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Thread: GP1200R rebuild

  1. #1

    GP1200R rebuild

    hi i was just wandering if someone could lead me in the right direction with some information. First off i just rebuilt my gp1200r motor complete stock rebuild. Also rebuilt the carbs with all mikuni rebuild kits. The ski is pretty much a stage II gp1200r: D-plate, flame arrestors, reed stuffers, free flow exhuast tube premixing 40:1 with amsoil dominator also. My first question is how do you go about tuning the high and low speed adjustments? how many turns out are they suppose to be at with these mods? i rode it yesterday for the first time since the rebuild and its having a low end hesitation or bog cant really determine what it is but once past 1/4 throttle its pretty good. havent done a compression test on all cylinders yet either.


  2. #2

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    http://www.mikuni.com/pdf/sbn_manual.pdf

    Your carbs are the Mikuni 44 "I" type carbs. They have a lean spot in them, and it is described well by Harry and Gerhart Klemm www.grouk.com . Perhaps this is why many have opted to rejet when they went to aftermarket flame arrestors.

    In any case, reading the mikuni manual and the specifics on your type of 44 carb at GroupK will give you much insight on how to tune. I will say that you MUST have a good tach.

    In a nutshell you will do the following (and more):

    set your low and high speed adjustments 2 turns out (each)
    set your idle speed at 1200 rpms
    slowly close the low speed jets approximately 1/8 turn at a time until your rpms in the idle speed stops increasing (you will have to keep reducing idle speed to maintain 1200 rpm)

    set your high speed adjustments by finding your highest rpms, then closing the jets screws 1/8 turn per run until you find your maximum rpms.

    When you find maximum rpms, it is best to increase the fuel by 1/4 turn to account for changes in barometric pressure, O2 concentrations, poor fuel quality, and other wise things that change from moment to moment that could cause you to run lean.

    This stuff can be intimidateing if you have never done it before, but if you err on the rich side, it will be hard to mess up.

    Good luck, and keep us posted on your progress.

  3. #3
    Thanks for the input as of now the highs are set at 1 3/4 and the lows are at about 1 1/4. the problem im having is that i previously bought the bike from a guy with all these mods done and am not sure if he rejetted for the mods or not . it didnt come across my mind until after i had rebuilt the motor myslef and the carbs that i would need to know that to tune the ski. also what tach would you suggest the tiny tach? im failry famliar with all this but just getting input from someone who knows more about skis is good to know.

  4. #4

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    You could try calling the guy and ask what he rejetted to. It is better than tearing it down again to figure what you have.

    Use a PET 2100dx. Carl at Long Island racing has a good price. The tiny tach does not update fast enough.

    You are on the right track, probably a little fat on both the high and low sides.....that is assumeing that it is rejetted properly. If it has been properly rejetted, you probably will not get hurt running that way, but I would not go any futher without a tach.

  5. #5
    i bought it in 2006 so i dont have a contact for the guy. what is usually the norm for the adjustments with these mods?

  6. #6
    silver93t's Avatar
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    You really need to know what jets you have. 110/120 should be about right per oside bill's recommendations.

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    Quote Originally Posted by silver93t View Post
    You really need to know what jets you have. 110/120 should be about right per oside bill's recommendations.
    95 gram spring with pop off press approxiately 43 psig.

    about 3/4 to 1& 1/4 turn out on the lows and 3/4 to 1 & 1/2 turn out on highs.

    A 95 to a 105 high speed jet in the return line from the carbs to the tank to help maintain back pressure. This jet should be placed into a 1/4 inch barbed (plastic) coupler. If you choose to use a brass coupler, then 1 end should be tapped with a 5m metric tap. No drilling needed. The jet will screw right in.

  8. #8
    yeah theres no way for me to tell if it is rejetted unless i tear them down again. might have to do that during the winter to get this thing right

  9. #9
    how would the ski run without being rejetted with theses mods?

  10. #10

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    You said "Stage II" set up.... this assumes 150 psig compression. Your "I" carbs would run lean in the midrange IMHO with stock jetting. The aftermarket flame arrestors reduces the signal or the ability of the carbs to pull fuel in with the air at the proper ratio.

    The stock system works fine, but with the stock flame arrestor, the signal is much stronger. When you switch to the aftermarket flame arrestors you must switch the jetting.

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