Need help tuning 750sx with dual 46mm carbs
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  1. #1
    PWCToday Newbie
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    Need help tuning 750sx with dual 46mm carbs

    I rebuilt my 93 small pin 750 motor. New crank, forged pistons, transfer ports, boost ports all raised 2mm, exhaust port widened to 59mm, 25cc head, cases ported, 46mm sbn carbs, 46mm intake, factory B pipe and exhaust manifold, aftermarket water box, MSD ignition enhancer. Just broke in the motor and noticed there is no top end. Bottom end feels ok, but will not rev up. Tried enriching carbs with no luck, and tried leaning carbs by adjusting mixture. Still cannot get the motor to preform. No bogging, no hesitation. The ski just flat lines at WOT. Any suggestions on jetting or timing? Anything will help. Thanks

  2. #2
    Resident Guru kartracer41's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs

    Who did the porting???
    RGR Racing.com

  3. #3
    Resident Guru JC-SuperJet's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs

    59mm wide Exhaust Port on an 80mm Bore (about 74% of Bore) is wide enough to cause premature Ring wear and maybe even snag the rings if the Chamfers aren't done right.

    Transfer/Booster Ports have both Horizontal and Vertical Angles that are very very sensitive to porting changes.

    Did the Porting Guy use a Two Stroke Design Program to model this set-up? What Software?

    The Ignition Timing should have been provided for you and so too the Jetting with recommended Adjustment Screw starting points.

    Post some photos of the Porting work for us to see.
    Last edited by JC-SuperJet; 10-16-2018 at 08:29 AM.

  4. #4
    Resident Guru JC-SuperJet's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs


  5. #5
    Resident Guru JC-SuperJet's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs

    What Impeller are you using? Maybe it's too tall?

  6. #6
    PWCToday.com Is My Home Away From Home Ragged Edge Racing's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs

    Best prop we had found was a Skat 10/18 with an 82mm exit nozzle and I had a 2mm stroker. It too was ported wide(not this wide) as I wasn't overly concerned about top speed, I wanted acceleration. It was about a 53mph ski and this was back in '95. I ran Blackjack 44's and that was more than enough for a 750. Had a Vilder ignition and flywheel as well running an R&D pipe.

  7. #7
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    Re: Need help tuning 750sx with dual 46mm carbs

    I did the porting myself, I have ported a few 550’s in the past along with a few motorcycle cylinders. I have a complete machine shop at my disposal and I’ve invested in costly porting tools years ago. I’m not saying I didn’t make a mistake but I do have experience and took my time for sure. I didn’t use any computer design, I simply made my own port maps using specs for others. I was very careful not to change any angles in the tunnels. I have gotten the ski to run tremendously better by lowering the pilot jet down to an 85 from 120, and the main down to a 145 from 150. I believe the pop off pressure is around 14 psi.

  8. #8
    Resident Guru JC-SuperJet's Avatar
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    Re: Need help tuning 750sx with dual 46mm carbs

    Well, raising the Transfer Ports by 2mm makes for an improved Bottom End hit. I suppose you traded some Top End for a better Bottom End hit.

    MCN6 knows all about these 750 cylinders. I think you used his recreational porting specs.

    Maybe he'll chime in and explain to us how the decrease in Blow-Down affects the Engine Top End performance. I'm guessing that at high rpms, the exhaust port opening time is too short and less efficient.

  9. #9
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    Re: Need help tuning 750sx with dual 46mm carbs

    I did use MCN6 specs on porting, hopefully he’ll chime in and give some advice. Thanks for all the responses!

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