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  1. #1
    BlueFishCrisis's Avatar
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    Polaris 780 Fuji No Fire on PTO Cylinder

    Ran into a strange problem this past weekend, but was able to trouble shoot and correct. Figured I would post the info for future searches as it is a bit of an oddity.

    1996 SLX with Fuji 780. Motor was recently rebuilt and re-assembled. When testing in the driveway after the rebuild, motor was running rough, and required excessive throttle to get to idle speed. Used a spark tester to determine a no spark condition on the CEN and PTO cylinders. Since Fuji ignitions (less the Pro) are wasted spark, I was a bit surprised to have no spark on only 2 out of 3 cylinders. Opened up ebox, and found that the PTO coil wire from the CDI had been pinched when the coil retaining plate was installed exposing the wire and shorting out to ground. Removed the pinch point, and stuffed everything back in the box, being careful to ensure no more pinched wires. After re-assembly, spark returned to all three plugs and the motor ran fine on the trailer. Took it to the lake, and fired up, idled fine, and ran it for about an hour without issue. Parked the ski for about two weeks.

    Took it back out this past Sunday for some more tuning. Started on the trailer, but once in the water, it fired up, and on the way out of the no wake zone it died. Would start up again for a few seconds, then died. Started again for a shorter time then died. Then would not start. Took the ski over to a shallow area, had someone hold it, and started it with the throttle opened slightly. I could get it to run, but had to really raise the idle speed to do so. Removed the CEN plug boot, and it got worse. Put the CEN plug boot back on and RPM jumped back up. Removed the PTO plug boot, no change. Pulled the PTO plug, wet piston, wet plug. Figured no spark on the PTO. Held the plug to the head, has spark. Held my thumb over the plug hole, and seemed to have good compression. So I was corn-fused. Pulled the ski out of the water, and headed home.

    Used a spark tester to check for spark. Had good spark on all three cylinders. Checked compression, 130+ on all holes, so that was good. Engine would run on the MAG cylinder alone. Would try, but would not stay running on the CEN alone. Wouldn't even fire on the PTO. I decided that the ground out condition on the coils may have somehow damaged them, so I swapped out the CEN and PTO coils. Everything back to normal. Running strong.

    So strangely enough, even though it appeared that I had Fuel, Compression, and spark, there was still an issue with the ignition coils.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Did you ohm test the 'bad' ignition coils?

    An open or partially shorted ignition coil can mess with the wasted spark system since each coil can affect the others.

    On this engine which ignition coil primary is 'farthest' from ground?

  3. #3
    BlueFishCrisis's Avatar
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    Quote Originally Posted by K447 View Post
    Did you ohm test the 'bad' ignition coils?

    An open or partially shorted ignition coil can mess with the wasted spark system since each coil can affect the others.

    On this engine which ignition coil primary is 'farthest' from ground?
    I have not yet tested the bad coils, but plan on it. The order for connecting the coils does not matter. Any of them can be furthest from ground, as they are connected via bullet connectors. I haven't paid close enough attention to determine which is connected to the black wire feed from the CDI at the factory, nor if this is a direct ground connection. I assume it is not...... The wiring diagram does not identify coil locations either.

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    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by BlueFishCrisis View Post
    Quote Originally Posted by K447 View Post
    Did you ohm test the 'bad' ignition coils?

    An open or partially shorted ignition coil can mess with the wasted spark system since each coil can affect the others.

    On this engine which ignition coil primary is 'farthest' from ground?
    I have not yet tested the bad coils, but plan on it. The order for connecting the coils does not matter. Any of them can be furthest from ground, as they are connected via bullet connectors.

    I haven't paid close enough attention to determine which is connected to the black wire feed from the CDI at the factory, nor if this is a direct ground connection. I assume it is not...... The wiring diagram does not identify coil locations either.
    Indeed, the wiring sequence normally does not matter for Fuji wasted spark ignition coils.

    But if a Fuji ignition coil primary has an internal (partial) short to ground then the wiring sequence does matter. The coil(s) in series between the 'bad' coil and ground can have weak or no spark while the coil(s) between the CDI primary ignition coil feed and the bad coil should have full spark.

    In this scenario the actual bad coil may have good spark, weak spark, or no spark, depending on how it has failed internally.

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