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  1. #1
    GP1300R XL1200 LTD GP1200R Yimihemi1200's Avatar
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    Jetting on Group K 1390 Engine

    Group K recommends the following on the 1390 (66V engine) :
    Carbs – OEM Triple Carbs Mikuni SBN-I Super 44 mm w/accelerator pump Jets:Main -117.5 Pilot 110 Spring - _95_ grams. N&S (Needle and Seat) __1.2___Jetted and modified by Group K. Top High Speed Screws 1.0 Turns out. Low Speed 1.5 Turns Out
    With Riva Flame Arrestors, Single Pipe and "D" Plate.

    How well will this work and what would other people recommend for performance? What would be my pop off pressure to use?
    Any help would be appreciated before I get this engine broken in.


  2. #2

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    no f***ing way
    some were in this mass of info there is -jetting I have used see if you can do search

  3. #3
    GP1300R XL1200 LTD GP1200R Yimihemi1200's Avatar
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    Main Jets 117.5, Pilot Jet 110
    Seems a little small for sucha big bore engine.

  4. #4

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    Group K likes to use the accellerator pump to transition you across the mid range lean spot that the "I" carburetors have (by design). Others favor removing it and use different jetting. I would bet that since Osidebill is on the west coast that he has had much experience with this engine and would be a great source of info for you.

    I have a Group K 1200 sleeperII which has a mild port. I chose to disable my accellerator pump and as a result, wound up with the following specs:

    high 117.5
    low 115
    80 gram spring
    pop off 33-34 psig
    2.0 N&S

    It is a torquey son-of-a-gun on the bottom, and I've had a lot of fun riding it through the years.

    My bet is that you are going to need a lot of fuel on the bottom. I hope that 110 can deliver.

    You should also talk to Harry or Gerhardt about your concerns. They know more about their engines than you or I ever though of.

  5. #5
    Happily Self-Employed WFO's Avatar
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    You should talk to Harry or Gerhardt about your concerns. They know more about their engines than you or I ever though of.
    +1

  6. #6

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    I've used the Group K's recommendations on my 1390 for several years, with great success. Some think the jetting is on the small side, however, Group K's porting is more conservative than some, thus there is greater vacuum(signal) on the carbs requiring less jet.

  7. #7
    Moderator DrewNJ's Avatar
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    Before I snapped my crank I was running 120/140 95g 1.5 all screws at 1 turn out on my 1390 GK motor. This was with proks, 3 degree key and no accel. pump. 1 layer of head gasket at about 135psi.
    This setup worked great for me at sea level even during the cold months of riding.

  8. #8
    Quote Originally Posted by DrewNJ View Post
    Before I snapped my crank I was running 120/140 95g 1.5 all screws at 1 turn out on my 1390 GK motor. This was with proks, 3 degree key and no accel. pump. 1 layer of head gasket at about 135psi.
    This setup worked great for me at sea level even during the cold months of riding.
    I miss that ski...

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