So whats going on with my motor guys? cant figure it out?
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  1. #1
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    So whats going on with my motor guys? cant figure it out?

    So I have a 760 62t/62t in my sj, fp blaster mod pipe, milled head- 175psi, rec cylinder porting, ported case, carbon tech reeds, stock 38's with prok fa's and a primer kit. compression is within 1psi each cylinder, hot and cold. rebuilt carbs to try and help chase down this issue so all parts are new also. 99% of the time it starts without primer or throttle. idle is consistant and smooth and I can easily change it with the idle screw.

    seems to have great response and overall runs good until I hold it at a constant rpm for more than about 15 sec? it physically runs ok but when I go wot I get what feels like to me a slight lean hesitation? I get a sporadic slight bog off idle also? I tried 3 diff springs on my n&s, going up and down on my mains and lows and it changed it some but my original jetting seemed the best (135m, 75p, 2.0ns, ls 1, hs 1.25).

    this is where it gets weird? as I jetted up the pto cylinders plug got way dark, the mag plug didn't seem to change much if at all? tried staggering the screws, and eventually the jets but I still couldn't get the plug to what I would call safe? any ideas whats going on? could I have some bad reeds? crank seel leaking? any ideas? let me know what you think.

    -Jesse

  2. #2
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    Re: So whats going on with my motor guys? cant figure it out?

    BTW im by no means a pro tuner so It could be a rich bog but from looking at the plugs im going with lean>

    here is a few pics of the plugs:



    Last edited by millnoff; 08-18-2013 at 10:58 PM.

  3. #3
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: So whats going on with my motor guys? cant figure it out?

    Personally, I don't give plug readings much weight.

    What is the normal peak rpm, and how did you determine the low speed and hi speed screw positions?


    Bill M.
    Horsepower == Speed, RPM != Speed



  4. #4
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    Re: So whats going on with my motor guys? cant figure it out?

    Quote Originally Posted by wmazz View Post
    Personally, I don't give plug readings much weight.

    What is the normal peak rpm, and how did you determine the low speed and hi speed screw positions?


    Bill M.
    well, just guessed mostly lol. started at 1.5 turns out on both, and with 70p, and 130m and just went up from there tell it felt better and closer to what the mikuni manual says. I tried going out 1/2 a turn on the lows, and in 1/2 turn, and the bog was still there. tried 115g spring, 95, and 80. lower I went the worse it seemed to get and the pto plug showed the same, it got darker as I went. mag stayed the same, that's what worried me. tried 75p, and 77.5 and just made the it feel less snappy with no real change in the 1/4-wot range. tried going up to 137.5m and kept them in there, seemed like it liked that better than the 135's, response was better and top end seemed better (no tach or speedo). so just to try and get the plugs more even I went to a 77.5p in the mag carb, and open both screws about a 1/2 turn maybe a little less and it seemed to run better randomly and the same other times? and of course the plug was still looking lean.

    im trying not to pay to much attention to the plugs while tuning but its hard now seeing that the mag looks so lean...

  5. #5
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: So whats going on with my motor guys? cant figure it out?

    1/4 Throttle is a problem are for our type of carbs. It is the point where manifold vacuum is rapidly decreasing,
    and air speed is slowly increasing. It creates a natural lean spot.

    Recreational porting normally has exhaust port durations near 186 degrees. Cutting exhaust ports above 180 deg
    duration decreases manifold vacuum and creates a bog or soft power around 1/4 to 1/2 throttle.

    Combine the normal 1/4 throttle problem with your porting, jetting problems, vacuum leaks, and the possibility the
    carbs are not properly synchronized, impeller size, and unknown peak rpm.

    You must be close, or your problems would be much worse.

    Quote Originally Posted by millnoff View Post
    but my original jetting seemed the best (135m, 75p, 2.0ns, ls 1, hs 1.25).
    Your jetting looks close. Use a 95gr spring. Work on vacuum leaks, carb synchronization, and learn how to do an
    idle drop test to set your pilot air screws. Get a SeaDoo or Polaris tachometer, and determine your impeller size.

    Depending on your porting, your problem maybe just a characteristic you need to live with, but in the future, maybe
    you can minimize it.


    Bill M.
    Horsepower == Speed, RPM != Speed



  6. #6
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    Re: So whats going on with my motor guys? cant figure it out?

    well I pulled the intake mani, tossed the 75 pilots back in, and 95g spring, new gaskets and swapped the reed cages to the opposite cylinders and it runs mint? so maybe had a slight air leak or something? plugs still look lean but the ski runs fine now? thanks for the help and info wmazz!!

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