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  1. #1
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    Default Mikuni Super 38 High Speed Screw Setting

    I'm rebuilding my MK Super BN 38's from my '96 SPX. The high speed needle screw has a cap on it which only allows 1/4 turn adjustment but I can't find where the initial position should be set at. IE: the low speed screw says to turn it all the way in and back out 1.25 turns for the SPX but it doesn't say anything about the high speed screw other than it only allows for 1/4 turn adjustment. I am assuming it calls to be screwed in all the way and the adjustment is only to open it up to 1/4 turn but I would like to be sure.

    TIA!

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  3. #2
    JoeZ's Avatar
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    I'll let other people chime in here too, but that is how I've done it, right or wrong..lol

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  4. #3
    seadoosnipe's Avatar
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    Cool Correct

    In another application I read about on the Mikuni carbs, the high speed is set at zero and only adjustable at 1/4 turn to rich or lean and that's it. Do not take off the plastic cap. Unless you removed the high speed screw, you shouldn't have to do anything. If you did, then turn it in till there is resistance, then start. If you need to adjust, it's only by 1/4 turn either way.
    Also, I thought sure I read that your model says to adjust the low speed screw 1 turn open, then corrections to + or - 1/4 turn.

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  5. #4
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    Default

    Well I totally disassembled the carbs for rebuilding. I should have noted the high speed screw positions before taking the caps off and the screws out. Actually I should have noted a lot of things because I ended up with 4 carbs on the table after buying two more off e-bay because the housing broke when removing one. I had to look at the manual and the carb kit diagrams more than a few times to get the PTO and MAG carbs right. Its weird how the PTO carb is missing the part of the body with the check valve disks.

    Sounds like I did the semi-right thing, we'll see soon.

    Thanks guys.

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  6. #5
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    Default

    Quote Originally Posted by seadoosnipe View Post
    Also, I thought sure I read that your model says to adjust the low speed screw 1 turn open, then corrections to + or - 1/4 turn.
    The manual says 1 1/4 turns for the low speed screw on the SPX snipe.

    Any tips on tuning with the low or high speed screws once they are back on? Or is it just best to leave them at spec?

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  7. #6
    seadoosnipe's Avatar
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    Default Useful info!

    O.K....I just went back through some information on your carbs. I looked again at the low speed setting for the Mikuni 38 and I did read once again that the initial setting is open 1 turn. Not just for your model SPX, but the XP, GSX, and GTX all run the same carb. No matter though, because it's only a 1/4 turn and it's also just a starting point.
    Once you get to that point, your going to start your motor. Checking the low speed screw, you'll look for the following. Does the engine idle smoothly? When pressing pulling on the throttle cable, is there quick, initial response? If not, make small adjustments until it smoothes out on you. If the screws are accessible while the engine is running, then try and adjust them that way. Listen to the sound of the engine. Try and adjust the carbs together, the same. But if your adjusting one and you hear the tone of the engine change, stop and adjust the other. Once you think you got it right, it sounds good, then back off on one carb till it sounds a little rough, then tighten back down till it's smooth again. This will put you on the side of error as rich. It's better to be on the side of rich, moreso than on the side of lean........
    The high speed screw starts to work on the carbs after being open, I believe it was 1/3. I'm already forgetting my info. But it did say maximum fuel delivery on the high speed screw is at 3 turns off of seat. Then you can go from there. If you start at three turns out, you start the motor and it runs smooth, low speed should be fine. Testing the high speed throttle will probably want to be done in the water. But when you open the throttle up, if it's running rough, stop ( and i may want to check the plugs to see how much fuel is on them) and turn the screw in a quator turn. Then try again till you find the sweet spot. As soon as you turn it in to the point it's running good, don't go any further because you don't want to lean her out at high speeds.....(eeeks, engine damage can occur).
    The Mikuni Carbs manual is in the premimum section and is a very informative and interesting read. It doesn't take that long to read it and in your application, it will probably give you a lot more info on the carbs than the actual seadoo shop manual!.......
    Last edited by seadoosnipe; 10-01-07 at 10:05 AM.

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  8. #7
    freebie fixer scooper77515's Avatar
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    Actually I should have noted a lot of things because I ended up with 4 carbs on the table after buying two more off e-bay because the housing broke when removing one.
    Sounds familiar...that is where I am right now...

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  9. #8
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    I can't believe I missed the Mikuni Carb Guide! Thanks!

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