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  1. #1

    Question Compression check failed, now what?

    I have a 1993 SL 650 that was having a hard time running. I did a compression check on all cylinders and the front cylinder was 90 and the back cylinders were 120. Where should I start to fix the front cylinder?

  2. #2
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Welcome to the Polaris side of the Hulk

    The links below will give you a good starting point.

    It sounds like your machine has the classic failure; fuel system has aged and not been properly maintained, front carburetor runs lean, and melts a hole through the front piston.

    The usual starting point is to remove the cylinder head(s), and check for damage. If the piston is holed or otherwise damaged, then at the very least you need to remove that MAG cylinder, and have it checked and possibly re-bored, then install a new piston and rings.

    If there is metal debris down where the crankshaft lives, then you may need to take the engine apart and clean the crank case, and check the crank shaft bearings.

    While you are working on the engine, you might consider also freshening the other two cylinders with new piston rings.

    In addition to repairing the damage, you need to clean up the fuel system. The carburetors should be rebuilt, all the fuel lines replaced (including any inside the fuel tank). It is recommended that the fuel pump be upgraded to the higher output triple outlet version.

    Tip: When replacing the fuel lines, don't lose track of the tiny fuel restrictor that lives inside the return fuel line from the carbs to the fuel tank.

    A pressure test of the sealed off engine will tell you if the front or rear crank shaft seals are leaking. If they cannot hold modest pressure or vacuum, then the crank seals need to be replaced.

    Polaris Fuji (blue) engine

  3. #3

    Join Date
    Sep 2006
    nicely put k447 but since he has compression wouldnt that just mean his rings are screwed ?

  4. #4
    Hayabusa's Avatar
    Join Date
    Sep 2006
    Trenton Michigan
    Compression of 90 is low, but not so low to think that the damage is severe. With a little luck, it might be an easy repair.

  5. #5
    ryandi2's Avatar
    Join Date
    Aug 2009
    conesus ny!
    Quote Originally Posted by Hayabusa View Post
    Compression of 90 is low, but not so low to think that the damage is severe. With a little luck, it might be an easy repair.
    this is my first impression as well... take the mag(front) cylinder off and take some pics of it.. and while its apart read threw the updates and do the fuel updates

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Welcome to the Hulk.

    With any luck, you can easily fix the low MAG compression.

    Remove the cooling rail.

    Remove the MAG cyl head.

    Remove the exh pipe.

    Remove the exh manifold.

    Remove the MAG cyl.

    If the piston is damged, you can buy a piston kit for around $70, and have the cyl bored oversized.

    If the piston is good, you may be able to get away with a cyl hone and a new set of rings.

    You'll need at least 1 cyl base gasket, 1 cyl head gasket, 1 exh manifold gasket, 1 exh pipe gasket and 3 cooling rail gaskets.

    I would still do the fuel system upgrades because of the age of the ski.

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