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  1. #1

    tuning mikuni carbs low speed circuit

    The info I found says to take the ski on a low speed ride and do a plug chop when adjusting the low adjustment screw. How fast is this exactly. I tried holding 1/4 throttle but it stated out at 9mph and ended up on plain at 20mph don't know if one was to fast and other was to slow can some one tell me where I should be at on this point of tuning. Mph, rpm, plain or no plain , or should I be just using the idle momentum?


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Somewhere on here is Randy's original Carburetor Tuning Tips for Girls post, which you may find useful. It may have since been renamed and reposted elsewhere...

    [With minor edits, this is Randy's generic tuning guide for the Mikuni carbs]...
    Quote Originally Posted by ph2ocraft View Post
    THANKS RANDY!!!

    Start with the carb screws all set at 1 turn out, and tune the high speed screw adjustment with the tachometer for peak RPM (at wide open throttle), then stop there and count the number of turns on the high speed screws. Write this down on a piece of paper..

    Then turn the high speed screws in all the way and open them to 1/4 turn open. (Now you will be testing starts only, so no more big high rpm trips around the course). You are only turning in the high speeds to allow you to easily tune the low speeds.

    Now check the low speed screws and set them at 1 turn and do a practice start;
    To test the low speed setting pretend that you just jumped the race start and you have to start with no holder. Hold the ski by yourself and hold as many RPM as you can (don't blow it out first for testing) and then nail the throttle hard.... NO FARTHER THAN 50 FEET, as you are only testing the start response

    If it blubbers like a schoolgirl trying to guzzle a quart of beer, then it's too rich on the low speed screws..*
    If it falls flat on the line (like someone punched you in the stomach).. It's too lean on the low speed screws.

    When you like the acceleration off the line, stop there and count the number of turns on the low speed screws. Write this down on a piece of paper.. Then re-set your high speed screw where they showed the best peak RPM earlier and try both acceleration from start and a peak RPM run...

    It will probably "blubber" a little off the line... DO NOT turn in the low speed screws... Instead, go back out on the course and fine tune for peak RPM again.. The blubbering off the line is caused by having too much fuel in the high speed circuit...

    Write down your new high speed setting on the piece of paper along with the best low speed setting that you found earlier, this will be your base-line carburetor settings.*

    If the high or low speed screws are open more than 1 1/4 turns, then you need to swap to the next bigger size jet.
    If the high or low speed screws are closed all the way, then you need to change to the next size smaller jet.

    *** If no matter what you try, it still won’t leave the line without falling on its face "lean bog"... Then try the next size lower pop-off spring in the carbs... Don't be afraid to try #95 or even #80 gram springs..
    Last edited by K447; 12-03-2014 at 01:49 AM.

  3. #3
    kensmith's Avatar
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    Quote Originally Posted by K447 View Post
    Somewhere on here is Randy's original Carburetor Tuning Tips for Girls post, which you may find useful. It may have since been renamed and reported elsewhere...

    [With minor edits, this is Randy's generic tuning guide for the Mikuni carbs]...
    Quote Originally Posted by ph2ocraft View Post
    THANKS RANDY!!!

    Start with the carb screws all set at 1 turn out, and tune the high speed screw adjustment with the tachometer for peak RPM (at wide open throttle), then stop there and count the number of turns on the high speed screws. Write this down on a piece of paper..

    Then turn the high speed screws in all the way and open them to 1/4 turn open. (Now you will be testing starts only, so no more big high rpm trips around the course). You are only turning in the high speeds to allow you to easily tune the low speeds.

    Now check the low speed screws and set them at 1 turn and do a practice start;
    To test the low speed setting pretend that you just jumped the race start and you have to start with no holder. Hold the ski by yourself and hold as many RPM as you can (don't blow it out first for testing) and then nail the throttle hard.... NO FARTHER THAN 50 FEET, as you are only testing the start response

    If it blubbers like a schoolgirl trying to guzzle a quart of beer, then it's too rich on the low speed screws..*
    If it falls flat on the line (like someone punched you in the stomach).. It's too lean on the low speed screws.

    When you like the acceleration off the line, stop there and count the number of turns on the low speed screws. Write this down on a piece of paper.. Then re-set your high speed screw where they showed the best peak RPM earlier and try both acceleration from start and a peak RPM run...

    It will probably "blubber" a little off the line... DO NOT turn in the low speed screws... Instead, go back out on the course and fine tune for peak RPM again.. The blubbering off the line is caused by having too much fuel in the high speed circuit...

    Write down your new high speed setting on the piece of paper along with the best low speed setting that you found earlier, this will be your base-line carburetor settings.*

    If the high or low speed screws are open more than 1 1/4 turns, then you need to swap to the next bigger size jet.
    If the high or low speed screws are closed all the way, then you need to change to the next size smaller jet.

    *** If no matter what you try, it still won’t leave the line without falling on its face "lean bog"... Then try the next size lower pop-off spring in the carbs... Don't be afraid to try #95 or even #80 gram springs..


    Good info !

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