was the piston but am not sure how much research went into it. So this is what i'm dealing with....I really need all the help i can get. Thanks guys!


  2. #2
    Moderator beerdart's Avatar
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    Start with compression check it should be around 120 and 10% of each.. Next would be to clean/rebuild the carbs and upgrade the fuel system to a three outlet type. All the info and the manual is in the tech section. Any more Qs just ask..

  3. #3
    Great thanks buddy i'll try that.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Right. A compression test will tell you if it's a piston related problem for sure. Usually the low reading will be the culprit.

    Take the spark plugs out and see how fast the engine cranks with the start button. Slow or not at all may mean a bad crank bearing.

    Also the jet pump may have seized. So if the engine doesn't want to turn over, seperate the pump from the engine by removing the driveshaft and try again.

    There are manuals posted in the tech section to help you diagnose and repair problems.

  5. #5
    cameron da river rat's Avatar
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    yeah, first things first......just see if you can even get the thing to turn over.

    if it does, check compression and for spark with the spark plugs in the boots and touching the engine somewhere. if alls well...

    pour a soda bottle cap full of gas down each carb and put the plugs back in and see if she'll fire off but thats if you got good compression and spark.

  6. #6
    Guys thanks again for the help. So here is where i'm at. I got the ski late yesterday and it is really in good shape. The inside is cleaner than some 02's i just looked at. So whoever told them the piston was bad had taken all the heads off and the third (rear) is exposed and easily viewable. You can see what i'll call pitting and or small chunks gone on the top of the piston. The other two are smooth. I assume that is what they were looking at when they diagnosed. So my question is that the problem and there is no need to do any more testing or should i put it all back together and compression test? Any help is appreciated. Thanks.

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    A few clear pictures of the engine would help.

    How long has it been sitting with the heads off?
    Was it sitting outside, or anywhere damp?

    How long ago was it last running?

  8. #8
    Here is a pic of the bad piston (I think, i'm learning as i go here....). It ran and had the problem in Aug. From a little research on this site i suspect it had the standard fuel pump/line issue and went lean. After that they winterized it and put in storage in a garage and covered it. I don't think it was ever damp or got wet. Please let me know if more pics would help. Thanks.
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  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by K447 View Post
    A few clear pictures of the engine would help...
    Quote Originally Posted by bigbadb View Post
    ...let me know if more pics would help...
    Set your camera to close-up mode, often depicted with a flower icon. That will allow it to focus better at short distances. Don't get too close, or your camera will not be able to focus properly, even in close-up mode.

    Post clearer photos of the piston top and sides, and the interior of the matching cylinder head.

    Lean (fuel starvation) operation typically results in a hole melted through the center of the piston.

    Detonation typically causes damage around the edges of the piston top surface, although similar damage can result from small metal pieces getting caught between the piston and the cylinder head dome.

    Here is a post listing some other things to be aware of.

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    That would be a problem for sure. More than likely from detonation.

    Just to be sure you're dealing with good parts, you may want to do a crank index. If one piston is bad from deto, and the others appear good, it may be an out of phase crank.

    Or another possibility may be an air leak. It's not uncommon for the rear crank seal to fail after years of service.

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