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  1. #1

    Help! GP1200R Group K 1390 Obliterated #3

    Ok. Took the ski out for a ride and it lost power i eased off and it locked up after about an hour of normal riding, doing fine. i just replaced the display as the overheat sensor was erroneously coming on on a cold start. Display works now but didn't at the time, that's why I assume I got no overheat alarm. G1200R groupK 1390, no power valves on penzoil premium plus 32:1 premix half avgas 100LL half 93 oct ethanol free. 26 hrs since GPK mod. 121 hrs total on the motor. First time running half avgas.

    Tore it down today. #3 went way lean. 1&2 look fine. Piston chewed itself up. Broke the tabs off the bottom off the cylinder. Chunks of metal all down in the crank area. Piston skirt broken off of one side. The bearings don't seem to have any end play. I ordered a new 1390 piston kit from GPK already and an air fuel separator kit.

    Long story short I wanted your opinion on what made the cyl go lean, how to clean the crank area, and can I re-sleeve the cylinder or will it need replacement?

    Either way ill have to send it to GPK. My opinion this far is that the #3 fuel pump diaphragm creased because of the lighter weight half avgas, leaned the cylinder out, heated the piston into pieces, caused some slap, The cyl walls being thinner on the GPK, snapped, then she seized. I will pull the carb rack and see later on what the diaphragms look like as I will eliminate all of them with the GPK air fuel sep kit. Click image for larger version. 

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  2. #2
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  3. #3

    Join Date
    Aug 2006
    I went with the air separator kit on mine and haven't had an issue. You didn't mention if you have removed the fuel tank flapper valve or confirmed the reserve line was not too long and touching the bottom of the tank well. That can restrict fuel when on reserve. Fuel pump issue is a common problem, especially if you haven't done the O'Side Bill drilled return mod,, listed in FAQ. Good insurance. Real seal leak is a possible culprit too. If you have the GK jetting, you may want to upgrade to the specs listed in Thrashers 1390 build. I just changed after 6 yrs of GK's jetting and wow, big difference. Runs much better, uses a bit more fuel but I know it has enough now. Just a thought.

  4. #4
    LT1, thanks a bunch for the info. This is my first pwc I've ever messed with and new to the forum, just saw the faq section Always 2 stroke street bikes until now. I didn't want to have to pull the motor as the guy I bought it from said he rebuilt the bottom end on the build 25 hrs ago. Didn't mention the return mod or flapper but I'm going to assume they weren't done. I may have to pull it and do this right. The jetting is GPK and all I was getting was 62mph, something seemed off and it must be jetting. I'll try that jetting when I do the mods. Thank you again for the info.

  5. #5
    MR-1 X 2's Avatar
    Join Date
    Feb 2007
    Rancho Santa Margarita, CA
    Sorry to hear about your engine failure. Once the skirt breaks off of a cylinder it usually becomes scrap metal.

    I have some Group K 1390 cylinders available if you are looking to save some $$.

    Please PM if interested.


  6. #6
    Well, pulled the motor today. The case is chewed up, reed is chewed up, and crank feels ok but has metal pressed into the oil slot on the big end rod. Also a large case repair on that cyl the PO didn't mention that doesn't look healthy. Not a happy camper here.

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