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  1. #1

    780 misfiring - Not limp mode

    Polaris SLT 780.
    Pulled the gray wire and it still will not rev up. Not a fuel issue. I hooked my spark tester up and get a good strong spark but I can see a misfire. Stator checked out ok. Is the CDI cooked?


  2. #2
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Trim a 1/4" off the spark plug wires. Is it misfiring on just one cyl? Doubt it's the CDI,but possibly a coil. Also double check the wiring in the box,that there is nothing loose or corroded.

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk.

    You got a good strong spark, but can see a misfire????

    What does a misfire look like?

    You may have a sheared woodruff key. Did your engine come to an abrupt stop at all? Something get caught in the impeller?

    Switch the plug wires around and see if the misfire follows.

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    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by X2guy View Post
    Polaris SLT 780.
    Pulled the gray wire and it still will not rev up. Not a fuel issue.

    I hooked my spark tester up and get a good strong spark but I can see a misfire.

    Stator checked out ok. Is the CDI cooked?
    Welcome

    On the Fuji 780 (and 650/750), all spark plugs fire together, whenever any cylinder needs spark (called 'wasted spark' ignition). So every plug should be firing three times per crank revolution.

    It is possible your CDI has a problem, but you should first eliminate any other possibility.

    This video might be helpful. This particular engine uses sequential spark, but the method can be helpful in finding a dead or weak cylinder.

    http://www.youtube.com/watch?v=UEzN67xol38

  5. #5
    Thanks for the welcome.
    I can see the missfire when using the spark tester. When turning it over at a consistant speed without starting it the spark frequency is not consistant. I've trimmed the wires back. All 3 coils are doing it so it's not an issue with a coil. I have now replaced the CDI with no success. I don't think the flywheel has spun but I will check when I tear into the stator. Possibly there is still somthing wrong with it even though it checking out OK.
    I'm repairning this for a customer and not familiar with polaris watercraft. Tho only other thing I noticed is that the display does not come on. If that may be a hint to anything.
    Thanks

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    It's sounding like a sheared woodruff key. Best look into the flywheel possibilty.

    Check the fuse inside the elec box for the MFD. It should be a 1/4 amp fuse. Radio Shack has them.

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    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by X2guy View Post
    ...The only other thing I noticed is that the display does not come on...
    If the MFD fuse is good, and the MFD connector is getting power and ground, but the MFD is still not actually working, then unplug the MFD connector behind the MFD.

    Sometimes a 'dead' MFD can cause strange problems with the ignition system.

    Have you ohm checked the stator coils?
    Polaris Fuji (blue) engine

    How to test CDI, Magneto stator coils & Ignition coils on Fuji engines

  8. #8
    The MFD was unplugged that's why it wasn't working. I still don't feel the flywheel has spun. I went to take another shot tonight and hooked the spark tester up while it's running. It seemes to idle fine but as soon as I rev it up I can only get 4800 RPM. I also see on the spark tester that the spark completely cuts out for a few revs then starts again. Remember I've changed the CDI and still have the gray wire unplugged. Is there anything else that could be throwing this thin in limp mode? When I tested the pulser coil on the stator it was a little lower than the value given but no tolerance was given either. What do you guys think?

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Tolerance for stator is around 10% at 70* ambient temp.

    Is it cutting out on all 3 plugs or just 1?

    Have you trimmed the plug wires back yet?

    Did you ohm test the coils?

    At some point, you're going to have to check that woodruff key though. Someone recently had one go bad.

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by X2guy View Post
    The MFD was unplugged that's why it wasn't working. I still don't feel the flywheel has spun.

    I went to take another shot tonight and hooked the spark tester up while it's running. It seems to idle fine but as soon as I rev it up I can only get 4800 RPM.

    I also see on the spark tester that the spark completely cuts out for a few revs then starts again.

    Remember I've changed the CDI and still have the gray wire unplugged. Is there anything else that could be throwing this thing in limp mode?...
    It may not have anything to do with limp mode.

    It could just be a failing ignition problem.

    Unplug the MFD from the 8-pin connector in back (not just the MFD power feed wire) - does the engine still limit RPM the same way?
    Sometimes a flaky MFD can cause weird ignition problems.

    A shifted flywheel key would not kill spark at a certain RPM. It would cause other run-ability problems, but not loss of spark at a certain RPM level.

    It is possible for a stator to ohm test good with the engine off, yet fail with engine vibration and/or heat.

    That gray wire that you unplugged from the terminal board is the gray wire coming from the CDI - correct?
    And it is taped so it cannot touch anything else?

    Are you 100% sure the installed CDI is good?

    Just on a hunch, unplug the Yellow wire from the stator where it connects to the terminal board. Tape it off, as it has battery power on it.
    Any change in the RPM symptom?

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