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  1. #1

    94 polaris sl 750 performance questions (new member)

    I have 2 polaris sl 750 skit with the blue Fuji engines I have spent Mitch time on this site gathering information and finally got one somewhat running.

    I started checking to find out about backfiring and frond a thread that was talking about compression and from what I have learned my skis have some performance mods done to it and have some questions. What I have is qa blue Fuji engine I have verified that it has weisco pistons in it no big deal but my compression is 157 across all 3 cylinders witch to my understanding is high for a stock engine.

    I know the ski has an aftermarket ride plate as well but have no other information.

    When I baught the ski I baught it as parts and found out the motor was good and decided to repair. I purchased stock carbs on ebay and a stock set of reeds and installed as they were missing upon purchase I also installed a muffler from ebay as it was missing also.

    The ski will crank and run on the trailer but the mag and center cyls will either back fire or ignite in the bottom end I can't tell wich it is but it only does it when first starting and idling once revving it it smooths out and stops.

    Could this be due to wrong reeds or improper carb jetting for the motor compression or am I not using hih enough octane for it any help would be great.
    Last edited by K447; 08-06-2014 at 10:07 PM.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Have you checked the ignition timing? It sounds like the timing is way off as there should never be fire in the crankcase.

    The most likely cause of wildly incorrect timing is the flywheel woodruff key has sheared and the flywheel has rotated on the crankshaft.

    What CDI is being used?

  3. #3
    It wouldn't seen to be timing because as soon as you give it a little throttle it smooths out and once it is warm it stops and runs perfect

  4. #4
    BlueFishCrisis's Avatar
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    How do you know it is backfiring in the bottom end?

    A lack of fuel or low octane would cause detonation. Take a look at the piston tops to see what condition they are in. I would imagine 157 psi is going to require higher octane than pump gas.....

  5. #5
    That islands more like my issue than anything else. I was trying to get it to run again this morning and now it is not firing not sure if it may be the Cdi or possibly coils or magneto I will post as soon as I find out something. If I get it running again what fuel should I use in it I am running 91 octane non ethanol right now and detonating any ideas.

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