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I have just purchased a 1999 Ski-doo MXZ 600. The sled runs fine, but only has 100 pounds compression exactly on both sides. The past owner has told me he had just changed a piston due to low jetting. You guys have any ideas where it should be and what I should check? Also, a few head botls were leaking antifreze...you think the gaskets are shot? He did not change them he tells me. The sled has only 1300 miles.
 

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not sure what elevation youi are at but mine has 128-130 lbs in each cylinder.  It now has over 7000 miles (recently rebuilt due to self-induced burndown) but has always been in that range (not sure about my own elevation mind you).
 

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I think you will find that the moly on the rings are burnt off or gone.   Try re-ringing the pistons and see what you get.   If that doesn't do it, you better check your piston wear.   This has happened to me numerous times.   You should have at least 115-125 per side.   Good luck, piston rings are the cheapest  fix at this point.   Also, a good way to tell for sure is put a few drops of oil in each spark plug hole and do another compression test, if it raises the compression your rings are definately bad and not sealing.   Piece out.
 

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mxz_600, you should back up that compression test  with a leak down test or at least check it against another comp gauge
 

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I `ve got 2 "high quality" gauges, one snap on one mac , they read 15lbs apart, what  `s interesting about your post is that the sled runs fine, kid had some good advice (as always) but 100 lbs is real low for a doo to run fine.
 
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