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  1. #1

    High Compression

    96 SLT 780 and the compression is 160 ish all three cylinders. Story is the couple the sled came from said it was rebuilt several years ago for their son and then sat up. Ive got it running again, on the trailer, waiting to back it down the ramp to run a little longer. Why would compression be so high? I have no idea what work was done. I do think it was put up with a fouling oil because it smoked like a train and the exhaust had a bunch of oil in it. I didn't pay anything for the sled so I guess if it blows up no big loss other then the carb kits I put in it and a steering cable.


  2. #2
    BlueFishCrisis's Avatar
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    Quote Originally Posted by scmoose View Post
    96 SLT 780 and the compression is 160 ish all three cylinders. Story is the couple the sled came from said it was rebuilt several years ago for their son and then sat up. Ive got it running again, on the trailer, waiting to back it down the ramp to run a little longer. Why would compression be so high? I have no idea what work was done. I do think it was put up with a fouling oil because it smoked like a train and the exhaust had a bunch of oil in it. I didn't pay anything for the sled so I guess if it blows up no big loss other then the carb kits I put in it and a steering cable.
    140s is normal for the 780. Take a look at the head gaskets... The 780 gaskets are doubled up with two small rivets holding them together. If it is a single gasket, it can bring the dome closer and increase compression. Could be other things too, tough to tell... Did you try a different gauge just to confirm yours is right?

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  4. #3
    Quote Originally Posted by BlueFishCrisis View Post
    140s is normal for the 780. Take a look at the head gaskets... The 780 gaskets are doubled up with two small rivets holding them together. If it is a single gasket, it can bring the dome closer and increase compression. Could be other things too, tough to tell... Did you try a different gauge just to confirm yours is right?
    Ha funny you mention a different gage. I bought a cheap Chinese gage from amazon and could only get 70 psi on all three cylinders. Tried the same gage on my outboard and got roughly half of what it should be. Borrowed another cheap Chinese gage and got 60 psi and same on my outboard. Finally bought a Mac compression tool and that’s where I’m getting the high readings, but normal on my outboard so I “ sort of” trust it. Thanks for the lead on the doubled up head gasket. It’s possible whom ever did the work only put one gasket on it.

  5. #4
    casey67's Avatar
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    Having oil in the cylinders will help seal the rings and cause a higher compression reading.

    Run it well/until full temperature to clear out oil, recheck when cool.
    Or clean out cylinders with carb cleaner.

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    I will add that you should be using high octane fuel until you know for sure about the compression.

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Another option is they rebuilt the engine with oversized pistons and never modified the domes

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